THE VERTEBRAL COLUMN The vertebral column (columna vertebralis) or the spine has a metameric structure (a feature connecting the vertebrates with the earliest invertebrates) and consists of separate bone segments, vertebrae, placed one over another in a series; they are short spongy bones. Function of the spine. The spine acts as the axial skeleton supporting the body. It protects the spinal cord enclosed in its canal and takes part in the movements of the trunk and head. The position and shape of the vertebral column are determined by the upright position of man. Common features of the vertebrae. In accordance with the three functions of the spine, each vertebra (Gk. spondylos1) has the following features an anterior part, which is responsible for support and which thickens in the shape of a short column, this is the body (corpus vertebrae); an arch (arcus vertebrae), which is attached to the posterior surface of the body by two pedincles (pedinculi arcus vertebrae) and contributes to the formation of the vertebral foramen (foramen vertebrate); a series of these foramina forms the vertebral or spinal canal (canalis vertebralis), which protects the spinal cord lodged in it from external injury. the arch also carries structures permitting movement on the vertebra called processes. A spinous process (processus spinosus) arises from the arch on the midline; a transverse process (processus transversus) projects laterally on each side; paired superior and inferior articular processes (processus articulares superiores and inferiores) project upward and downward. The articular processus bind notches on the posterior aspect; these are the paired incisurae vertebrates superiores and inferiores from which the intervertebral foramina (foramina intervertebralia) form when one vertebra is placed on another. The foramina transmit the nerves and vessels of the spinal cord. The articular processes serve for the formation of intervertebral joints at which movement of the vertebrae is accomplished; the transverse and the spinous processes serve for the attachment of ligaments and muscles which make the vertebrae move. Some parts of the vertebrae in the different parts of the spine have a distinctive size and shape and the following vertebrae are consequently distinguished: cervical (seven), thoracic (twelve), lumbar (five), sacral (five), and coccygeal (three to five).

INDIVIDUAL TYPES OF VERTEBRAE Cervical vertebrae (vertebrae cervicales). Since the load suffered by the cervical vertebrae is lighter than that suffered by the more distally located spinal segments, their bodies are correspondingly smaller. The bodies are transverse-oval in shape, and the upper and lower surfaces are concave. Each transverse process is characterized by the presence of a hole (foramen transversarium), which forms as a result of fusion of the transverse process with the rib rudiment. The canal which forms from a series of these foramina protects the vertebral artery and the vein that it transmits. This fusion is manifested on the ends of the transverse processes as two tubercles (tuberculum anterius and posterius). The anterior tubercle of the sixth vertebra is enlarged and is called the carotid tubercle (luberculum caroticum) (the carotid artery can be compressed against it to arrest bleeding). The spinous process is bifid, with the exception of that of the sixth and seventh vertebrae. The seventh cervical vertebra is distinguished by a large spinous process, and for that reason it is called the vertebra prominens. This vertebra is easily located in a living person and is often helpful in making diagnosis. The first and second cervical vertebrae have a specific shape because they form the mobile articulation with the skull. Most of the body of the first vertebra, the atlas, remains separate and joins the second vertebra as a tooth-like process, the dens. The anterior (arcus anterior) and posterior (arcus posterior) arches of the atlas are connected to each other by lateral masses (massae laterales). On the inner surface of the anterior arch is a depression(fovea dentis)for articulation with the dens of the second vertebra. The superior and inferior surfaces of each arch articulate with the adjoining bones: the convex superior articular facet (fovea articularis superior) receives the corresponding condyle of the occipital bone; the flattened inferior articular facet (fovea articularis inferior) receives the articular surface of the second cervical vertebra. The outer surfaces of the anterior and posterior arches carry tubercles (tuberculum anterius and posterius). The second cervical vertebra, the axis (consequently the axial vertebra) s. epistropheus (BNA) (Gk. epistrephomai pivot, consequently the pivotal vertebra) differs sharply from all the other vertebrae by the presence of the tooth-like process, the dens. The dens has articular surfaces, which on the anterior aspect serve to articulate with the anterior arch of the atlas and on the posterior aspect to attach the transverse ligament. Another distinction of the axis is that its superior articular surfaces articulating with the atlas are not on the arch but on the superior surface of the body to the sides of the dens. 2. Thoracic vertebrae (vertebrae thoracicae) articulate with the ribs. Their distinctive feature, consequently, is the presence of articular facets for the ribs, costal facets (fovea costales), on the body of each vertebra close to the base of the arch. Since the ribs usually articulate with two adjoining vertebrae, most vertebral bodies have two incomplete (half) costal facets: one on the superior edge of the vertebra (fovea costalis superior) and the other on the inferior surface (fovea costalis inferior). When one vertebra is placed on the other, the two half-facets form a single whole articular facet, which receives the head of the rib. In accordance with the greater weight they bear, the bodies of the thoracic vertebrae are larger than the bodies of the cervical vertebrae. The articular processes are positioned frontally. The transverse processes are directed laterally and to the back. They have a small articular surface, transverse costal facet (fovea costalis processus transversus) for articulating with the tubercle of the ribs. The spinous processes of the thoracic vertebrae are long and are inclined sharply downward, as a result of which they overlie one another like tiles, mainly in the middle of the thoracic part of the spine. Such direction of the processes limits extension of the spine here, which is a protective accommodation for the heart. 3. Lumbar vertebrae (vertebrae lumbales) are distinguished by a massive body since they carry weight that is still greater than that borne by the part of the spine proximal to them. The spinous processes are short and flat, directed horizontally, to the back. The articular processes are in the sagittal plane. 4. Sacral vertebrae (vertebrae sacrales) fuse in youth to form a single bone, the sacrum. This fusion is an adaptation to the considerable load carried by the sacrum because of the upright posture of the human. The sacrum is triangular in shape, and its base (basis ossis sacri) faces upward, while the apex (apex ossis sacri) faces downward. The anterior border of the base together with the body of the last lumbar vertebra forms an angle projecting forward, a prominence (promontorium). The ventral or pelvic surface of the sacrum (facies pelvina) is concave. The sites of the fusion of the vertebral bodies are seen on it as transverse lines (lineae transversae) with the anterior sacral foramina (foramina sacralia pelvina) at their ends. On the dorsal surface there are, correspondingly, the posterior sacral foramina (foramina sacralia dorsalia). Five crests formed by fusion of different parts of the vertebrae stretch lengthwise on the dorsal surface: an unpaired spinous tubercles of the sacrum on the median line (crista sacralis mediana) formed as the result of fusion of the spinous processes; the articular tubercles of the sacrum (cristae sacrales intermediae) from by fusion of the articular processes and lateral to these, the transverse tubercles of the sacrum (cristae sacrales laterales) (sites of fusion of the transverse processes). Lateral to the sacral foramina are the lateral parts of the sacrum (partes laterales). They have on their lateral aspect an articular surface curved like the auricle, which is called the auricular surface (facies auriculares). It serves for joining with the iliac bone. At the back of each auricular surface is the sacral tuberosity (tuberositas sacralis) (the site of attachment of muscles and ligaments). The sacral canal (canalis sacralis) passes in the sacrum. It is a continuation of the vertebral canal. As a consequence of the disappearance of the tail and reduction of the tail musculature in man, the corresponding parts of the sacral vertebrae are reduced, therefore, the sacral canal is not closed in its distal part but opens as the sacral hiatus (hiatus sacralis). Lateral to this hiatus are the sacral cornua (cornua sacralia), remnants of the last sacral vertebra, which articulate with similar cornua of the coccyx. 5. Coccygeal vertebrae (vertebrae coccygeae s. caudales) are remnants of the tail and rudimentary structures fusing at middle age to form a single bone, the coccyx (os coccygis).

THE THORACIC CAGE Joining posteriorly with the thoracic vertebrae and anteriorly with the unpaired breast-bone, or sternum, the ribs form the thoracic cage, the thorax, or the chest. THE STERNUM The breast-bone (sternum) resembling a dagger in shape, consists of three parts: the upper part, the manubrium sterni, the middle part, the body (corpus sterni), and the lower part, the xiphoid process (processus xiphoideus). The manubrium has a jugular notch (incisura jugularis} on its superior border and lateral to this, a clavicular notch (incisura clavicularis) on each side, by means of which the sternum articulates with the sternal end of the clavicle. The inferior border of the manubrium and the superior border of the body join at an anteriorly protruding angle, the angle of the sternum (angulus sterni). The lateral border of the sternal body has costal facets (incisurae costales) for articulation with the cartilages of the ribs, beginning with the second rib. THE RIBS There are twelve ribs on each side, and all articulate by their posterior ends with the bodies of the thoracic vertebrae. The anterior ends of the upper seven ribs are joined directly to the sternum by cartilages. These are the true ribs (costae verae). The next three ribs, eighth, ninth, and tenth, are joined by their cartilages not to the sternum but to the cartilage of the rib next above, and are referred to as false ribs (costae spuriae). The anterior ends of the eleventh and twelfth ribs lie free, and these ribs are called floating ribs (costae fluctuantes). The ribs (costae) are narrow curved plates which in their longest, posterior part are formed of bone, os costale, belonging to the group of long spongy bones, and in the narrower anterior part, of cartilage, cartilago costalis. Posterior and anterior ends and a shaft of the rib (corpus costae) between them are distinguished in each bony rib. The posterior end is slightly enlarged to form the head of the rib (caput costae), which has an articular facet; the rib articulates by means of this facet with the vertebral bodies. A narrowed part, the neck of the rib (collum costae), immediately succeeds the head. At the junction of the neck and shaft, there is a tubercle of the rib (tuberculum costae), which carries a facet for joining with the articular surface of the transverse process of the corresponding vertebra. Lateral to the tubercle, the rib curvature is greatly accentuated and the angle of the rib (angulus costae) is located here posteriorly on the shaft. A groove (sulcus costae) is detectable on the inferior border of the inner surface of the middle ribs; it conducts the intercostal vessels.

THE SKELETON OF THE UPPER LIMB THE SHOULDER GIRDLE The shoulder girdle (cingulum membri superioris) consists of two paired bones, the clavicle (collar bone) and the scapula (shoulder blade). THE CLAVICLE The clavicle (clavicula) is the only bone fastening the upper limb to the skeleton of the trunk. It is of high functional importance because it holds the shoulder joint at the needed distance from the thoracic cage and thus permits greater freedom of movement of the limb. The clavicle is a short tubular bone, and a body (shaft) and two ends, are distinguished in it. The thickened sternal end (extremitas sternalis) carries a saddle-shaped articular surface for uniting with the sternum. The acromial end (extremitas acromialis) has a flat articular surface for articulating with the acromial process of the scapula. On the inferior surface of the acromial end is the conoid tubercle (tuberculum conoideum). THE SCAPULA The shoulder blade or scapula is a flat triangular bone lying on the posterior surface of the thoracic cage in the space between the second and seventh ribs. According to the shape of the bone, three borders are distinguished in it: the medial border (margo medialis), facing the spine, the lateral border (margo lateralis), and the superior border (margo superior). The. three borders meet at three angles, one of which, the inferior angle (angulus inferior) is directed downward while the other two, the superior angle (angulus superior) and lateral angle (angulus lateralis) are at the ends of the superior border of the scapula. The lateral angle is greatly thickened and supplied with a shallow depression, a laterally positioned articular glenoid cavity (cavitas glenoidalis) wich articulates with the head of the humerus. The coracoid process (processus coracoideus) arises from the superior border of the scapula in the vicinity of the glenoid cavity. The anterior, costal surface of the scapula (facies costalis) forms a hollow depression called the subscapular fossa (fossa subscapularis) which gives attachment to the subscapular muscle. The spine of the scapula (spina scapulae) projects from the dorsal surface of the scapula. The spine of the scapula stretches laterally and is continuous with the acromion overhanging the glenoid cavity at the back and above.

THE SKELETON OF THE FREE UPPER LIMB The skeleton of the free upper limb consists of the humerus, two forearm bones, and the bones of the hand. The Humerus The humerus is a long lever of movement, which develops like a typical long tubular bone. In conformity with this function and development, it is made up of a diaphysis, metaphyses, epiphyses, and apophyses. Its upper end is supplied with a spherical head (caput humeri) (the proximal epiphysis), which articulates with the glenoid cavity of the scapula. The head is separated from the rest of the bone by a narrow groove called the anatomical neck (collum anatomicum). Directly below it are two tubercles for attachment of the muscles. The greater tubercle (tuberculum majus) is on the lateral side; the lesser tubercle (tuberculum minus) is a little to the front of it (apophyses). The intertubercular groove (sulcus intertubercularis) passes between both tubercles and lodges the tendon of the long head of the biceps muscle. The part of the humerus directly below the tubercles at the junction with the diaphysis is called the surgical neck (collum chirurgicum) (where fractures often occur). The upper part of the body (shaft) of the humerus has cylindrical outlines, but its lower part has a distinctly trihedral shape. About the middle of the shaft, is the deltoid tuberosity (tuberositas deltoidea). The widened distal part of the humerus, the condyle (condylus humeri) is bent slightly forward and has two rough projections on its sides, the medial and lateral epicondyles (epicondylus medialis and epicondylus lateralis), Two parts are distinguished in the condyle. Medially is the pulley-shaped trochlea; it articulates with the ulna and laterally of the trochlea is an articular surface the shape of a sphere, the capitulum of the humerus (capitulum humeri) for articulation with the radius. .Above the trochlea are two fossae, one, the coronary fossa (fossa coronoidea) in front, and the other, the olecranon fossa (fossa olecrani) behind. A small radial fossa (fossa radialis) lies above the capitulum in front.

BONES OF THE FOREARM The forearm bones belong to the group of long tubular bones. There are two of them, the ulna, which is the medial bone, and the radius, the lateral bone. The Ulna The ulna (s. cubitus). The upper (proximal) thickened end of the ulna is separated into two processes: the thicker, posterior process, olecranon and the smaller anterior, coronoid process (processus coronoideus). Between these two processes is the trochlear notch (incisura trochlearis) for articulation with the trochlea of the humerus. In front, under the coronoid process, there is the tuberosity of the ulna (tuberositas ulnae). The lower (distal) end of the ulna carries a spherical head with a flat inferior surface (caput ulnae) (epiphysis) from the medial surface of which the styloid process (processus styloideus) (apophysis) projects. The head carries on its circumference an articular surface (circumferentia articularis), by means of which it articulates with the adjacent radius. The Radius The radius, in contrast to the ulna, has a distal end that is thicker than the proximal end. The proximal end forms a rounded head (caput radii), which has a concave surface for articulation with the head of the humerus. One third or one half of the head circumference is also occupied by an articular surface (circumferentia articularis) articulating with the ulna. The head of the radius is separated from the rest of the bone by a neck (collum radii) directly below of which is the radial tuberosity (tuberositas radii), providing attachment for the biceps muscle of the arm. The lateral border of the distal end of the radius is continuous with the styloid process (processus styloideus). The carpal articular surface (facies articularis carpea) on the distal epiphysis serves for articulation with the carpal bones. The medial border of the distal radial end has a small ulnar notch (incisura ulnaris) for articulation with the ulnar head. THE HONES OF THE HAND The bones of the hand are subdivided into the carpal and metacarpal bones and the bones which are the components of the fingers, the phalanges. The Carpus The carpus is an aggregate of eight short, spongy bones, carpal bones (ossa carpi) arranged in two rows of four bones each. The Metacarpus The metacarpus consists of five metacarpal bones (ossa metacarpalia), which are related in type to short tubular bones and are numbered in sequence, beginning with the thumb: first, second, third, fourth, and fifth. Each carpal bone has a base (basis), a diaphysis (body, shaft) (corpus) and a rounded head (caput). Bones of the Fingers The bones of the fingers (ossa digitorum manus), called phalanges, are small, short tubular bones. Each finger, with the exception of the thumb, is made up of three phalanges: proximal (phalanx proximalis), middle (phalanx media), and distal (phalanx distalis) or ungual phalanx. The thumb has only two phalanges, the proximal and the distal phalanx. In all animals the thumb is less developed than the other fingers; it is highly developed only in man. THE SKELETON OF THE LOWER LIMB THE PELVIC GIRDLE The pelvic girdle is made up of the paired hip or innominate bone the hip bone (os coxae) is a flat bone concerned with the function of movement (takes part in articulations with the sacrum and femur), protection (shields the pelvic organs), and support (transfers the weight of the whole proximal part of the body to the lower limbs). The latter function prevails, and this determines the complex structure of the hip bone and its formation from fusion of three separate bones, the ilium (os ilium), the pubis (os pubis) and the ischium (os ischii). These bones fuse in the region bearing the greatest weight, namely, in the region of the acetabulum, the articular cavity of the hip joint, by means of which the pelvic girdle is connected to the free lower limb. The ilium is above the acetabulum, the pubis below and to the front of it, and the ischium is below and to the back of the acetabulum. In individuals under 16 years of age these bones are separated one from another by layers of cartilage which in an adult undergo ossification, i.e. synchondrosis changes to synostosis. As a result the three bones fuse to form a single bone possessing great strength necessary for bearing the weight of the whole trunk and head. The acetabulum ("vinegar curet" from L acetum vinegar) is on the lateral surface of the hip bone and serves for articulation with the head of the femur. It is a rather deep, cup-shaped cavity with a high rim, in the medial side of which is a notch (incisura acetabuli). The smooth articular surface of the acetabulum (facies lunata) is crescent- shaped: the centre, the acetabular fossa (fossa acetabuli) and the part nearest to the notch are rough. THE ILIUM The ilium fuses by means of its short, thick, inferior part, called the body (corpus ossis ilii), with the other parts of the hip bone in the region of the acetabulum; the superior fan-shaped and fairly thin part of the ilium forms the wing or ala (ala ossis ilii).The superior free border of the wing, for instance is a sinuous crest (crista iliaca) to which three broad abdominal muscles are attached. The crest ends anteriorly and posteriorli by iliac spines. Below the posterior inferior spine is the deep greater sciatic notch (incisura ischiadica major).. The inner (medial) surface of the iliac wing is smooth, slightly cancave, and forms the iliac fossa (fossa iliaca) produced from supporting the viscera in vertical posture of the body. Posterior to and below the fossa is an ear-shaped articular surface, auricular surface (facies auricularis) the site of articulation with the corresponding surface of the sacrum. Behind and above the auricular surface is the iliac tuberosity (tuberositas iliaca) giving attachment to the interosseous sacro- iliac ligaments. Occasionally conspicuous rough lines, marks of the origin of the gluteal muscles, are seen on the external (lateral) surface of the iliac wing. THE PUBIS The pubic bone (os pubis) has a short thickened body (corpus ossis pubis) adjoining the acetabulum, and the superior and inferior rami (ramus superior and ramus inferior ossis pubis) forming an angle. At the apex of the angle facing the midline is an oval symphyseal surface (facies symphysialis) for articulation with the contralateral pubic bone. A small pubic tubercle (tuberculum pubicum) lies 2 cm lateral of this surface. THE ISCHIUM The ischium (os ischii) has, like the pubis, a body (corpus ossis ischii), which forms part of the acetabulum, and a ramus (ramus ossis ischii). The body and the ramus meet at an angle, the apex of which is greatly thickened and is the ischial tuberosity (tuber ischiadicum). On the posterior border of the body, upward from the ischial tuberosity, is the lesser sciatic notch (incisura isciadica minor) separated from the greater sciatic notch (incisura ischiadica major) by the ischial spine (spina ischiadica). The ischial ramus branching from the ischial tuberosity fuses with the inferior pubic ramus. As a result the rami of the pubis and ischium surround the obturator foramen (foramen obturatum) which is located medial to the acetabulum and is triangular with rounded angles. THE SKELETON OF THE FREE LOWER LIMB The skeleton of the lower limb consists of the femur, or thigh bone, two leg bones, and the bones of the foot. Besides, a small (sesamoid) bone, the patella, adjoins the thigh bone. The Femur The femur, or thigh bone, is the largest and thickest long tubular bone. Like all such bones it is a long lever of movement and has a diaphysis, metaphyses, epiphyses, and apophyses in accordance with its development. The upper (proximal) end of the femur carries a spherical articular head (caput femoris), a little downward from the centre of the head is a small rough depression (fovea capitis femoris), where the ligament of the head is attached. The head is connected with the rest of the bone by a neck (collum femoris), which meets the axis of the femoral shaft at an obtuse angle (about 130 degree's); in the wider female pelvis this angle is closer to 90 degrees. Two bony prominences called trochanters (apophyses) are found at the junction of the neck with the shaft of the femur. The greater trochanter (trochanter major) is the upper end of the femoral shaft. The lesser trochanter (trochanter minor) is at the inferior margin the neck on the medial surface and a little to the back. Both trochanters are joined on the posterior surface of the femur by an oblique intertrochanteric crest (crista intertrochanterica) and on the anterior surface by the intertrochanteric line (linea intertrochanterica). The body (shaft) of the femur is slightly convex forward and has a rounded trihedral shape; its anterior and lateral surfaces are smooth, while the posterior surface bears a mark of attachment of the thigh muscles. The lower (distal) thickened end of the femur forms two rounded, posteriorly turned medial and lateral condyles (condylus medialis and condylus lateralis) (epiphysis). Despite the difference in the size of these two condyles, however, they are located on the same level because the femur in its natural position stands obliquely with its lower end closer to the midline than the upper end. Anteriorly the articular surfaces of the condyles blend with each other to form a small concavity in the sagittal direction. This common part of the articular surfaces is called facies patellaris because the posterior surface of the patella abuts against it in extension at the knee joint. On the posterior and inferior surfaces, the condyles are separated by a deep intercondylar fossa(fossa intercondylaris). Rough prominences are found on the sides of each condyle above the articular surface. These are the medial and lateral epicondyles (epicondylus medialis and epicondylus lateralis). The Patella The patella, or knee-cap, is none other than a large sesamoid bone lodged in the tendon of the quadriceps femoris muscle stretching over the anterior surface of the knee joint. THE SKELETON OF THE LEG The skeleton of the leg consists of two bones of unequal thickness, the tibia and the fibula. The first is on the medial and the second on the lateral side. Only the tibia articulates with the femur by means of the knee joint.

The Tibia The proximal end of the tibia (epiphysis) forms two condyles, medial (condylus medialis) and lateral (condylus lateralis). On the surface facing the femur the condyles are supplied with shallow articular surfaces (facies articularis superior) for articulation with the condyles of the femur. The articular surfaces of the tibial condyles are separated one from the other by an intercondylar eminence (eminentia intercondylaris). The eminence has two small intercondylar depressions on its ends: area intercondylaris anterior on the anterior end and area intercondylar is posterior on the posterior end (all these structures formed from attachment of the intra-articular ligaments). On the anterior surface of the tibia, there is a rather massive, roughened convexity called tuberosity of the tibia (tuberositas tibiae) for attachment of the patellar ligament . A small flat articular surface (facies articularis fibularis) is in the posterolateral part of the lateral condyle. It is the site of articulation with the head of the fibula. The body (shaft) of the tibia is trihedral. The lower, distal end of the tibia (epiphysis) has a strong process, the medial malleolus (malleolus medialis) below on its medial side. The Fibula The fibula (Gk. perone) is a long thin bone with thickened ends. The upper (proximal) epiphysis forms the head (caput fibulae) which articulates with the lateral condyle of the tibia. THE BONES OF THE FOOT The tarsus, metatarsus, and the bones of the toes are distinguished in the foot. The Tarsus The tarsus is made up of seven short spongy bones (ossa tarsi), which are arranged in two rows similar to the carpal bones. The proximal row is formed of two comparatively large bones, the talus and the calcaneus lying below it. The distal row is formed of five bones; by the navicular and three cuneiform bones and by a single cuboid bone. The talus (ankle bone) consists of a body (corpus tali), which extends anteriorly as a constricted neck (collum tali). The. neck is continuous with an oval convex head (caput tali), carrying a surface for articulation with the navicular bone. The body of the talus has a trochlea tali on its superior surface for articulation with the leg bones. The calcaneus (heel bone) has two articular surfaces (anterior and posterior) on its superior surface, which correspond to the inferior articular facets on the talus. The calcaneus gives off a medial process called the systenaculum (L support) tali, so called because it supports the head of the talus

The Metatarsus The metatarsus consists of five metatarsal bones (ossa metatarsalia) related to the short tubular bones and resembling the carpals of the hand. Just as in the hand, a proximal end, or base (basis), a middle part, or body (corpus), and a distal end, or head (caput), are distinguished in each metatarsal. They are numbered from the medial border of the foot. Bones of the Toes The bones of the toes, the phalanges (phalanges digitomm pedis) are short tubular bones.

GENERAL INFORMATION ON BONE ARTICULATIONS (GENERAL SYNDESMOLOGY) Two types of bone joining developed in the process of phylogenesis: the elementary, compact joining with a limited range of movements and a later, interrupted type allowing wide movements. According to development, structure, and function, all bone articulations can be divided into two large groups 1. Contiguous articulations, synarthroses (BNA), earliest in development, fixed in function or allowing slight movement. 2. Interrupted (synovial) articulations, diarthroses (BNA), latest in development and permitting more movements functionally. Between these forms there is a transitional form, from contiguous to interrupted or vice versa. It is characterized by the presence of a small gap which does not have the structure of a true articular cavity, and because of this the articulation is called a half-joint, or hemiarthrosis. Schematically Representation of Bone Joining

CONTIGUOUS ARTICULATIONS (SYNARTHROSES) The following three types of synarthroses are distinguished I. If connective tissue remains in the space between the bones after birth, they become joined by means of this tissue and is called fibrous or syndesmosis. II. If the intermediate connective tissue transforms to cartilaginous tissue, which remains after birth, the bones become joined by means of cartilaginous tissue, and is called cartilaginous or synchondrosis (chondros cartilage). III. Finally, if the connective tissue between the bones transforms to bone tissue or first to cartilaginous and then to bone tissue, the bones become joined by means of bone tissue, and is called synostosis (BNA). TYPES OF SYNDESMOSES Syndesmosis (articulatio fibrosa) is contiguous joining of bones by means of connective tissue. 1. If the connective tissue fills a large space between the bones, the articulation acquires the form of interosseous membranes (membrana interossea), for instance between the bones of the forearm or leg. 2. If the connective tissue between the bones has the structure of fibrous bundles, fibrous ligaments (ligamenta) form in all the joints. 3. Until wide remnants of the primary connective tissue remain between the bones of the skull-cap, the joints are called fontanelles (fonticuli). 4. When the intermediate connective tissue takes the character of a thin layer between the skull bones, sutures (suturae) form. The following sutures are distinguished according to the shape of the articulating bone margins: (a) serrate (sutura serrata), when the projections on the margin of one bone fit between the projections on the opposing bone (most bones of the skull-cap articulate in this fashion); (b) squamous (sutura squamosa), when the margin of one bone overlaps that of the opposing, bone (articulation between the margins of the temporal and parietal bones); (c) plane (sutura plana), apposition of smooth margins (joining of the bones of the facial cranium) TYPES OF SYNCHONDROSES The following synchondroses are distinguished according to the duration of their existence: 1. Temporary, existing only to a definite age after which they are replaced by synostoses, for instance, synchondrosis between the three bones of the pelvic girdle which fuse to form a single pelvic bone. 2. Permanent, existing throughout life, for instance, synchondroses between the pyramid of the temporal bone and the sphenoid bone, or between the vertebral bodies ther are intervertebral discs. INTERRUPTED (SYNOVIAL) ARTICULATIONS, JOINTS, DIARTROSES A joint is an interrupted, cavitary, mobile articulation (articulatio synovialis) (Gk. arthron joint, hence arthritis, inflammation of a joint). Articular surfaces of the articulating bones, the articular capsule enclosing the articular ends of the bones, and a joint cavity within the capsule between the bones are distinguished in each joint. They are the obligative components of the the joint. 1. The articular surfaces (facies articulares) are covered by articular cartilage (cartilago articularis), which is hyaline, 0.2-0.5 mm thick. 2. The articular capsule (capsula articularis) encloses the joint cavity hermetically and is attached to the articulating bones along the margin of the articular surfaces or at some distance from them. It consists of an outer fibrous membrane (membrana fibrosa) and an inner synovial membrane (membrana synovialis). The synovial membrane surface facing the joint cavity is lined with a layer of endothelial cells which makes it smooth and shiny. The membrane secretes a sticky, clear synovial fluid (synouia) into the cavity of the joint. 3. The joint cavity (cavitas articularis) is a closed, air-tight slit bounded by the articular surfaces and the synovial membrane. Under normal conditions it is not an empty cavity but contains synovial fluid which moistens and lubricates the articular surfaces and lessens the friction between them. JOINT BIOMECHANICS The following types of movements at the joints are distinguished: 1. Movement on the frontal axis: flexion (flexio), i.e. a reduction of the angle formed by the articulating bones, and extension (extensio), i.e. increase of this angle. 2. Movements on the sagittal axis: adduction (adductio}, i.e. drawing towards the median plane, and abduction (abductio), i.e. drawing away from it. 3. Movements on the vertical axis, i.e. rotation (rotatio}, inward and outward or to the right and to the left. 4. Movement in a circular manner, circumduction (circumductio), by changing from one axis to another with one end of the bone describing a circle and the whole bone describing the figure of a cone. CLASSIFICATION AND GENERAL CHARACTERISTICS OF JOINTS The classification of joints can be based on the following principles: (1) the number of articular surfaces; (2) the shape of the articular surfaces; and (3) function. The following joints are distinguished according to the number of articular surfaces 1. Simple joint (art. simplex), which has only two articular surfaces, e.g. the hip joint. 2. Compound joint (art. composita), which has more than two articulating surfaces, e.g. the elbow joint. 3. Complex joint (art. complexa) contains an intra-articular cartilage in the articular capsule. This cartilage divides the joint into two compartments (a bilocular joint) either completely (if the intra-articular cartilage is shaped like a disc, e.g. in the temporomandibular joint) or incompletely (if the cartilage is a crescentic meniscus, e.g. in the knee joint). 4. Combined joint is a combination of several isolated joints, located separately but functioning together. Such are, for example, the two temporomandibular joints, the proximal and distal radio-ulnar joints. According to shape and function, joints are classified as follows. The function of a joint is determined by the number of axes on which the movement occurs. The number of axes on which movements are accomplished in the given joint, in turn, depends on the shape of the articulating surfaces. UNIAXIAL JOINTS 1. Trochoid joint (art. trochoidea). This is a cylindrical or wheel-like articular surface (Gk. trochos wheel, eidos form) whose axis is a vertical line running parallel to the long axis of the articulating bones or to the vertical body axis; it permits movements on one vertical axis, i.e. rotation; it is also called the pivot joint. Such are, for example, the atlantoaxial median joint. 2. Hinge joint (ginglymus) (Gk. ginglymos hinge). An example are the interphalangeal joints of the fingers and toes. The articular surface is a cylinder stretching transversely whose long axis is a transverse line running in the frontal plane perpendicular to the long axis of the articulating bones; as a result movements at a hinge joint are made on this frontal axis (flexion and extension). BIAXIAL JOINTS 1. Ellipsoid joint (articulatio ellipsoidea) (for instance, the radiocarpal or wrist joint). The articulating surfaces are segments of an ellipse: one of them is convex, oval and unequally curved in two directions; the other is correspondingly concave. They allow movements on two horizontal axes which are perpendicular to each other: flexion and extension on the frontal axis and adduction and abduction on the saggital axis. 2. Condyloid joint (articulatio condylaris} (e.g. the knee joint). A condyloid joint has a convex articular head in the form at a protruding rounded process which resembles an ellipse in shape and is called a condyle (condylus) (Gk. kondylos knuckle). 3. Saddle joint (articulatio sellaris) (e.g. the carpometacarpal joint of the thumb). This joint is formed by two saddle-shaped articulating surfaces, one "astride" the other moving lengthwise and across the other. As a result this joint allows movement on two mutually perpendicular axes: frontal (flexion and extension) and sagittal (abduction and adduction). Biaxial joints also permit a change of movement from one axis to another, i.e. movement in a circular manner (circumduction).

MULTIAXIAL JOINTS 1. Ball-and-socket joint (art. spheroidea) (e.g. the shoulder joint). One of the articular surfaces forms a convex spherical head, the other a correspondingly concave articular cavity. Theoretically, movements can occur on many axes which correspond to the radii of a sphere. Practically, however, only the following three main axes, perpendicular to each other and intersecting in the center of the head, are distinguished: (1) the frontal axis, the site of forward flexion, anteflexion, when the moving part together with the frontal plane forms an angle open to the front, and of backward flexion, retroflexion, when the angle is open to the back; (2) the sagittal axis, on which abduction and adduction are accomplished; and (3) the vertical axis, on whose circumference inward and outward rotation occurs. Circumduction takes place when movement changes from one axis to another. The ball-and-socket joint is the most freely mobile joint. A variety of the ball-and-socket articulation is the cotyloid joint (art. cotylica) . Its articular fossa is deep and embraces the greater part of the head. As a result movement is more restricted at the joint than at a typical ball-and-socket joint. An example of this type of articulation is the hip joint. 2. Plane joints (art. plana) (e.g. the intervertebral joints). The articular surfaces in these joints are almost flat. They can be regarded as the surfaces of a sphere with a very large radius. They consequently allow movement on all three axes, but the range of movement is small because the articular surfaces differ only slightly. JOINTS BETWEEN THE VERTEBRAL BODIES The vertebral bodies forming the vertebral column proper which supports the trunk unite one with another (and also with the sacrum) by means of synchondroses called intervertebral cartilages or discs (disci intervertebrales) or by means of hemiarthroses if there are clefts between them. Each disc is a fibrocartilaginous plate whose periphery is formed of concentric layers of connective-tissue fibres. These fibres form a very strong peripheral fibrous ring (anulus fibrosus), while the central part of the plate is a gelatinous nucleus (nucleus pulposus) consisting of soft fibrous cartilage. This nucleus is under considerable pressure and continuously tends to distend (on cross-section of the disc it protrudes markedly above the level of the section) and is therefore resilient and acts as a buffer. The column of vertebral bodies united by the intervertebral discs is re-enforced by two longitudinal ligaments running in front and back on the median line. The anterior longitudinal ligament (lig. longitudinale anterius) stretches on the anterior surface of the vertebral bodies and discs. This ligament prevents abnormal backward extension of the spine. The posterior longitudinal ligament (lig. longitudinale posterius) extends downward on the posterior surface of the vertebral bodies in the vertebral canal. It hinders flexion and is a functional antagonist of the anterior longitudinal ligament. JOINTS BETWEEN THE VERTEBRAL ARCHES The arches are united by joints and ligaments located both between the arches themselves and between their processes 1. Union between the articular processes is accomplished by the intervertebral joints (articulationes intervertebrales). Since these are flat, tight joints, which are limited in movement, they limit flexibility of the spine and give it a definite direction in conformity with the position of the articular surfaces in the different parts of the spine (multiaxial joints). 2. The spaces between the arches are filled by elastic fibres of yellow colour which are, therefore, called yellow ligaments (ligamenta flava) 3. The ligaments between the spinous processes, the interspinous ligaments (ligamenta interspinalia), are developed most markedly in the lumbar region. A roundish band continuous with the interspinous ligaments in the back stretches over the apices of the spinous processes as a long supraspinous ligament (lig. supraspinale). In the cervical part of the spine the interspinous ligaments stretch beyond the apices of the spinous processes and form the sagittal nuchal ligament (lig. nuchae). It is triangular. 4. Ligaments between the transverse processes, the intertransverse ligaments (ligamenta intertransversaria), limit lateral movements of the spine to the contralateral side. THE UNION OF THE VERTEBRAL COLUMN WITH THE SKULL The vertebral column is joined to the skull by a combination of several joints permitting movement on three axes as in a ball-and-socket joint. 1. The atlanto-occipital joint (art. atlantooccipitalis) is a simple, condyloid joint. It is formed by two condyles of the occipital bone, and the superior concave articular surfaces of the atlas. Both pairs of the articulating surfaces are enclosed in separate articular capsules but move simultaneously, forming a single combined joint. There are the following auxiliary ligaments: (1) the anterior atlanto-occipital membrane (membrana atlantooccipitalis anterior) stretched between the anterior arch of the atlas and the occipital bone; (2) the posterior atlanto-occipital membrane (membrana atlantooccipitalis posterior) located between the posterior arch of the atlas and the posterior margin of the foramen magnum. The atlanto-occipital joint allows movement on two axes, the frontal and the sagittal 2. The atlas and the axial vertebra unite by means of three joints. Two lateral atlanto-axial joints (articulationes atlantoaxiales laterales) are formed by the inferior articular surfaces of the atlas and the similar superior surfaces of the axis contiguous to them, thus making up a combined joint. The median atlanto-axial joint (art. atlanto-axialis mediana) is a trochoid joint .The odontoid process (dens axis) located in the middle is joined with the anterior arch of the atlas and the transverse ligament of the atlas (lig. transversum atlantis) stretched between the inner surfaces of the lateral masses of the atlas. Two fibrous bands which had separated from the posterior longitudinal ligament of the spine arise from the superior and inferior edges of the transverse ligament. Together with the transverse ligament these two bands form the cruciform ligament of the axis (lig. cruciforme atlantis). An accessory ligament of the joints described is the apical ligament of the odontoid process (lig. apicis dentis) extending from the tip of the dens to the anterior edge of the foramen magnum. Two strong ligaments, alar ligaments of the odontoid process (ligamenta alaria) pass from the lateral surfaces of the dens. The atlanto-axial joints permit only a single type of movement, rotation of the head on the vertical axis (turning right and left) passing through the dens of the axial vertebra; the head moves about the dens together with the atlas (trochoid,pivot joint). Movements occur the same time at the lateral atlanto-axial joints. The Shoulder Joint The shoulder joint (articulatio humeri) connects the humerus, and through it, the whole free upper limb, with the shoulder girdle, the scapula in particular. The head of the humerus contributing to the formation of the joint is spherical. The glenoid cavity of the scapula articulating with it is shallow. On the circumference of the cavity is a cartilaginous glenoid lip (labrum glenoidale), which increases its depth without limiting the range of movements and also absorbs the jerks and shocks during movement of the head. The articular capsule of the shoulder joint is free and thin; it is attached to the bony edge of the scapular glenoid cavity, embraces the humeral head, and terminates on the anatomical neck.. The coracohumeral ligament (lig. coracohumerale}, is a slightly thicker bunch of fibres serving as an accessory ligament. The shoulder joint, typical ball-and-socket joint, is distinguished by freedom of movement. As with all joints of this general type, movement at the shoulder joint takes place on three main axes: frontal, sagittal, and vertical. Circumduction is also possible. The Elbow Joint The elbow joint (articulatio cubiti). Three bones articulate in the elbow joint; the distal end of the humerus and the proximal ends of the ulna and radius. The articulating bones form three joints invested in a common capsule (a compound joint): the humeroulnar articulation (articulatio humeroulnaris), the humeroradial articulation (articulatio humeroradialis), and the proximal radioulnar articulation (articulatio radioulnaris proximalis). The latter functions with the distal radioulnar articulation, thus forming a combined joint. The humeroulnar articulation is a hinge joint with spirally deviating articular surfaces. The articular surface of the humerus is the trochlea; wich is joined with the ulnar trochlear notch. The humeroradial articulation is formed by union of the humeral head with the concave surface of the radial head. Although this articulation is a ball-and-socket joint in shape, it actually permits movement on only two axes at the elbow joint since it is merely a part of this joint and is connected to the ulna, which limits its movement. The proximal radioulnar articulation is formed by the articulating surfaces, the circumferentia articularis radii and the incisura radialis ulnae and is cylindrical. In front and behind the capsule is free, but on its sides are accessory ligaments, the ulnar collateral (lateral) and the radial collateral (medial) ligaments (lig. collaterale ulnare and collaterale radiale), the anular ligament of the radius (lig. anulare radii). Movements at the elbow joint are of two kinds. Firstly, flexion and extension of the forearm on the frontal axis occur; these movements take place at the articulation of the ulna with the trochlea of the humerus. The second movement consisting in rotation of the radius on the longitudinal axis occurs in the humeroradial articulation as well as in the proximal and distal radioulnar articulations, which are thus a combined pivotal joint from the mechanical standpoint. The movement during which the rotating radius crosses the ulna at an angle while the hand turns so that its dorsal surface faces upward (with the limb extended forward) is called pronation. The opposite movement, in which the forearm bones are parallel while the hand is turned with the palm facing upward, is called supination The Hip Joint The hip joint (art. coxae) is formed by the cup-like acetabulum of the hip bone, by its facies lunata to be more precise, and the femoral head fitting into it. A fibrocartilaginous ring (labrum acetabulare) is attached to the whole rim of the acetabulum and makes the cavity deeper so that its depth is more that half a spheroid. The fibrocartilaginous rim bridges the acetabular notch and forms the transverse ligament of the acetabulum (lig. transversum acetabuli). The articular capsule of the hip joint is attached along the whole rim of the acetabulum. The articular capsule is attached to the femur in front along the entire distance of the intertrochanteric line and behind to the femoral neck parallel to and medial of the intertrochanteric crest. The hip joint has two other intra-articular ligaments, the above mentioned transverse ligament of the acetabulum and the ligament of the head (lig. capitis femoris) whose base is attached to the edges of the acetabular notch and to the transverse ligament of the acetabulum. The apex is attached to the fovea capitis femoris and transmits the vessels to the femoral head. The hip joint is a ball-and-socket joint of the limited type (cotyloid joint) and therefore permits movement, though not as freely as a free ball-and-socket joint, on three main axes: frontal, sagittal, and vertical. Circumduction is also possible. The external ligaments of the joint are arranged in accordance with the three main pivotal axes: three longitudinal ligaments (ligamenta iliofemorale, pubofemorale and ischiofemorale), which pass perpendicular to the horizontal axes (frontal and sagittal), and one circular (zona orbicularis), which is perpendicular to the vertical axis. 1. The iliofemoral (Bertin's) ligament (lig. iliofemorale) is on the anterior aspect of the joint. 2. The pubofemoral ligament (lig. pubofemorale) is on the inferomedial aspect of the joint. 3. The ischiofemoral ligament (lig. ischiofemorale) begins on the posterior aspect 4. The orbicular zone (zona orbicularis) consists of circular fibres in the deep layers of the articular capsule under the longitudinal ligaments described above. These fibres embrace the femoral neck like a loop The Knee Joint The knee joint (art. genus) is the largest and, at the same time, the most structurally complicated joint in the body. Three bones form the knee joint: the lower end of the femur, the upper end of the tibia, and the patella. The articular surfaces of the femoral condyles, uniting with the tibia, are convex in the transverse and sagittal directions and are segments of an ellipsoid. The tibial facies articularis superior articulating with the femoral condyles consists of two shallow facets covered with hyaline cartilage; these facets are complemented by two intra-articular cartilages, lateral and medial semilunal cartilages (menisci lateralis and medialis) interposed between the femoral condyles and the articular surfaces of the tibia. Each meniscus is a trihedral plate bent along the edge; the thickened peripheral edge is attached to the articular capsule, while the sharpened edge directed into the joint is free. The ends of both menisci are attached anteriorly and posteriorly to the intercondylar eminence. A fibrous bundle, the transverse ligament of the knee (lig. transversum genus) stretches in front between the menisci. The articular capsule is attached at some distance from the edges of the femoral, tibial and patellar articular surfaces. The medial and lateral ligaments stretch on the sides of the joint perpendicular to their frontal axis: ligamentum collaterale tibiale stretches on the medial side,; ligamentum collaterale fibulare passes on the lateral side.. On the posterior aspect of the knee-joint capsule are two ligaments merging with its posterior wall, the arcuate ligament of the knee (lig. popliteum arcuatum) and the oblique ligament of the knee (lig. popliteum obliquum). The tendon of the quadriceps muscle of the thigh is on the anterior aspect of the knee joint. It encloses the patella as a sesamoid bone and is then continuous with a thick and strong patellar ligament (lig. patellae). Besides the extra-articular ligaments described above, the knee joint has two intra-articular ligaments too called the anterior cruciate ligament (lig. cruciatum anterius) and the posterior cruciate ligament (lig. cruciatum posterius). Two types of movement occur at the knee joint: flexion and extension and then rotation. The knee is a typical condylar joint. Flexion and extension take place on the frontal axis passing through the femoral condyles. In flexion the menisci straighten out, while the collateral ligaments relax because the points of their attachment come closer to each other; as a consequence, rotation on the longitudinal axis becomes possible when the knee is flexed. Joints of the Bones of the Foot 1. The talocrural or ankle joint (art. talocruralis} is formed by the articular surfaces of the distal ends of both leg bones, which fit over the trochlea of the talus like a fork; the lower articular surface of the tibia articulates with the facies articularis superior of the trochlea, while the articular surface of the malleoli articulate with the articular surfaces on the sides of the talus. The articular capsule is attached to the cartilaginous margin of the articular surfaces and covers part of the neck of the talus in front. Accessory ligaments run on the sides of the joint from the malleoli to the adjacent tarsal bones. In the character of its structure, the ankle joint is a hinge joint. Movement takes place on the frontal axis passing through the trochlea of the talus, during which the foot is now raised with the toes upward (dorsiflexion), now lowered (plantar flexion).

THE SKELETON OF THE HEAD The head is related to the locomotor system only in part. Its skeleton (the skull, cranium) is primarily the receptacle for the most highly developed part of the nervous system, the brain and the sensory organs connected with it; moreover, it encloses the initial part of the digestive and respiratory tracts communicating with the external environment. In accordance with this, the skull of all vertebrates is divided into two parts: the cerebral cranium (neurocranium) and the visceral cranium (cranium viscerale). The skull-cap, or vault (calvaria) and the base (basis) are distinguished in the cerebral cranium. The human cerebral cranium consists of the unpaired occipital, sphenoid, frontal, and ethmoid bones and the paired temporal and parietal bones. The visceral cranium is formed by the paired maxilla, inferior turbinate, palatine, zygomatic, nasal, and lacrimal bones and the unpaired vomer, mandible, and hyoid bones. Development of the skull. The human skull goes through three developmental stages in ontogenesis: (1) connective-tissue; (2) cartilaginous, and (3) bony. The change from the second to the third stage, i.e. the formation of secondary bones in cartilage, occurs throughout man's life. Remnants of cartilaginous tissue persist between the bones as their cartilaginous joints (synchondroses) even in adults. The calvaria, which serves only for protection of the brain, develops directly from the membranous skull without going through the cartilage stage. The conversion of connective tissue to bony tissue also occurs here throughout man's life. The remnants of unossified connective tissue persist between the skull bones as fontanelles in the newborn and as sutures in children and adults. THE BONES OF THE CEREBRAL CRANIUM THE OCCIPITAL BONE The occipital bone (os occipitale) forms the posterior and inferior walls of the brain case and is thus a part of the calvaria and a part of the base of the skull. It is made up of four parts, which are laid down separately and fuse to form a single bone only between the ages of 3 and 6. These parts, which form the borders of the foramen magnum (where the spinal cord is continuous with the medulla oblongata and passes from the vertebral canal into the cavity of the skull), are as follows: anteriorly, the basilar part (pars basilaris) (os basilare in animals); laterally, the condylar parts (partes laterales) and posteriorly, the squamous part (squama occipitalis). The squamous part of the occipital bone (squama occipitalis) is shaped like a plate, improperly rounded, with a convex external surface and a concave internal surface. The attachment of muscles and ligaments lends it the external relief. The external occipital protuberance (protuberantia occipitalis externa) is in the center of the external surface. A curved superior nuchal line (linea nuchae superior) passes laterally from the protuberance on each side. The external occipital crest (crista occipitalis externa) extends from the occipital protuberance downward on the midline to the posterior edge of the foramen magnum. The inferior nuchal lines (lineae nuchae inferiores) pass laterally from the middle of the crest. The relief of the internal surface is determined by the shape of the brain and the attachment of its meninges; as a result this surface is divided by two crests intersecting at a right angle into four fossae. These two crests form the cruciate eminence (eminentia cruciformis) and the internal occipital protuberance (protuberantia occipitalis interna) at the site of their intersection. The lower half of the longitudinal crest is sharper and is called the internal occipital crest (crista occipitalis interna) while the upper half of this crest and both halves of the transverse crest are supplied with clearly pronounced sulci: sagittal groove (sulcus sinus sagittalis superioris) and groove for the transverse sinus (sulcus sinus transversi) (prints of venous sinuses of the same name which are lodged here). Each lateral part (pars lateralis) contributes to the union of the skull with the spine and therefore carries on its inferior surface the occipital condyle (condylus occipitalis), the place of articulation with the atlas. The hypoglossal canal (canalis hypoglossi) penetrates the bone approximately at the middle of the occipital condyle. Behind the condyle is the condylar fossa (fossa condylaris) on whose floor an opening of a condylar canal (canalis condylaris) is sometimes present (for the transmission of a vein). The jugular process (processus jugularis) projects laterally to the condyle. The sigmoid groove (sulcus sinus sigmoidei) is on the superior surface of pars lateralis, next to the jugular process, while the jugular notch (incisura jugularis) is on its margin. The basilar part (pars basilaris) fuses with the sphenoid bone by the age of 18 to form a single bone in the center of the cranial base. A sloping area, clivus, made up of two fused parts, is located on the superior surface of this; it lodges the medulla oblongata. The groove for the inferior petrosal sinus (sulcus sinus petrosi inferioris) is seen on the lateral edges of the basilar part of the occipital bone; it lodges the inferior petrosal sinus. The inferior surface, which is a component of the superior pharyngeal wall, carries the pharyngeal tubercle (tuberculum pharyngeum) to which the fibrous capsule of the pharynx is attached. THE SPHENOID BONE The sphenoid bone (os sphenoidale) is an unpaired bone whose structure is even more complex than that of the occipital bone. The sphenoid bone resembles a bat or a flying insect, which explains the names of its parts (wings, pterygoid processes, Gk. pterygoin wing). The name "sphenoid" probably appeared by mistake. In Galen’s manuscript this bone was called sphecoid (resembling a wasp), but apparently the copyist made an error and wrote sphenoid (wedged, Gk. sphen wedge). The following parts can be distinguished: (1) the body (corpus), (2) the greater wings (alae majores), (3) the lesser wings (alae minores) and (4) the pterygoid processes (processus pterygoidei). The body (corpus) has on the midline of its superior surface a depression, the shape of a Turkish saddle, the sella turcica, on the floor of which is a depression for the cerebral hypophysis (fossa hypophysialis). To the front of the depression is an elevation, the tuberculum sellae, on which the sulcus chiasmatis lodging the crossing (chiasma) of the fibres of the optic nerve passes transversely. The optic foramina (canales optici) transmitting the optic nerves from the orbital cavity into the cranial cavity, are found at the ends of the sulcus chiasmatis. The sella turcica is bounded posteriorly by a bony plate the dorsum sellae. A curved carotid groove (sulcus caroticus) lodging the internal carotid artery passes on the lateral surface of the body. A ridge, the crest of the sphenoid (crista spenoidalis) is seen on the antenor surface of the body. The crest continues down to become a pointed vertical prominence, the rostrum of the sphenoid (rostrum sphenoidale), which fits between the wings of the vomer. The sphenoidal crest articulates in front with the perpendicular plate of the ethmoid bone. Irregularly shaped openings, apertures of the sphenoidal sinus (aperturae sinus sphenoidalis) are seen to the sides of the crest. They open into an air cavity, the sphenoidal sinus (sinus sphenoidalis) located in the body of the sphenoid bone.The sinus communicates with the nasal cavity by means of these openings. The lesser wings (alae minores) are two flat triangular plates arising by two roots from the anterosuperior edge of the body of the sphenoid bone and extending forward and laterally. The superior orbital fissure (fissura orbitalis superior) leading from the cranial into the orbital cavity, is between the lesser and greater wings. The greater wings (alae majores) spring from the sides of the body laterally and upwards. A round opening (foramen rotundum) leading in front into the pterygopalatine fossa, is located close to the body, to the back of the superior orbital fissure; it transmits the second branch of the trigeminal nerve. It is the spinous foramen (foramen spinosum) through which the middle meningeal artery passes. A much larger oval foramen (foramen ovale) is seen to the front of it; it transmits the third branch of the trigeminal nerve. The greater wings have the following four surfaces: cerebral (facies cerebralis); orbital (facies orbitalis); temporal (facies temporalis), and maxillary (facies maxillaris). Their names indicate which cranial surface they face. The last two surfaces are separated by the infratemporal crest (crista infratemporalis). The pterygoid processes (processus pterygoidei), drop vertically downward from the junction of the greater wings and the body of the sphenoid bone. Their base is pierced by a pterygoid canal (canalis pterygoideus) directed sagittaly, which transmits the pterygoid nerve and vessels. Its anterior opening communicates with the pterygopalatine fossa. Each process is made up of two plates, one medial and one lateral (lamina medialis and lamina lateralis), between which the pterygoid fossa (fossa pterygoidea) forms posteriorly. The inferior portion of this fossa is continuous with the pterygoid fissure (fissura pterygoidea). In the intact skull the pyramidal process of the palatine bone fits into this notch.

THE TEMPORAL BONE The temporal bone (os temporale) is a paired bone whose structure is very complicated and consists of three parts: (1) squamous part (pars squamosa); (2) tympanic part (pars tympanica), and (3) petrous part (pars petrosa). A fourth part of the temporal bone, mastoid (pars mastoidea) was previosly distinguished. Within the first year of life the parts fuse into a single bone and thus form the external acoustic meatus (meatus acusticus externus) with the squamous part to the top, the petrous part in a medial position, and the tympanic part to the back, bottom, and front. The traces of fusion of the separate parts of the temporal bone persist throughout life in the form of sutures and fissures namely: the tympanosquamous fissure (fissura tympanosquamosa) in the depth of the mandibular fossa separated by the process of the petrous part into fissura petrosquamosa, and petrotympanic fissure (fissura petrotyrnpanica) (through which the chorda tympani nerve passes). The squamous part (pars squamosa) contributes to the formation of the lateral walls of the skull. The cerebral surface (facies cerebralis) of the squamous part bears marks of the brain, impressions for cerebral gyri (impressiones digitatae) and an ascending groove lodging the middle meningeal artery (a. meningea media). The smooth external surface of the squama contributes to the formation of the temporal fossa and is therefore called the temporal surface (facies temporalis). It gives rise to the zygomatic process (processus zygomaticus), which passes forward to join the zygomatic bone. The zygomatic process has two roots at its origin, an anterior and a posterior root, with a depression articular fossa (fossa mandibularis) for articulating with the lower jaw between them. The inferior surface of the anterior root carries an articular tubercle (tuberculum articulare), which prevents anterior dislocation of the head of the mandible when the mouth is opened very wide. The tympanic part (pars tympanica) of the temporal bone forms the anterior, inferior, and .part of the posterior border of the external acoustic meatus. The external auditory meatus (meatus acusticus externus) is a short canal directed medially and somewhat anteriorly and leading into the tympanic cavity. The superior edge of its external opening, porus acusticus externus, and part of the posterior edge are formed by the squama of the temporal bone. The other edges are formed by the tympanic part of the bone. The external acoustic meatus is incompletely formed in the newborn because the tympanic part is an incomplete ring (anulus tympanicus) closed by the tympanic membrane. Since the tympanic membrane is so near the external environment, newborns and infants often suffer from diseases of the membrane. The tympanic ring grows and is converted to a tube during the first year of life; this tube pushes the petrous part medially and forms most of the bony external acoustic meatus whose roof is the squamous part. The tympanic membrane, now moves deeper into the external acoustic meatus and separates it (i.e. the external ear) from the tympanic cavity, cavum tympani (and becomes external in relation to the tympanic cavity). The petrous part (pars petrosa) (Gk. petros stone) is an important component of the temporal bone. It is so named because its bony substance is strong. It is a part of the base of the skull and at the same time is a bony encasement for the organs of hearing and equilibrium, which have a very fine structure and must be protected reliably from injuries. This part is also called the pyramid because it is shaped like a trihedral pyramid with the base facing externally and the apex facing anteriorly and internally toward the sphenoid bone. The pyramid has three surfaces: anterior, posterior, and inferior. The anterior surface of the pyramid lias a small depression near its apex. This is the trigeminal impression (impressio trigemini), which lodges the ganglion of the trigeminal nerve. Lateral to it pass two small grooves, a medial sulcus of the greater petrosal nerve (sulcus n. petrosi majoris) and a lateral sulcus of the lesser petrosal nerve (sulcus n. petrosi minoris). They lead to two openings of the same name, a medial opening (hiatus canalis n. petrosi majoris) and a lateral opening (hiatus canalis n. petrosi minoris). The arcuate eminence (eminentia arcuata) is lateral to these openings; it forms due to prominence of the vigorously developing labyrinth, particularly the superior semicircular canal. The bone surface between the petrosquamous fissure and the arcuate eminence forms the roof of the tympanic cavity (tegmen tympani). In about the middle of the posterior surface of the pyramid is the porus acusticus internus leading into the internal auditory meatus (meatus acusticus internus), which transmits the facial and auditory nerves and the internal auditory artery and veins. The inferior surface of the pyramid that faces the base of the skull gives off a slender tapering styloid process (processus styloideus) for attachment of the muscles. Between the styloid and mastoid processes is the stylomastoid foramen (foramen stylomastoideum) transmitting the facial nerve and one of the arteries. The deep jugular fossa (fossa jugularis) is medial to the styloid process. To the front of the jugular fossa is the external opening of the carotid canal (foramen caroticum externum). They are separated by a sharp ridge, which has a small depression called fossulla petrosa. The pyramid has three edges: anterior, superior, and posterior. The short anterior edge forms a sharp angle with the squama, in which is found the musculotubal canal (canalis musculotubarius) leading to the tympanic cavity. The canal is divided by a septum into two parts: superior and inferior. The superior, smaller semicanal (semicanalis m. tensoris tympani) lodges the tensor tympani muscle, while the lower, larger semicanal (semicanalis tubae auditivae) is the bony part of the auditory tube for the conduction of air from the pharynx to the tympanic cavity. The superior edge of the pyramid that separates the anterior and posterior surfaces bears a clearly detectable groove, groove for the superior petrosal sinus (sulcus sinus petrosi superioris) lodging the superior petrosal venous sinus. The posterior edge of the pyramid joins the pars basilaris of the occipital bone to the front of the jugular fossa and together with this bone forms the groove for the inferior petrosal sinus (sulcus sinus petrosi inferioris) lodging the inferior petrosal sinus. The external surface of the base of the pyramid provides attachment for the muscles; this determines its relief (process, notches, areas of roughness). Its lower end stretches out to form the mastoid process (processus mastoideus) to which the sternocleidomastoid muscle is attached. The medial surface of the mastoid process bears a deep mastoid notch (incisura mastoidea), the site of attachment of m. digastricus, and, still closer to the midline, a small occipital groove (sulcus a. occipitalis). A smooth triangle on the external surface of the base of the mastoid process is the operative approach to the air cells of the process when they are filled with pus. The mastoid process contains compartments or cells (cellulae mastoidea), which are air cavities separated by bone trabeculae. They receive air from the tympanic cavity with which they communicate by means of the mastoid antrum (antrum mastoideum). A deep sigmoid groove (sulcus sinus sigmoidei) is found on the cerebral surface of the base of the pyramid. The canal of the venous emissary opens into this sulcus; its external opening, mastoid foramen (foramen mastoideum), varying greatly in accordance with the size of the canal. The canals of the temporal bone. The largest is the carotid canal (canalis caroticus), which transmits the internal carotid artery. It begins as the external carotid foramen (foramen caroticum externum) on the interior surface of the pyramid, then ascends and bends at a right angle and opens by its internal foramen (foramen caroticum internum) at the apex of the pyramid medial to the canalis musculotubarius. The canal for the facial nerve (canalis facialis) begins in the depth of porus acusticus internus and then passes at first forward and laterally to the hiatus in the anterior surface of the pyramid. There the canalis facialis, still horizontal, bends at a right angle laterally and backward to form the geniculum of the facial canal (geniculum canalis facialis); it then descends and ends as the (foramen stylomastoideum) on the inferior surface of the pyramid of the temporal bone. Canaliculus chordae tympani begins 2-3mm above of the stylomastoid foramen, passes through the tympanic cavity and ends on the petrotympanic fissure (fissura petrotyrnpanica) (through which the chorda tympani nerve passes). Canaliculus tympanicus begins from the depth of the the fossulla petrosa, passes through the tympanic cavity, finishies on the hiatus canalis n. petrosi minoris. THE PARIETAL BONE The parietal bone (os parietale) is a paired bone forming the middle part of the vault of the skull. It is a quadrangular plate with convex external and concave internal surfaces. Its four borders articulate with the adjoining bones, namely: the frontal border (margo frontalis) with the frontal bone; the posterior, occipital border (margo occipitalis) with the occipital bone; the superior, sagittal border (margo sagittalis) with the contralateral bone, and the inferior, squamosal border (margo squamosus) with the squama ol the temporal bone. The first three borders are serrated, while the last is adapted for the formation of a squamous suture. The four angles are as follows: the frontal angle (angulus frontalis) unites with the frontal bone; the sphenoidal angle (angulus sphenoidalis) joins with the sphenoid bone; the occipital angle (angulus occipitalis) articulates with the occipital bone; and the mastoid angle (angulus mnstoideus) unites with the mastoid process of the temporal bone. The relief of the external convex surface is determined by the attachment of muscles and fasciae. In its centre is a prominence, the parietal eminence (tuber parietalis). Below it are two curved temporal lines (linea temporalis superior and inferior) for attachment of the temporal fascia and muscle. An opening, the parietal foramen (foramen parietale) for the artery and the venous emissary is seen near to the superior border. The relief of the internal concave surface (facies interna) is determined by the brain and especially the dura mater, which fit close to it. The sites of attachment of the dura mater to the bone are marked by a sagittal groove (sulcus sinus sagittalis superioris) (lodging the superior sagittal sinus) on the superior border, and a sigmoid groove (sulcus sinus sigmoidei) (lodging the sigmoid sinus), in the region of the angulus mastoideus. THE FRONTAL BONE The frontal bone (os frontale) an unpaired, membrane bone, contributes to the formation of the vault of the skull and develops in connective tissue. The frontal bone is made up of two parts: a vertical part, squama (squama frontalis) and a horizontal part. According to its relation to the organs of vision and smell, the paired orbital part (pars orbitalis) and an unpaired nasal part (pars nasalis) are distinguished in the horizontal part. As a result, the following four parts are distinguished in the frontal bone. 1. The frontal squama (squama frontalis) as any membrane bone, has the shape of a plate, externally convex and internally concave. It ossifies from two ossification points, which are apparent even in an adult as two frontal tubers (tuberas frontalia) on the external surface (facies externa). They are pronounced only in man due to the development of the brain. They are absent not only in anthropoid apes but also in extinct forms of man. The inferior border of the squama is called the supraorbital border (margo supraorbitalis). Approximately at the junction of the medial and middle third of this border is the supraorbital notch (incisura supraorbitalis) (which transforms sometimes into a foramen supraorbitale), transmitting the supraorbital arteries and nerve. Eminences, the superciliary arches (arcus superciliares) varying greatly in size and length, are seen immediately above the supraorbital border; they are continuous medially on the midline with a more or less prominent area, the glabella, the superior part of the bridge of the nose. The glabella is an important feature in distinguishing the skull of modern man from a fossil skull. The lateral end of the supraorbital border stretches out to form the zygomatic process (processus zygomaticus), which articulates with the zygomatic bone. A clearly detectable temporal line (linea temporalis) extends upward from the process; this line delimits the temporal surface (facies temporalis) of the squama. A small groove, sagittal groove (sulcus sinus sagittalis superioris) runs on the midline of the internal surface (facies interna) from the posterior border and is continuous at the lower end with the frontal crest (crista frontalis), which ends by the blind opening (foramen caecum). These structures provide attachment for the dura mater. 2 and 3. The orbital parts (partes orbitales) are two horizontal plates whose inferior concave surface faces the orbit.. The superior cerebral surface bears marks of the brain; namely cerebral ridges of cranium (juga cerebralis) (BNA) and digitate impressions (impressiones digitatae). The inferior surface (facies orbitalis) forms the superior orbital wall and bears marks of adjacent accessories of the eye: the lacrimal fossa (fossa glandulae lacrimalis) near the zygomatic process. The orbital parts are separated by the ethmoid notch (incisura ethmoidalis), which in an intact skull is filled by the ethmoid bone. 4. The nasal part (pars nasalis) occupies the anterior part of the ethmoid notch on the midline. A projection ending as a sharp process, the nasal spine (spina nasalis), is found here; it helps to make up the nasal septum. To the front of it is an opening leading into the frontal sinus (sinus frontalis) located in the thickness of the bone to the back of the superciliary arches. The frontal sinus contains air and is separated by the septum . THE ETHMOID BONE The ethmoid bone (os ethmoidale) is an unpaired bone usually described in the group of bones of the cerebral cranium, although most of it helps to make up the visceral cranium. The ethmoid bone is located centrally between the bones of the face and comes in contact with most of them to form the nasal cavity and orbit. Formed of thin bone plates surrounaing the air sinuses, it is light and fragile. The ancient Egyptians made use of these properties when they removed the brain from the skull through the ethmoid bone for embalment. The bony plates of the ethmoid bone are arranged in the form of the letter "T" in which the vertical line is the perpendicular plate (lanina perpendicularis) and the horizontal is the cribriform plate (lamina cribrosa). From the lamina cribrosa, on either side of the perpendicular plate, hang the ethmoidal labyrinths (labyrinthi ethmoidales). As a result four parts are distinguished in the ethmoid bone. 1. Lamina cribrosa is a rectangular plate fitting into the ethmoid notch of the frontal bone. It is perforated by small openings like a sieve, hence its name (Gk. ethmos sieve). These perforations transmit the branches of the olfactory nerve (about 30 of them). The crista galli projects upward from the midline of the cribriform plate (for attachment of the dura mater). 2. Lamina perpendicularis is a part of the nasal septum. 3 and 4. Labyrinthi ethmoidales make up a paired complex of bony air cells, cellulae ethmoidales, covered laterally by a thin orbital plate (lamina orbitalis) which forms the medial wall of the orbit. On the medial surface of the labyrinth are two nasal conchae (conchae nasales superior and media), although sometimes there is a third, highest nasal concha (concha nasalis suprema). The conchae (Gk. konche shell) are thin, curved plates

THE BONES OF THE VISCERAL CRANIUM THE UPPER JAW BONE The upper jaw bone (maxilla) is a paired bone of a complex structure (determined by the diversity of its functions. The maxilla consists of a body and four processes. A. The body (corpus maxillae) contains a large maxillary air sinus (sinus maxillaris s. antrum Highmori, BNA) (hence highmoritis, inflammation of the maxillary sinus), which communicates with the nasal cavity by a wide opening, the maxillary hiatus (hiatus maxillaris). The following four surfaces are distinguished on the body. The anterior surface (facies anterior), is concave in modern man since his food is prepared and the function of mastication is consequently weaker. Inferiorly it is continuous with the alveolar process, in which a series of depressions (juga alveolaria) between the ridges of the tooth roots are seen. The ridge corresponding to the canine tooth is most pronounced. The canine fossa (fossa canina) is above and lateral to it. Superiorly the anterior surface of the maxilla is separated from the orbital surface by the infraorbital margin (margo infraorbitalis). Immediately below it is the infraorbital foramen (foramen infraorbitale) through which the infraorbital nerve and artery leave the orbit. The medial border of the anterior surface is formed by the nasal notch (incisura nasalis) whose edge extends forward to form the anterior nasal spine (spina nasalis anterior). The infratemporal surface (facies infratemporalis) is separated from the anterior surface by the zygomatic process and carries several small perforations (transmitting the nerves and vessels to the upper teeth), the maxillary tuber (tuberosity of maxilla) (tuber maxillae) and the greater palatine sulcus (sulcus palatinus major). The nasal surface (facies nasalis) is continuous inferiorly with the superior surface of the palatine process. The conchal crest (crista conchalis) is seen on it. To the back of the frontal process is the lacrimal groove (sulcus lacrimalis) which, with the lacrimal bone and the inferior nasal concha, is converted into the nasolacrimal canal (canalis nasolacrimalis). The smooth, flat orbital surface (facies orbitalis) is triangular. On its medial border, behind the frontal process, is the lacrimal notch (incisura lacrimalis) lodging the lacrimal bone. The infraorbital groove (sulcus infraorbitalis) originates near the posterior border of the orbital surface and is converted anteriorly into the infraorbital canal (canalis infraorbitalis), which opens into the anterior surface of the maxilla by means of the infraorbital foramen mentioned above. B. Processes. 1. The frontal process (processus frontalis) projects upward and joins the pars nasalis of the frontal bone. Its lateral surface is divided into two parts by a vertical lacrimal crest (crista lacrimalis anterior), which is continuous downward with the infraorbital margin. The medial surface carries the ethmoidal crest (crista ethmoidalis) for attachment of the middle nasal concha. 2. The alveolar process (processus alveolaris) carries on its inferior border, alveolar arch (arcus alveolaris), dental sockets (alveoli dentales) for the eight upper teeth; the sockets are separated by septa interalveolaria. 3. The palatine process (processus palatinus) forms most of the hard bony palate (palatum osseum) by joining the contralateral process in the midline. Where they meet, the nasal crest (crista nasalis) rises on the superior surface facing the nasal cavity and articulates with the inferior edge of the vomer. Near the anterior end of the nasal crest on the superior surface is an opening that leads into the incisive canal (canalis indsivus). The superior surface of the process is smooth, whereas the infrior surface, facing the oral cavity, is rough (impressions of the musocal glands) and carries longitudinal palatine grooves (sulci palatini) lodging the nerves and vessels. The incisive suture (sutura incisiva) is often seen in the anterior part. It delimits the os incisivum which fuses with the maxilla. In many animals this bone exists as an independent bone (os intermaxillare), but in man it is rarely encountered. 4. The zygomatic process (processus zygomaticus) articulates with the zygomatic bone to form a thick support through which pressure produced during mastication is transmitted to the zygomatic bone. The maxillary sinus, a large pyramidal cavity has thin walls corresponding to orbital, alveolar, facial, nasal and infratemporal aspects of the maxilla. Its apex extends into the zygomatic process; its baseis medial and is the lateral wall of the nasal cavity (midial wall), appearing in the maxillary hiatus in a disarticulated bone. The maxillary sinus connects only with the middle meatus, usually by two small holes, one of which is usually closed by mucous membrane. Its posterior wall forms infratemporal surface of the maxilla and contains alveolar canals, conducting posterior superior alveolar nerves and vessels to molar teeth; these canals may ridge the sinus. The superior wall is formed by the orbital surface of the maxilla. The floor is formed by the maxilla’s alveolar process corresponding to the roots of the first and second molar teeth and is usually about 1.25 cm below the level of the nasal floor. Several conical elevations of the roots of the first and second molar teeth project into the floor which they sometimes perforate. The roots of the first and second premolars and third molar, and occasionally the canine root, may also project into the sinus. The anterior wall of the sinus is formed by the facial surface of the bone. The maxillary sinus can be infected from the nasal cavity but the infection may also spread from the teeth, resulting in the loss of mucosal cilia and an impaired flow of mucus. Because its opening is high above its floor, the natural drainage of the maxillary sinus is hindered but may be effected by puncture in the lateral wall of the inferior nasal meatus or through the canine fossa on the anterior surface of the maxilla, which is nearer to its floor. THE LOWER JAW BONE The lower jaw bone or the mandible (mandibula) is a mobile skull bone. The mandible consists of a horizontal part or body (corpus mandibulae), which carries the teeth, and a vertical part in the form of two rami mandibulae, which serve for the formation of the temporomandibular joint and for attachment of the muscles of mastication. The horizontal and the vertical parts meet at an angle called the angle of the mandible (angulus mandibulae), on the external surface of which the masseter muscle is inserted into the masseteric tuberosity (tuberositas masseterica). On the inner surface of the angle is the pterygoid tuberosity (tuberositas pterygoidea) for insertion of another muscle of mastication, m. pterygoideus medxalis. The structure and relief of the body of the mandible are determined by the teeth and by the fact that the mandible takes part in the formation of the mouth. For instance, the upper, alveolar part of the body (pars alveolaris) bears teeth as a consequence of which its border, the alveolar arch (arcus alveolaris) has sockets for the teeth (alveoli dentalis), with interalveolar septa (septa interalveolaria) and corresponding depressions on the external surface (juga alveolaria). The rounded massive and thick inferior border of the body forms the base of the mandible (basis mandibulae). At old age when the teeth are lost, the alveolar part atrophies and the whole body of the mandible becomes thin and low. The ridge on the symphysis on the midline of the body is continuous with a triangular mental protuberance (protuberantia mentalis), the presence of which is characteristic of modern man. Among all mammals only man, and modern man at that, has a pronounced chin. On each side of this protuberance is a mental tubercle (tuberculum mentale). On the lateral surface of the body, in the space between the first and second premolars, is the mental foramen (foramen mentale), which is an opening of the mandibular canal (canalis mandibulae), transmitting a nerve and vessels. An oblique line (linea obliqua) runs to the back and upward from the mental tubercle. Two mental spines (spinae mentales) project from the inner surface of the symphysis; these are the sites of attachment of the tendon of the genioglossus muscle.On both sides of the mental spine, nearer to the inferior border of the mandible is the site for attachment of the digastric muscle, the digastric fossa (fossa digastrica). Further to the back is the mylohyoid line (linea mylohyoidea), running backward and upward; it serves for attachment of the mylohyoid muscle. The ramus of the mandible (ramus mandibulae) rises on each side from the posterior part of the body of the mandible. On its inner surface is the mandibular foramen (foramen mandibulae).The medial edge of this foramen projects as the lingula of the mandible (lingula mandibulae), to which is attached the sphenomandibular ligament; the lingula is developed more in man than in apes. The mylohyoid groove (sulcus mylohyoideus) originates behind the lingula and runs downward and forward; it lodges the mylohyoid nerve and vessels. Superiorly the ramus of the mandible terminates as two processes, anterior coronoid process (processus coronoideus) (it forms under the effect of traction exerted by the strong temporal muscle) and a posterior condylar process (processus condylaris). A crest for the attachment of the buccinator muscle (crista buccinatoria) runs on the inner surface of the ramus upward from the surface of the alveoli of the last molars, towards the coronoid process. The condylar process has a head (caput mandibulae) and a neck (collum mandibulae). On the anterior surface of the neck is the pterygoid pit (fovea pterygoidea) for attachment of the lateral pterygoid muscle. The vertical crest on the medial surface of the ramus serving for the attachment of the temporal muscle is designated as temporal crest. Between it and the anterior border of the ramus is a depression, the retromolar fossa, variable in its width and depth. Another ridge (crista colli) on the inner surface of the mandibular condyle crosses the mandibular neck, and continues to the region above and in front of the mandibular foramen, where it fuses with the temporal crest to form mandinular torus. THE PALATINE BONE The palatine bone (os palatinum) is a paired bone. It is a thin bone consisting of two plates uniting at a right angle and supplementing the maxilla. 1. The horizontal plate (lamina horizontalis) complements the maxillary palatine process posteriorly to form the hard palate (palatum osseum). Its medial border meets the medial border of the contralateral bone to form the hard palate (palatum osseum). On the inferior surface of the horizontal plate is the greater palatine foramen (foramen palatinum majus), through which palatine vessels and nerves leave the canalis palatinus major. 2. The perpendicular plate (lamina perpendicularis) adjoins the nasal surface of the maxilla. Along its lateral surface runs the greater palatine sulcus (sulcus palatinus major), which together with the maxillary sulcus of the same name forms the canalis palatinus major. The medial surface has two crests for two nasal conchae, the middle (crista ethmoidalis) and the inferior (crista conchalis). The palatine bone has three processes. One of them, the pyramidal process (processus pyramidalis) projects backward and laterally from the junction of the horizontal and perpendicular plates. In an intact skull the pyramidal process fits into the pterygoid fissure of the sphenoid bone. Nerves and vessels penetrate it vertically through the lesser palatine canals (canales palatini minores). The other two processes project from the superior edge of the perpendicular plate and form the sphenopalatine notch (incisura sphenopalatina), which meets the body of the sphenoid bone to form the sphenopalatine foramen (foramen sphenopalatinum) transmitting the sphenopalatine vessels and nerves. The anterior process forms the posterior corner of the orbit and is therefore known as the orbital process (processus orbitalis); the posterior process adjoins the inferior surface of the body of the sphenoid bone and is called the sphenoid process (processus sphenoidalis). ' THE INFERIOR NASAL CONCHA The inferior nasal concha or inferior turbinate bone (concha nasalis inferior) is a paired bone. As distinct from the superior and middle nasal conchae, which are components of the ethmoid bone, the inferior nasal concha is an independent bone. It is a thin, curled plate of bone whose upper edge is attached to the lateral wall of the nasal cavity; The nasa1 bone (os nasale) joins the contralateral bone to form the ridge of the nose at its root. The lacrimal bone (os lacrimale), a paired bone, is a thin plate found in the medial wall of the orbit immediately behind the frontal process of the maxilla. Its lateral, surface carries the crest of the lacrimal bone (crista lacrimalis posterior). To the front of the crest runs the lacrimal groove (sulcus lacrimalis), which meets the sulcus of the frontal process of the maxilla to form the fossa of the lacrimal sac (fossa sacci lacrimalis). The vomer , an unpaired bone, is an irregularly quadrangular plate, which resembles a plowshare (hence its name: L vomer plowshare) and forms part of the bony nasal septum. Its superior border is split into two wings (alae vomeris), which fit over the rostrum of the sphenoid bone. The upper half of the anterior edge articulates with the perpendicular plate of the ethmoid bone, and the lower part with the cartilaginous nasal septum. The inferior edge articulates with the nasal crests of the maxilla and palatine bone. The free, posterior edge is the posterior border of the bony nasal septum separating the posterior openings of the nasal cavity, choanae by means of which the nasal cavity communicates with the nasopharynx. THE ZYGOMATIC BONE The zygomatic bone (os zygomaticum) is a paired bone, the strongest bone of the skull. According to the location of the bone, three surfaces and two processes are distinguished in it. The lateral surface (facies lateralis) is shaped like a four-point star and bulges slightly. The smooth posterior surface faces the temporal fossa and is called the temporal surface (facies temporalis). The third orbital surface (facies orbitalis) takes part in the formation of the orbital walls. The superior frontal process of the zygomatic bone (processus frontalis) articulates with the zygomatic process of the frontal bone and the greater wing of the sphenoid bone. The lateral temporal process (processus temporalis) articulates with the zygomatic process of the temporal bone to form the zygomatic arch, the site of origin of the masseter muscle. THE HYOID BONE The hyoid bone (os hyoideum) is situated at the base of the tongue between the mandible and the larynx. It consists of a body (corpus) and two pairs of horns (cornua). The greater horns (cornua majora) are continued from both ends of the body. The lesser horns (cornua minora) arise from the junction of the body and the greater horns. THE TEMPOROMANDIBULAB JOINT The temporomandibular joint (articulatio temporomandibularis) is formed by the head of the mandible and the mandibular fossa of the temporal bone. The articulating surfaces are complemented by a fibrous articular disc (discus articularis) located between them. The edges of the disc are joined to the articular capsule as a result of which the joint cavity is separated into two isolated compartments. The articular capsule is attached along the border of the mandibular fossa up to the petrotympanic fissure and thus encloses the articular tubercle and embraces the neck of the mandible inferiorly. Near the temporomandibular joint are three ligaments only one of which, the lateral ligament (lig. laterale), is directly related to the joint. The lateral ligament prevents excessive movement of the articular head to the back. The remaining two ligaments (lig. sphenomandibulare and lig. stylomandibulare) are at a distance from the joint and are actually not ligaments but artificially separated areas of fascia, which form a loop- like structure to help to suspend the mandible. Both temporomandibular joints function simultaneously and are therefore a single combined articulation from the mechanical standpoint. The temporomandibular articulation is a condyloid joint, but because of the articular disc, it permits movements in three directions. The mandible makes the following movements: (1) downward and upward movements with opening and closure of the mouth; (2) forward and backward movements; (3) lateral movements (rotation of the mandible to the right and to the left as it occurs in chewing). THE SKULL AS A WHOLE The external surface of the skull. Part of the external surface of the skull examined from the front (norma facialis s. frontalis) consists of the forehead superiorly and two orbits, with the piriform aperture of the nose between them. The orbits, or eyesockets (orbitae) contain the organ of vision and are cavities in the shape of somewhat rounded, four-sided pyramids. The base of the pyramid corresponds to the opening into the orbit (aditus orbitae), while the apex is directed backward and medially. The medial orbital wall (paries medialis) is formed by the frontal process of the maxilla, the lacrimal bone, the orbital plate of the ethmoid bone, and the body of the sphenoid bone to the front of the optical canal. The orbital surfaces of the zygomatic bone and greater wings of the sphenoid bone form the lateral wall (paries lateralis). The superior wall (paries superior) or the roof of the orbit is formed by the orbital part of the frontal bone and lesser wings of the sphenoid bone; the inferior wall (paries inferior) or floor of the orbit is made up of the zygomatic bone and maxilla, and in the poslerior portion by the orbital surface of the orbital process of the palatine bone. Two openings are seen at the apex of the pyramid: a large lateral opening, the superior orbital fissure (fissura orbitalis superior), and a smaller round medial opening, the optic canal (canalis opticus); by means of both openings the orbit communicates with the cranial cavity. In the corner formed by the lateral and inferior orbital walls is the inferior orbital fissure (fissura orbitalis inferior), which is bounded laterally by the greater wing of the sphenoid bone and medially by the edge of the maxilla; its posterior end leads into the pterygopalatine fossa and the anterior end into the infratemporal fossa. The fossa of the lacrimal sac (fossa sacci lacrimalis) is in the anterior part of the medial wall, which leads into the nasolacrimal canal (canalis nasolacrimalis). The other end of the lacrimal canal opens into the inferior nasal meatus. Further to the back, in the suture between the frontal and ethmoid bones, are two openings, the anterior and posterior ethmoidal foramina (foramina ethmoidale anterius and posterius), transmitting the anterior and posterior ethmoidal vessels and nerves. The anterior foramen loads into the cranial cavity, the posterior foramen into the nasal cavity. On examination of the skull from the side (norma lateralis), the temporal lines (lineae temporales) (superior and inferior), strike the eye first of all. Each line rises at the zygomatic process of the frontal bone, curves upward and to the back to intersect the coronary suture, and then passes over to the parietal bone on which it runs in the direction of the mastoid angle and, curving anteriorly, extends to the temporal bone. It marks the attachment of temporal muscle and fascia. The following depressions merit special description because of their important topographical relations: (1) the temporal fossa (fossa temporalis); (2) the infratemporal fossa (fossa infratemporalis); and (3) the pterygopalatine fossa (fossa pterygopalatina) The temporal fossa (fossa temporalis) is bounded superiorly and posteriorly by the temporal line, inferiorly by the infratemporal crest and the inferior margin of the zygomatic arch. The temporal fossa lodges the temporal muscle. The infratemporal fossa (fossa infratemporalis) is continuous downward with the temporal fossa, and their borderline is the infratemporal crest of the greater wing of the sphenoid bone. The medial wall of the infratemporal fossa is formed by the lateral plate of the pterygoid process. The anterior wall is formed by the infratemporal surface of the maxilla and the lower part of the zygomatic bone. The superior wall is formed by the inferior surface of the greater wing of the sphenoid bone and the oval and spinous foramina in it, as well ap by a small area of the squamous part of the temporal bone. The infratemporal fossa is covered partly on the external surface by the mandibular ramus, wich contains mandibular foramen.. It communicates with the orbit through the inferior orbital fissure and with the pterygopalatine fossa through the pterygomaxillary fissure. The pterygopalatine fossa (fossa pterigopalatina) is located between the maxilla and the the pterygoid process. The anterior wall is the tuber of the maxilla, posterior wall is a base of the pterygoid process and its medial wall is the vertical plate of the palatine bone isolating the pterygopalatine fossa from the cavity of the nose. The following five openings are found in the pterygopalatine fossa: (1) a medial opening, the sphenopalatine foramen (foramen sphenopalatinum) leading into the nasal cavity and transmitting the sphenopalatine nerve and vessels; (2) a round opening (foramen rotundum) leading into the middle cranial fossa and transmitting the second branch of the trigeminal nerve which leaves the cranial cavity; (3) the inferior orbital fissure (fissura orbitalis inferior) leading into the orbit and transmitting the nerves and vessels; (4) the greater palatine canal (canalis palatinus major) leading into the oral cavity; it is formed by the maxilla and the greater palatine sulcus of the palatine bone and is a funnel-shaped narrowing of the lower part of the pterygopalatine fossa and transmits the nerves and vessels leaving this fossa; (5) the pterygoid canal (canalis pterygoideus) transmitting the vegetative nerves (n. canalis pterygoidei) and leading to, the external base of the skull. On examination of the skull from above (norma verticalis), its roof and the sutures are seen: the sagittal suture (sutura sagittalis) between the medial border of the two parietal bones; the coronal suture (sutura coronalis) between the frontal and the two parietal bones; and the lambdoid suture (sutura lambdoidea) (from its resemblance to the Greek letter lambda) between the two parietal bones and the occipital bone. The external surface of the base of the skull (basis cranii externa)extends from the incisors anteriorly to the superior nuchal line (linea nuchae superior) posteriorly. Three parts are distinguished in the external surface of the base of the skull: anterior, middle, and posterior. The anterior part is formed of the hard palate (palatum osseum) and the alveolar arch of the maxilla; a transverse suture (sutura transversa) is seen in the posterior part of the hard palate at the junction of its components, the palatine process of the maxilla and the horizontal plate of the palatine bone. A median palatine suture (sutura mediana) joining the paired parts of the hard palate runs on the midline and its anterior end is continuous with the incisive foramen. In the posterior part of the hard palate, near to the alveolar arch, is the greater palatine foramen (foramen palatinum majus), the exit from the greater palatine canal; still further to the back, on the inferior surface of the pyramidal process, are the openings of the lesser palatine canals. The middle part extends from the posterior edge of the hard palate to the anterior margin of foramen magnum. On the anterior border of this part are openings, choanae. In the posterior part of the base of the skull is the jugular foramen (foramen jugulare), formed by the jugular fossa of the temporal bone and the jugular notch of the occipital bone. The jugular foramen transmits the ninth, tenth, and eleventh cranial nerves, and the jugular vein originates here.The openings and anatomical components of the external base are; for. lacerum, for. ovale, for. spinosum, for. stylomastoideum, processus styloideus, condylus occipitalis and canalis hypoglossalis, external opening of the carotid canal,fossa mandibularis, fossulla petrosa, canalis pterygoideus, porus acusticus externus and others. The upper surface of the base of the skull (basis cranii interna) can be examined only on a horizontal or sagittal section of the skull. The internal or superior surface of the base of the skull is separated into three fossae. The anterior and middle fossae lodge the cerebrum, while the posterior fossa lodges the cerebellum. The posterior edges of the lesser wings of the sphenoid bone are the borderline between the anterior and middle fossae, and the superior edge of the pyramids of the temporal bones is the borderline between the middle and posterior fossae. The anterior cranial fossa (fossa cranii anterior) is formed by the orbital part of the frontal bone, the cribriform plate of the ethmoid bone, and the lesser wings of the sphenoid bone. It is distinguished by pronounced digitate impressions and cerebral ridges (juga cerebralia). Ther are the crista galli, for. cecum, crista frontalis interna. The middle cranial fossa (fossa cranii media) is located deeper than the anterior fossa. Its median part is formed by the sella turcica. The openings of the middle fossa are as follows: the optic canal, the superior orbital fissure, foramen rotundum, the oval foramen, the spinous foramen, the for. lacerum (is between the sphenoid, the occipital and the temporal bones), and the anatomical components of the anterior surface of the pyramis. The posterior cranial fossa (fossa cranii posterior) is the deepest and largest of the three fossae. Its components are the occipital bone, the posterior parts of the body of the sphenoid bone, the petrous part of the temporal bone, and the inferoposterior angle of the parietal bone. The following openings are found in it: foramen magnum, hypoglossal canal, jugular foramen, condylar canal (sometimes absent), mastoid foramen (occurring most regularly), porus acusticus internus (on the posterior surface of the pyramid). The cavity of the nose (cavum nasi) is the initial part of the respiratory tract and lodges the organ of olfaction. The piriform aperture leads into the cavity in front, and the paired openings, the choanae, connect it with the cavity of the pharynx. The bony septum of the nose (septum nasi osseum) divides the nasal cavity into two halves, which are not quite symmetrical, in most cases the septum deviates to one of the sides from the sagittal plane. Each half of the nasal cavity has five walls: superior, inferior, lateral, medial, and posterior. The lateral wall is the most complex in structure; it is formed of the following (from front to back) bones: the nasal surface of the body and frontal process of the maxilla, the lacrimal bone, the labyrinth of the ethmoid bone, the perpendicular plate of the palatine bone and the medial plate of the pterygoid process of the sphenoid bone. The medial wall, or the osseus nasal septum (septum nasi osseum) is formed by the perpendicular plate of the ethmoid bone and the vomer. The superior wall is made up of a nasal part of the frontal bone, the cribriform plate of the ethmoid bone, and partly the sphenoid bone. The inferior wall, or floor, is formed by the palatine procces of the maxilla and the horizontal plate of the palatine bone which make up the bony hard palate (palatum osseum); the opening of the incisive canal (canalis incisivus) is seen in its front part. Three nasal conchae project downward into the nasal cavity from the lateral wall, they separate the three nasal meatuses – superior, middle, and inferior – from each other. The superior nasal meatus (meatus nasi superior) is between the superior and middle conchae of the ethmoid bone; it is half the length of the middle meatus and is found only in the posterior part of the nasal cavity; it communicates with the sphenoid sinus and sphenopalatine foramen and the posterior air cells of the ethmoid bone open into it. The middle nasal meatus (meatus nasi medius) passes between the middle and inferior canchae. The anterior and middle cells of the ethmoid bone, the maxillary sinus and the frontal sinus open into it. These anatomical communications explain the spread of the inflammatory process to the frontal sinus (frontitis) in rhinitis. The inferior nasal meatus (meatus nasi inferior) is between the inferior nasal concha and the floor of the nasal cavity. The nasolacrimal and the incisive canales open into its anterior part; through this canal the tears flow into the nasal cavity. That is the reason the amount of nasal discharge increases when a person cries and, conversely, the eyes “water” in rhinitis. The space between the conchae and the nasal septum is known as the common meatus of the nose (meatus nasi communis). AGE FEATURES OF THE SKULL The skull of the newborn is characterized by a small visceral cranium as compared to the cerebral part. The fontanelles (fonticuli) are another characteristic feature of a newborn’s skull. The following fontanelles are distinguished: (1) a rhomboid anterior fontanelle (fonticulus anterior) located on the midline at the intersection of four sutures (saggital, frontal, and two halves of the coronary sutures); it closes in the second year of life; (2) a triangular posterior fontanelle (fonticulus posterior), at the posterior end of the sagittal suture between the two parietal bones in front and the squama of the occipital bone at the back; it closes in the second month after birth, (3) paired lateral fontanelles, two on each side; the anterior one is called sphenoidal (fonticulus sphenoidalis) and the posterior one is called mastoid (fonticulus mastoideus). The sphenoidal fontanelle is at the junction of the mastoid angle of the parietal bone, the greater wing of the sphenoid hone, and the squama of the temporal bone; it closes in the second or third mounth of life. The mastoid fontanelle is between the mastoid angle of the parietal bone, the base of the pyramid of the temporal bone, and the squama of the occipital bone. The sphenoidal and mastoid fontanelles are mostly found in premature infants; some full-term infants may also have no occipital (posterior) fontanelle.

MYOLOGY

THE MUSCLES OF THE HEAD

All the muscles of the head can be divided into the following two groups: 1. Muscles of mastication 2. Muscles of facial expression

MUSCLES OF MASTICATION The four muscles of mastication on each side are related morphologically (they are all attached to the mandible which they move when they contract), and functionally (they accomplish the chewing movements of the mandible, which determines their location) The masseter muscle (m. masseter) is thick and quadrangular, stretches form the zygomatic arch to the outer surface of the mandibular ramus. The action of the muscle is that of a powerful elevator of the lower jaw closing the jaws and exerting pressure upon the teeth, especially in the molar region. The temporal muscle (m. temporalis) is wide at its origin and occupies the whole temporal fossa of the skull up to the temporal line. The insertion of the muscle occupies the coronoid process and reaches down to the ramus of mandible. The temporal muscle is mainly an elevator of the mandible. The lateral pterygoid muscle (m. pterygoiaeus lateralis) arises with two heads. The larger inferion head originates from the outer surface of the lateral pterygoid plate and the smaller superior head from the infratemporal surface of the greater sphenoid wing. The muscle attaches to the articular capsule of the temporomandibular joint and inserts to a mandibular neck. The muscle pulls the head of the mandible and the articular disc forward, downward, and inward. The medial pterygoid muscle (m. pterygoideus medialis), its origin is in the pterygoid fossa. The fibres of the muscle run downward, backward, and outward and are inserted to the medial surface of the mandibular angle. The muscle is a synergist of the masseter muscle, and, therefore, an elevator of the mandible. Action. The temporal, masseter, and medial pterygoid muscles pull the mandible to the maxilla when the mouth is open and thus close the mouth. On simultaneous contraction of both lateral pterygoid muscled the mandible protrudes forward. Movement in the opposite direction is accomplished by the posterior fibres of the temporal muscle which pass almost horizontally forward. Unilateral contraction of the lateral pterygoid muscle displaces the mandible to the contralateral side of the mouth. The temporal muscle is associated with articulate speech; it sets the mandible in a definite position when a person speaks.

MUSCLES OF FACIAL EXPRESSION The muscles of facial expression, as distinct from the skeletal muscles, are not doubly attached to the bones but always interlace at one or both ends with the skin or mucous membrane. As a result they are devoid of fasciae and move the skin upon contraction. When they relax, the skin returns to its former state due to its elasticity. The muscles of facial expression are small, thin muscle boundles grouped around the natural orifices (the mouth, nose, palpebral fissure, and ear). These muscles take part in closing or widening, the orifices.Sphincters are usually arranged annularly around the orifices they close, while the dilators which widen the orifices, are arranged radially. .MUSCLES SURROUNDING THE EYES The orbicularis oculi muscle (m. orbicularis oculi) surrounds the eyelids; its wide peripheral orbital part (pars orbitalis) is on the bony margin of the orbit, while the central palpebral part (pars palpebralis) is on the eyelids. The palpebral part closes the lids gently, while strong contraction of the orbital part closes them tightly. Isolated contraction of the upper fibres of this part draws the skin of the forehead and the eyebrow downward; as a result the eyebrow is straightened out, and the transverse wrinkles on the forehead are smoothed out. In this respect it is an antagonist of the venter frontalis. MUSCLES AROUND THE MOUTH The levator labii superions muscle (m. levator labii superioris) raises the upper lip . The depressor anguli oris muscle (m. depressor anguli oris). It pulls down the angle of the mouth. Depression of the angles of the mouth produces the expression of grief. The levator anguli oris muscle (m. levator anguli oris).It raises the angle of the mouth. The depressor labii inferioris muscle (m. depressor labii inferioris) It pulls the lip down and a little laterally, as occurs, for example, in expressions of disgust. The mentalis muscle (m. mentalis) It raises the skin of the chin (with the formation of small dimples in it) and pushes the lower lip upward, pressing it to the upper lip. The buccinator muscle (rn. buccinator).The action of this muscle consists in the expulsion of the contents of the vestibule of the mouth, as, for example, the expulsion of air when playing a trumpet; hence the name of the muscle, buccinator (L trumpeter). The orbicularis oris muscle (m. orbicularis oris) lies in the thickness of the lips around the rima oris.Contraction of the peripheral part of the orbicularis oris purses the lips and pushes them forward, as in kissing. During contraction of the part under the vermilion border, the lips are drawn tightly together and inverted; as a result the vermilion border is hidden.

MUSCLES OF THE NECK The muscles of the neck are grouped as follows. 1. Superficial muscles (platysma, m. sternocleidomastoideus). 2. Medial muscles, or muscles of the hyoid bone: (a) muscles located above the hyoid bone or the suprahyoid muscles(mm. mylohyoideus, digastricus, stylohyoideus, geniohyoideus); (b) muscles located below the hyoid bone the infrahyoid muscles (mm. stemohyoideus, sternothyroideus, thyrohyoideus, omohyoideus). 3. Deep muscles: (a) lateral, attached to the ribs (mm. scaleni anterior, medius and posterior); (b) prevertebral muscles (m. longus colli, m. longus capitis, m. rectus capitis anterior and lateralis).

SUPERFICIAL MUSCLES 1.Platysma is a subcutaneous muscle of the neck lying directly under the fascia thin sheet. Action. Pulling the skin of the neck, the muscle protects the subcutaneons veins from, compression; it can also depress the angle of the mouth, which is important for facial expression. 2.The sternocleidomastoid muscle (m. sternocleidomastoideus) lies immediately under the platysma and is separated from it by the cervical fascia. Action. In unilateral contraction, the muscle flexes the cervical segment of the spine to the same side; the head is raised at the same time, and the face turned to the opposite side. In bilateral contraction, the muscles hold the head in a vertical position (head-holder); that is why the muscle itself and the place of its attachment (the mastoid process) are most developed in man, who walks erect. THE MIDDLE MUSCLES, OR MUSCLES OF THE HYOID BONE These muscles of suprahyoid localization lie between the mandible and the hyoid bone. The mylohyoid muscle (m. mylohyoideus) is a flat muscle with parallel fibres that arise from the mandibular mylohyoid line. Both mylohyoid muscles meet and form the floor of the mouth (diaphragma oris) which closes the bottom of the oral cavity. The digastric muscle (m. digastricus) consists of two; anterior and posterior bellies connected by a round intermediate tendon. The stylohyoid muscle (m. stylohyoideus) descends obliquely from the styloid process of the temporal bone. The geniohyoid muscle (m. geniohyideus) lies above the mylohyoid muscle laterally of the raphe. It stretches from the spina mentalis of the mandible to the body of the hyoid bone. Action. All the four muscles described above raise the hyoid bone. When the bone is steadied, three muscles (mylohyoid, geniohyoid and digastric) lower the mandible and thus are antagonists of the muscles of masticatiori. Muscles located below the Hyoid Bone. The sternohyoid muscle (m. sternohyoideus) originates from the posterior surface of the sternal manubrium. Between the medial borders of both sternohyoid muscles is a narrow vertical space closed by fascia; this is the linea alba cervicalis. Action. Pulls the hyoid bone downward. The sternothyroid muscle (m. sternothyroideus) lies under the sternohyoid muscle and is broader.It then ascends and attaches to the lateral surface of the thyroid cartilage (to its linea obliqua).Action. Lowers the larynx. The thyrohyoid muscle (m. thyrohyoideus) seems to be a continuation of the sternothyroid muscle from which it is separated by a tendinous intersection. It stretches from the oblique line of the thyroid cartilage to the body and greater horn of the hyoid bone. Action. Pulls the larynx upwards when the hyoid bone is steadied. The omohyoid muscle (m. ornohyoideus) is a long narrow muscle consisting of two; superior and inferior bellies joined almost at a right angle by an intermediate tendon. Action. The omohyoid muscle lies in the thickness of the cervical fascia which it tightens on contraction and thus aids in dilation of the large veins situated under the fascia. It also pulls the hyoid bone downwards. THE DEEP MUSCLES The three scalene muscles are altered intercostal muscles, wlich explains their attachment to the ribs. The scalenus anterior muscle (m. scalenus anterior). The scalenus medius muscle (m. scalenus rnedius). The scalenus posterior muscle (rn. scalenus posterior). Action. The scalene muscles raise the upper ribs and act as muscles of inspiration. TOPOGRAPHY OF THE NECK The neck (collum) is divided into four regions: posterior, lateral, the region of the sternocleidomastoid muscle, and the anterior region. The posterior region (regio colli posterior) is behind the lateral border of the trapezius muscle and is the nape, or nucha. The lateral region (region colli lateralis) is behind the sternocleidumastoid muscle and is bounded in front by this muscle, below by the clavicle, and behind by the trapezius muscle. The sternocleidomastoid region (regio stemocleidomastoidea) corresponds to the projection of this muscle. The anterior region (regio colli anterior) is in front of the sternocleidomastoid muscle and is bounded posteriorly by this muscle, in front by the midline of the neck, and above by the border of the mandible. A small area behind the mandibular angle and in front of the mastoid process is called the fossa retromandibularis. It lodges the posterior part of the parotid gland, nerves, and vessels. The anterior and lateral regions are divided into a number of triangles by the omohyoid muscle descending obliquely from front to back and crossing the sternocleidomastoid muscle. Two triangles are distinguished in the anterior region of the neck: (1) the fossa carotica, or carotis trigone (trigonum caroticum) (transmitting the carotid artery), formed by the sternocleidomastoid muscle posteriorly, the posterior belly of the digastric muscle in front and above, and the superior belly of the omohyoid muscle in front and below, (2) the submandibular trigone (trigonum submandibulare) (lodging the submaxillary gland), formed by the inferior border of the mandible above and the two bellies of the digastric muscle. In the submandibular trigone is distinguished trigonum linguale formed by the posterior margin of the mylohyoid muscle anteriorli, the posterior belly of the digastric muscle below and the hypoglossal nerv above (passes a lingualis). Two triangles are distinguished in the posterior region of the neck: (1) The omoclavicular trigone or subclavian triangle (trigonun omoclaviculare) is distinguished in the lateral region of the neck; it is bounded by the sternocleidomastoid muscle in front, the inferior belly of the omohyoid muscle above, and the clavicle below and (2) omotrapezoid trigone(trigonun omotrapezoideum) ; it is bounded by the sternocleidomastoid muscle in front, the inferior belly of the omohyoid muscle below, and the trapezius muscle behind. Triangular slits or spaces form between the scalene muscles; they transmit nerves and vessels of the upper limb.Between the anterior and middle scalene muscles and the first rib below is spatium interscalenum (it transmits the subclavian artery and the brachial plexus).

THE CAVITY OF ТНЕ MOUTH

The cavity of the mouth, oral cavity (cavitas oris) is divided into two parts, the vestibule of the mouth (vestibulum oris) and the cavity of the mouth proper (cavitas oris proprium). The vestibule of the mouth is the space bounded by the lips and cheeks externally and bу the teeth and gingivae internally. By means of the opening of the mouth, the oral fissure (rima oris), the vestibule opens into the external environment. The lips (labia oris) are fibres of the orbicular muscle of the mouth covered on the outside by the skin and lined inside with mucous membrane. At the angles of the oral fissure the lips come together bу means of commissures (commissurae labiorum). The skin on the lips is continuous with the mucous membrane of the mouth; extending from the upper lip оf the surface of the gum (gingiva) the mucous membrane forms on the midline а rather conspicuous fold, frenulum labii superioris. Frenulum labii inferioris is usually hardly noticeable. Epithelial villi (torus villosus) are seen in the region of the angle of the mouth and on the posterior margin of the vermillion border of the lip in the newborn; they help the infant tо grasp and hold in the mouth the mother's nipple during sucking. The cheeks (bиссае) are similar tо the lips in structure but instead of m. orbicularis oris, the buccinator muscle (m. buccinator) is situated here. The fat lodged in the thickness of the cheeks (corpus adiposum bиссае) is developed much better in the child than in an adult and is conducive to а decrease of pressure on the part of the atmosphere during sucking. Cavitas oris proprium extends from the teeth anteriorly and laterally to the entry into the pharynx posteriorly. The oral cavity is bounded superiorly by the hard palate and the anterior part of the soft palate; the floor is formed bу the diaphragm of the mouth (diaphrogma oris) (the paired mу1оhyoid muscles) and is occupied by the tongue. When the mouth is closed, the tongue comes in contact with the palate so that the oral cavity becomes narrow slit-like space between them. The mucous membrane extending to the inferior surface of the tip of the tongue forms on the midline the frenulum of the tongue (frenulum linguae). On each side of the frenulum is а noticeable eminence, caruncula sublingualis, with the opening of the ducts of the submandibular and sublingual salivary glands. The sublingual fold (plica sublingualis) stretches on each side laterally and posteriorly of the sublingual caruncle; it is formed by the sublingual salivary gland situated here.

THE PALATE

The palate (palatum) consists of two parts. Its anterior two thirds have a bony foundation, palatum osseum (the palatine process of the maxilla and the horizontal plate of the palatine bone); this is the hard palate (palatum durum). The posterior third, the soft palate (palatum molle) is a muscular structure with a fibrous foundation. During quiet breathing through the nose it hangs obliquely downward and separates the oral cavity from the pharynx. A seam, raphe palati, is seen on the midline of the palate. At the anterior end of the raphe is a row of transverse ridges (about six of ihem), plicae palatinae transversum (rudiments of palatine ridges contributtng in some animals to the mechanical treatment of the food). The mucous membrane covering the inferior surface of the hard palate adheres closely to the periosteum by means of dense fibrous tissue. The soft palate (palatum molle) is a duplication of the mucous membrane in which muscles are lodged together with a fibrous plate, the palatine aponeurosis, as well as glands. Its anterior margin is attached to the posterior edge of the hard palate, while the posterior part of the soft palate (velum palatinum) extends freely downward and to the back and has on the midline a tongue-like projection, the uvula. Laterally, the soft palate is continuous with folds, or arches. The anterior palatoglossal arch (arcus palatoglossus) passes to the lateral surface of the tongue, the posterior palatopharyngeal arch (arcus palatopharyngeus) stretches for some distance on the lateral wall of the pharynx. A depression forms between the anterior and posterior arches which lodges the palatine tonsil (tonsilla palatina). Each palatine tonsil is an oval-shaped mass of lymphoid tissue. The tonsil occupies the greater inferior part of a triangular depression between the arches (sinus tonsillaris s. fossa tonsillaris). The nearest important blood vessel in the facial artery which sometimes (when it is tortuous) comes very close to the pharyngeal wall at this level. This must he borne in mind in operation for removal of the tonsils. The internal carotid artery passes at a distance of about 1 cm from the tonsil. The soft palate is composed of the following muscles. 1. M. palatopharyngeus descends to the pharynx in the thickness of arcus palatopharyngeus. It pulls the velum pallatinum downwards and the pharynx upwards, in which case the pharynx becomes shorter and presses the soft palate to the posterior pharyngeal wall. 2. M. palatoglossus descends in the thickness of arcus palatoglossus. It lowers velum palatinum and during this movement both palatoglossal arches become tense ant the opening of the fauces is narrowed. 3. M. levator veli palatini. It raises velum palatinum. 4. M. tensor veli palatini. It tenses velum palatinum in the transverse direction. 5. M. uvulae. It shortens the uvula. The uvula is present only in man because it is necessary to create a condition of air-tightness in the oral cavity, which prevents the jaw from hanging in the erect position of the body. The aperture by means of which the oral cavity communicates with the pharynx is called the fauces, s. isthmus faucium. It is bounded on the sides by the palatoglossal arches, above by the soft palate, and below by the back of the tongue.

THE TONGUE

The tongue (lingua) (Gk glossa tongue, hence glossitis, inflammation of the tongue) is mainly a muscular organ (striated fibres). The changes in its shape and position are significant in the acts of mastication and speech, while due to the presence of specific nerve endings in the mucous membrane the tongue is also the organ of taste. Three parts are distinguished in the tongue: the larger part, or the body (corpus linguae) facing anteriorly, the tip (apex) and the postero-inferior part, or the root (radix linguae) by means of which the tongue is attached to the mandible and the hyoid bone. Its convex superior surface faces the palate and pharynx and is called the back (dorsum). The inferior surface of the tongue (facies inferior linguae) is free only in the anterior part; the posterior part is occupied by muscles. On the sides the tongue is bounded by margins (margo linguae). Two parts are distinguished on the dorsum of the tongue: the anterior, larger (accounting for about two thirds of the tongue), part is situated almost horizontally on the floor of the oral cavity; the posterior part is almost vertical and faces the pharynx. At the junction of the anterior and posterior parts of the tongue on the midline is a depression (foramen cecum linguae), which is the remnant of the tubular projection from the floor of the primary pharynx, from which the thyroid isthmus develops. Forward and lateralward from this foramen extends on either side the terminal sulcus of the tongue (sulcus terminales linguae). The trace of the fusion of the paired germ remains throughout life on the dorsum of the tongue in the form of a median sulcus (sulcus medianus linguae) and within the tongue as a fibrous septum of the tongue (septum linguae). The mucous membrane of the anterior part of the tongue is supplied with numerous papillae and bears the median sulcus mentioned above. In the posterior part, the mucous membrane is thicker and smoother because there are no papillae, but it has a bulging appearance due to the presence of lymphoid follicles. The aggregation of the lymphoid structures of the posterior part of the tongue is called the lingual tonsil (tonsilla lingualis). Three folds of the mucous membrane, glosso-epiglottic medial fold (plica glosso-epiglottica mediana) and two glosso-epiglottic lateral folds (plicae glosso-epiglotticae laterales) stretch from the posterior part of the tongue to the epiglottis; two depressions, valleculae epiglotticae, are seen between the median and lateral folds. The tongue papillae (papillae lingualis) are of the following types. 1. Filiform and conical papillae (papillae filiformis and conicae) are the smallest but most numerous. They occupy the superior surface of the anterior part of the tongue and lend its mucous membrane a rough or velvety appearance. The filiform and conical papillae evidently act as tactile organs. 2. Fungiform papillae (papillae fungiformes) are less in number and are found mainly at the apex and on the margins of the tongue. They are supplied with taste buds and it is therefore accepted that they are concerned with the sense of taste. 3. Vallate (walled) papillae (papillae vallatae) are surrounded by a bank. They are the largest and are arranged in the shape of the letter V immediately in front of the foramen cecum and the terminal sulcus, with the apex facing posteriorly. Their number varies between 7 and 12. Each papilla consists of a central cylindrical part (1.0-2.5 mm in diameter) and a surrounding deep narrow groove. Very many taste buds are embedded in each papilla. 4. Foliate papillae (papillae foliatae) are located on the margins of the tongue. The taste papillae are also encountered on the free edge and nasal surface of the palate and on the posterior surface of the epiglottis. Peripheral nerve endings, the receptors of the taste analyser, are embedded in the taste papillae. The muscles of the tongue constitute its muscular bulk, which is divided into two symmetric halves by a longitudinal fibrous septum (septum linguae). It is more correct, to distinguish three groups of muscles according to structure and action: (1) muscles arising on the derivatives of the first visceral arch (on the mandible), these are the genioglossus muscle and its continuation, the vertical muscle. Their contraction moves the tongue forward and frattens it. (2) muscles arising on the derivatives of the second visceral arch (on the sty1oid process and lesser horns of the hyoid bone): these are the styloglossus and the superior and inferior longitudinal muscles. Their contraction moves the tongue backward and shortens it. (3) muscles arising on the derivatives of the third visceral (first branchial) arch (on the body and greater horns of the hyoid bone): these are the hyoglossus and the transverse muscles. The palatoglossus muscle is also related to this group. Their contraction reduces the transverse dimension of the tongue. In unilateral contraction, the tongue moves to the side of the contracting muscles, in bilateral contraction it moves downward and backward.

GLANDS OF THE ORAL CAVITY

The ducts of three pairs of large salivary glands open into the oral cavity: the parotid, submandibular, and sublingual glands. Besides these, there are numerous small glands in the mucous membrane of the mouth, which, according to their location, are called as follows: the labial, buccal, palatine, and lingual glands. According to the character of the secretion, the glands may be serous mucous or mixed. These glands are as follows. 1. The parotid (glandula parotis) is the largest of the salivary glands and is a serous gland. It is situated on the lateral side of the face in front of and a little below the ear and penetrates into the retromandibular fossa. The duct of the gland (ductus parotideus) is 5-6 cm long an arises from the anterior border of the gland, passes on the surface of the masseter muscle, curves around its anterior border, and after passing through the fatty tissue of the cheek pierces the buccinator muscle. It then enters the oral cavity under the mucous membrane and opens into the vestibule of the mouth by a small opening opposite the second upper moral. 2. The submandibular gland (glandula submandibularis) is of a mixed character, compound alveolar- tubular in structure, and the second largest after the parotid. The submandibular gland is situated in the submandibular fossa, emerges from under the border of the mandible and is covered here by the skin, the platysma muscle, and the fascia of the neck. The submandibular duct (ductus submandibularis), which passes over the mylohyoid muscle on the floor of the oral cavity and opens on caruncula sublingualis. 3. The sublingual gland (glandula sublingualis) is of the mucous type and alveolar-tubular in structure. It is situated over the mylohyoid muscle on the floor of the oral cavity, and, covered only by the mucous membrane, forms a sublingual fold (plica sublinguales) between the tongue and the inner surface of the mandible. In front of the frenulum of the tongue it comes in contact with the contralateral gland. The ducts of some lobules (18-20 in number) open independently into the oral cavity along the sublingual fold; they are called the smaller sublingual ducts (ductus sublinguales minores). The principal duct of the sublingual gland (ductus sublingualis major) passes next to the duct of the submandibular gland and opens by means of a single opening common to both ducts or by its own opening along side the opening of the submandibular duct.

THE PHARYNX

The pharynx is that part of the alimentary canal and respiratory tract, which is a connecting link between the cavity of the nose and mouth and the oesophagus and trachea. It stretches from the level of the base of the skull to that of the sixth or seventh cervical vertebra. The space within the pharynx is the pharyngeal cavity (cavitas pharyngis). The pharynx is situated behind the nasal and oral cavities and the larynx and in front of the basilar part of the occipital bone and the upper six cervical vertebrae. In accordance with the organs situated in front of the pharynx, three parts can be distinguished in it: pars nasalis, pars oralis, and pars laryngea. The superior wall of the pharynx, which adjoins the base of the skull, is called the vault of the pharynx (fornix pharyngis). The nasal part of the pharynx, or the nasopharynx (pars nasalis pharyngis) is a purely respiratory part functionally. As distinct from the other parts of the pharynx, its walls do not collapse because they are immobile. The anterior wall of the nasal part is occupied by the choanae. On either lateral wall is a funnel-shaped pharyngeal opening of the auditory tube (part of the middle ear), ostium pharyngeum tubae. Superiorly and posteriorly the opening of the tube is bounded by torus tubarius formed due to the projection of the cartilage of the auditory tube here. At the junction of the superior and posterior pharyngeal walls on the midline is an accumulation of lymphoid tissue, the pharyngeal tonsil, or adenoids (tonsilla pharyngea s. adenoidea) (in adults it is hardly noticeable, or disappears entirely). Another accumulation of lymphoid tissue, but paired, is located between the pharyngeal opening of the tube and the soft palate; this is the tube tonsil (tonsilla tubaria). Thus, almost a complete ring of lymphoid structures is found at the entry into the pharynx: the lingual tonsil, two palatine tonsils, two tube tonsils, and one pharyngeal tonsil (Pirogov's lymphoepithelial ring). The oral part (pars oralis) is the middle part of the pharynx communicating with the oral cavity in front through the isthmus faucium, its posterior wall corresponds to the third cervical vertebra. The oral part is mixed in function because the alimentary and respiratory tracts intersect here. The laryngeal part (pars laryngea) begins behind the trachea and extends from the opening into the larynx to the opening into the oesophagus. In a state of rest, when there is no swallowing, the anterior and posterior walls of this art remain in contact and separate only when food passes in it. On the anterior wall is the opening into the pharynx bounded in front by the epiglottis and on the sides by the aryepiglotic folds (plicae aryepiglotticae). On either side of the folds lie paired pear- shaped fossae in the pharyngeal wall (recessus piriformes). The foundation of the pharyngeal wall is a well developed layer of fibrous tissue. This fibrous coat is lined inside with mucous membrane (mucous coat) and covered from the outside by red by a thinner layer of fibrous tissue connecting the pharyngeal wall with the adjoining organs. The fibrous coat of the pharynx, pharyngobastlar fascia (fascia pharyngobasilaris) is attached above to the basal part of the occipital bone. The mucous coat of the nasal part of the pharynx is covered with ciliated epithelium in accordance with the respiratory function of this part of the pharynx, whereas in the inferior parts it is covered with stratified squamous epithelium. The mucous coat of the inferior parts of the pharynx fuses with the underlying tissue and acquires a smooth surface, which promotes gliding of the bolus during swallowing. This is also aided by the secretion of mucous glands embedded in the mucosa and by the pharyngeal muscles arranged longitudinally (dilators) and circular (constrictors). The circular layer is much stronger and consists of three constrictors arranged in three storeys: the superior constrictor muscle of the pharynx (m. constrictor pharyngis superior), the middle constrictor muscle of the pharynx (m. constrictor pharyngis medius), and the inferior constrictor muscle of the pharynx (m. constrictor pharyngis inferior). The lower fibres of the inferior pharyngeal cobstrictor are, closely connected with the muscle fibres of the oesophagus. The longitudinal muscle fibres of the pharynx run in the following two muscles. 1. The stylopharyngeus muscle (m. stylopharyngeus) arises from the styloid process and descends to be inserted partly in the pharyngeal wall itself and partly on the superior edge of the thyroid cartilage. 2. The palatopharyngeus muscle (m. palatopharyngeus) is described above (see "Soft Palate"). The act of swallowing, deglutition. Since the respiratory and digestive tracts intersect in the pharynx, special devices exist, which separate these two tracts during swallowing. By contraction of the tongue muscles the bolus is pressed against the hard palate by the dorsum of the tongue and then pushed through the fauces. During this process, the soft palate is pulled upward (by contraction of the levator veli palatini and the tensor veli palatini muscles) and brought nearer to the posterior pharyngeal wall (by contraction of the palatopharyngeus muscle). In this manner, the nasal (respiratory) part of the pharynx is completely separated from the oral part. At the same time, the suprahyoid muscles pull the pharynx upward, while the root of the tongue is pulled downward by contraction of the hyoglossus muscle; the root of the tongue presses against the epiglottis and depresses it and closes the opening into the larynx (into the respiratory tract) in this way. Then the pharyngeal constrictors contract in succession as a result of which the bolus is pushed toward the oesophagus. The longitudinal pharyngeal muscles act as elevators, they pull the pharynx to meet the bolus.

THE OESOPHAGUS

The oesophagus is a narrow and long actively functioning tube inserted between the pharynx and the stomach. It aids the passage of food toward the stomach. The oesophagus begins at the level of the sixth cervical vertebra, which corresponds to the level of the inferior border of the cricoid cartilage of the pharynx, and ends at the level of the eleventh thoracic vertebra. Arising in the region of the neck, the oesophagus passes further into the thoracic cavity and, penetrating the diaphragm, enters the abdominal cavity; three parts are consequently distinguished in it: cervical (pars cervicalis), thoracic (pars thoracica), and abdominal (pars abdominalis). The oesophagus is 23 to 25 cm in length. The total length of the tract from the anterior teeth, through the oral cavity, the pharynx, and the oesophagus, ranges from 40 to 42 cm (in collecting the gastric secretions for tests the rubber gastric tube must be advanced into the oesophagus for this distance plus another 3.5 cm). Structure. On a transverse section, the oesophageal lumen is seen as a transverse gap in the cervical part (due to pressure exerted by the trachea) but it is rather round or stellate in the thoracic part. The oesophageal wall consists of the following coats: the innermost, the mucous coat (tunica mucosa), the middle, muscular coat (tunica muscularis) and the outer coat of a connective-tissue character, the adventitious coat (tunica adventitia). The mucous coat contains mucous glands whose secretions facilitate glidillg of the bolus during swallowing. When the oesophagus is not dilated, the mucosa gathers to form longitudina folds, which can straighten out when food passes through the oesophagus. The muscular coat (tunica muscularis) corresponds to the tubular shape of the oesophagus which, to accomplish the function of propelling the bolus, must dilate and contract. The muscular coat is arranged in two layers; the outer, longitudinal layer (dilating the oesophagus) and an inner, circular layer (constricting the oesophagus). In the upper third of the oesophagus, both layers consist of striated muscles but distally they are gradually replaced by smooth fibres so that in the lower half of the oesophagus they are composed almost exclusively of smooth fibres. Of the two oesophageal muscular layers, the outer, longitudinal one, is more developed. The adventitious coat (tunica adventitia) invests the oesophagus and serves as a sort of fascia for the muscular coat. The abdominal part of the oesophagus is covered by the peritoneum. The oesophagus has a number of constrictions and dilatations, which arc significant in making the diagnosis of pathological processes. Three anatomical constrictions are clearly seen in a cadaver. They persist also in an isolated oesophagus: (1) pharyngeal (at the origin of the oesophagus), (2) bronchial (at the level of the tracheal bifurcation ), and (3) a diaphragmatic constriction (where the oesophagus pierces the diaphragm).

THE ABDOMINAL CAVITY

Beginning with the stomach, the parts of the digestive tract with its large glands (liver and pancreas) as well as the spleen and the urogenital system are located in the abdominal cavity. The abdominal cavity (cavitas abdominis) is the space in the trunk below the diaphragm; it is completely filled with the abdominal organs. The diaphragm, serving as the superior wall of the abdominal cavity, separates it from the thoracic cavity. The anterior wall of the abdominal cavity is formed by the tendinous expansions of the three broad abdominal muscles and tbe straight abdominal muscles. The components of the lateral walls are the muscular portions of the three broad muscles of the abdomen. The posterior wall is formed by the lumbar segment of the spine and the psoas major and quadratus lumborum muscles. Below are the iliac bones and the pelvic diaphragm. The abdominal cavity is subdivided into the abdominal cavity proper and the pelvic cavity (cavitas pelvis). The pelvic cavity is bounded posteriorly by the anterior surface of the sacrum covered on the sides by the piriform muscles, anteriorly and laterally by parts of the hip bones with the overlying internal obturator muscles, which are lined with fasciae. The floor of the pelvic cavity is formed by the pelvic diaphragm composed of two pairs of muscles, the levatores ani and the coccygeal muscles. The abdominal cavity is lined with a serous membrane called the peritoneum, which covers to a lesser or greater extent the abdominal viscera. As a result a serous fold of two layers forms. Peritoneal folds passing from the wall of the abdominal cavity to parts of he intestinal canal are called the mesenterium, while those passing from the wall to the organs (e.g. the liver) are called ligament (ligamentum). An organ invested in the peritoneum is said to have an intraperitoneal position (e.g. the small intestine); a mesoperitoneal position is that when an organ is covered by the peritoneum on three sides (one side is devoid of a covering, e.g. the liver). If an organ is covered by the peritoneum only in front, its position is called extraperitoneal (e.g. the kidneys). Being smooth due its epithelial covering and moist because of the presence of a capillary layer of serous fluid, the peritoneum makes movement of the organs in relation to one another much easier by relieving friction between the contacting surfaces.

THE STOMACH

The stomach (ventriculus [gaster]) is a sac-Iike expansion of the digestive tract. After passing through the oesophagus food accumulates in the stomach and undergoes here the first stages of digestion during which the hard components are converted to a liquid or pasty mixture. An anterior wall (paries anterior) and posterior wall (paries posterior) of the stomach are distinguished. The concave border of the stomach facing upward and to the right is called the lesser curvature of the stomach (curvatura ventriculi minor); the convex border facing downward and to the left is the greater curvature (curvatura ventriculi major). A gastric notch (incisura angularis) is seen on the lesser curvature, nearer to the caudal than to the cranial end of the stomach, where the two parts of the curvature meet at a sharp angle to form the angulus ventriculi. The following portions are distinguished in the stomach: the opening of the oesophagus into the stomach, the cardiac orifice (ostium cardiacum) (Gk kardia heart; the opening of the oesophagus into the stomach is closer to the heart than the opening of the stomach into the duodenum); the adjoining, cardiac portion, of the stomach (pars cardiaca); the distal aperture of the stomach, pylorus and the adjacent pyloric portion of the stomach (pars pylorica); the dome-shaped part of the stomach to the left of ostium cardiacum is called the fundus, or to be more exact, the fornix. The body of the stomach (corpus ventriculi) stretches from the fundus to the physiological sphincter (sphincter antri) seen in a live person, and to the pars pylorica in a cadaver. Pars pylorica is in turn divided into the pyloric antrum (antrum pyloricum) and the pyloric canal (canalis pyloricus), a narrower tube-like part immediately adjoining the pylorus. Topography of the stomach. The stomach is situated in the epigastrium; its greater portion (about five sixths) is to the left of the median plane, from 11-th thoracic vertebra to 1 lumbar vertebra. Structure. The wall of the stomach consists of the following four layers; (1) the mucous coat (tunica mucosa); (2) a submucous (tela submucousa); (3)a muscular coat (tunica muscularis); (4)a serous coat (tunica serosa). The mucous coat (tunica mucosa) is built according to the principal function of the stomach, namely chemical treatment of the food in an acid medium. In view of this the mucosa contains special gastric glands, which produce gastric secretion, or gastric juice (succus gastricus) containing hydrochloric acid.. The close contact of the bolus with the mucous membrane and its adequate saturation with the gastric juice occur because the mucosa is capable of gathering into folds (plicae gastricae) due to contraction of its own musculature (lamina muscularis mucosae) and because of the presence of loose submucous tissue (tela submucosa) containing vessels and nerves and allowing the mucosa to smooth out and then to gather into folds of different directions. The folds on the lesser curvature are longitudinal and form the "gastric path", which, on contraction of the gastric muscles, may transform into a canal at. the given moment in which the liquid parts of the food (water, salt solutions) pass from the oesophagus into the pylorus, by-passing the cardiac part of the stomach. In addition to folds, the stomach has rounded elevations (1-6 mm in diameter) called gastric areas (areae gastricae) on whose surface numerous tiny openings (0.2 mm in diameter) of the gastric pits (foveolae gastricae) can be seen. The gastric glands open into these pits. In the region of the pyloric orifice (ostium pylorica) is a circular mucosal fold separating the acid medium of the stomach from the alkaline medium of the intestine; it is called valvula pylorica (BNA). The muscular coat (tunica muscularis) is composed of unstriated muscle fibres, which contribute to the mixing and movement of food; according to the sac shape of the stomach, they are arranged not in two layers, like the fibres in the oesophageal tube, but in three: an external longitudinal layer (stratum longitudinale); a middle circular layer (stratum circulare), and an internal oblique layer (stratum obliquae). The longitudinal fibres are continuation of similar fibres of the oesophagus. The circular layer is stronger than the longitudinal layer and is continuous with the circular fibres of the oesophagus. Toward the distal opening of the stomach, the circular layer increases in thickness and at the junction of the pylorus and the duodenum it forms a ring of muscular tissue, the pyloric sphincter (m. sphincter pylori). Valvula pylorica, which corresponds to the pyloric sphincter, on contraction of the sphincter isolates completely the cavity of the stomach from the cavity of the duodenum. The pyloric sphincter and valvula pylorica constitute a special adjustment regulating the passage of food from the stomach into the duodenum and preventing its return; otherwise the normal acid gastric medium would be neutralized. The oblique muscle fibres (fibrae obliquae) are arranged in bundles, which loop over the cardiac orifice on the left and form a "supporting loop" that serves as the punctum fixum for the oblique muscles. The oblique muscles descend obliquely on the anterior and posterior surfaces of the stomach and on contraction pull the greater curvature toward the cardiac orifice. The outermost layer of the gastric wall is formed by the serous coat (tunica serosa), which is a part of the peritoneum. The serous coat is closely attached to the whole surface of the stomach except for the curvatures where large vessels pass between the two peritoneal layers. The peritoneum, which passes from the hepatic porta to the lesser curvature of the stomach forms tow ligamenta; the hepatogastric ligament (lig. hepatogastricum), and to the part of the duodenum nearest to the stomach as the hepatoduodenal ligament (lig. hepatoduodenale). The hepatoduodenal and hepatogastric ligaments form together the lesser omentum (omentum minus). On the lesser curvature of the stomach both layers of the lesser omentum separate, one covers the anterior and the other the posterior surface of the stomach.

THE SMALL INTESTINE

The small intestine (intestinum tenue) (Gk enteron, hence enteritis, inflammation of the intestinal mucosa) begins at the pylorus, makes a series of looped curves, and ends at the beginning of the large intestine. The small intestine is about 7 m long in male cadavers and about 6.5 m in female cadavers; its length exceeds the length of the body 4.3 times. It is always longer in cadavers than in live subjects due to postmortem relaxation of the muscles. Mechanical (movement) and further chemical treatment of food under conditions of an alkaline reaction occurs in the small intestine, as well as absorption of the nutrients. Accordingly, special adjustments for the secretion of digestive juices (glands situated both in and outside the intestinal wall) and for the absorption of the digested substances, are present in it. Three parts are distinguished in the small intestine: (1) the duodenum, the part nearest to the stomach, 25-30 cm long; (2) the jejunum, which accounts for two fifth of the small intestine with the exception of the duodenum; and (3) the ileum, composing the remaining three fifth. There is no clearly manifest anatomical boundary between the jejunum and ileum and their separation is therefore relative. The duodenum runs around the head of the pancreas like a horseshoe. Four main parts are distinguished in it: (1) the superior part (pars superior) passes at the level of the first lumbar vertebra to the right and posteriorly and, forming a descending bend, the superior flexure of the duodenum (flexura duodeni superior) is continuous with (2) the descending part of the duodenum (pars descendens), which passes downward on the right of the spine to the level of the third lumbar vertebra where a second bend, the inferior flexure of duodenum (flexure duodeni inferior), is formed, the intestine passing to the left and forming (3) the horizontal (inferior) part (pars horizontalis), which stretches horizontally in front of the vena cava inferior and the aorta and (4) the ascending part (pars ascendens), which rises to the level of the first or second vertebra on the left and in front. At the junction of the ascending part of the duodenum with the jejunum on the left side of the first, or most frequently, the second lumbar vertebra, the intestinal tube bends sharply to form the duodeno-jejunal flexure (flexura duodenojejunalis) with the initial part of the jejunum passing downward, anteriorly, and to the left. This flexure is fixed in position by the peritoneum and a band of unstriated muscle fibres, the suspensory muscle of the duodenum (m. suspensorius duodeni). The jejunum and ileum. The jejunum and ileum are embraced under the common name intestinum tenue mesenteriale because, in distinction from the duodenum, this segment is fully invested in the peritoneum and attached to the posterior abdominal wall by the mesentery. Though there is no manifest boundary between the jejunum (in a cadaver this part is usually found to be empty, hence the name, L jejunum empty) and the ileum, as it is pointed out above, the typical segments of both parts (the superior segment of the jejunum and the inferior segment of the ileum) are clearly distinct: the jejunum is larger in diameter, its walls are thicker, and it is richer in vessels (the difference on the part of the mucous coat is pointed out below). The loops of the mesenteric part of the small intestine occupy the mesogastrium and hypogastrium for the most part, those of the jejunum are mostly to the left of the midline, while the loops of the ileum lie mostly to the right of the midline. Structure. The mucous coat (tunica mucosa) of the small intestine has a lustreless, velvety appearance due to the numerous intestinal villi (villi intestinalis) covering it. The villi are projections of the mucous membrane about 1 mm in length. The villi are concerned with the absorption of nutrients. The number of villi is greater in the jejunum, where they are thinner and longer. The large molecules of the nutrients cannot enter these submicroscopic villi and undergo routine digestion in the cavity of the intestine. The small molecules, however, penetrate these villi and disintegrate while the products of this disintegration are immediately absorbed. The absorbing surface of the mucous membrane of the small intestine is considerably enlarged due to the presence of the transverse, or circular folds (plicae circulares). These folds are formed only by the mucous coat and the submucous layer (the muscular coat does not take part in their formation) and are permanent structures, which do not disappear even when the intestinal tube is distended. The circular folds differ in character in the different parts of the small intestine; they are absent in the initial part of the duodenum close to the pylorus, in the rest of the duodenum and in the upper part of the jejunum they are high and placed closely to one another, while more distally they become lower and are set less closely and disappear completely in the end of the ileum. Besides the circular folds, the mucous membrane of the duodenum has longitudinal duplications in its initial part (in the region of the bulb) and a longitudinal fold (plica longitudinalis duodeni) on the medial wall of the descending part; this fold has the appearance of an elevation, which terminates as a papilla, the greater duodenal papilla (papilla duodeni major). The opening of the conjoined common bile duct and the pancreatic duct is on the papilla. Proximal to the greater duodenal papilla is a smaller duodenal papilla (papilla duodeni minor) (the accessory pancreatic duct opens on it). The small intestine has a lymphatic apparatus, which renders harmful substances and micro- organisms harmless. It consists of solitary lymphatic nodules (folliculi lymphatici solitarii) and their aggregations called Peyer's patches (folliculi lymphatici aggregati [Peyeri]). The solitary lymphatic nodules are scattered throughout the small intestine as whitish elevations the size of a millet. Peyer's patches are only found in the ileum. These are flat elongated patches whose longitudinal diameter coincides with the longitudinal axis of the intestine. Peyer's patches are arranged along the antimesenteric border of the ileum. The muscular coat (tunica muscularis), in accordance with the tubular shape of the small intestine, is composed of two layers of smooth muscles, an outer, longitudinal, and an inner, circular layer, which is developed better than the outer layer. The muscular coat is thinner in the distal segments of the intestine. An opinion exists to the effect that the circular layer of muscles contains also spiral muscle fibres forming a continuous layer of spiral musculature here and there. The contractions of the muscle fibres are peristaltic in character and spread successively toward the distal end of the intestine; the circular fibres constrict the lumen, while the longitudinal fibres shorten and so contribute to its distension (distal to the contracted ring of fibres). The spiral fibres promote movement of the peristaltic wave along the axis of the intestinal tube. Contractions in the opposite direction are called antiperistaltic. The serous coat (tunica serosa) invests the small intestine except for Ia narrow strip on the posterior surface between the two layers of the mesentery where nerves and blood and lymphatic vessels approach the intestine.

THE LARGE INTESTINE

The large intestine (intestinum crassum) extends from the end of the small intestine to the anus and is divided into the following parts: (1) caecum, the blind gut, with the vermiform process (appendix vermiformis); (2) ascending colon (colon ascendens); (3) transverse colon (colon transversum); (4) descending colon (colon descendens); (3) sigmoid (pelvic) colon (colon sigmoideum), and (6) rectum. The total length of the large intestine varies from 1.0 to 1.5 m. Its width reaches 7 cm in the region of the caecum and then reduces gradually to about 4 cm in the descending colon. The large intestine is distinguished from the small intestine not only by a much larger diameter but also by its appearance characterized by the presence of: (1) specific longitudinal muscle strips, or bands, teniae coli; (2) characteristic sacculations, haustra coli, and (3) fat-containing protrusions of the serous coat, appendices epiploicae. Teniae coli, three in number, begin at the base of the vermiform process and extend to the beginning of the rectum at approximately equal distances from each other. (To find the vermnorm process in operation for appendicitis it is therefore necessary to find on the caecum the site where all the three bands as if come together.) Teniae substitute for the longitudinal muscular layer of the colon which does not form a single layer here but separates into three bands: (1) tenia libera, (2) tenia mesocolica ,(3) tenia omentalis. Haustra coli are seen from the inside as sacculations; from the outside they appear as protrusions between the bands. They promote the treatment of undigested food remnants. Haustra disappear if the teniae are removed because they are produced by the adaptation of the length of the teniae to the somewhat (by one sixth) longer length of the colon itself. Appendices epiploicae are protrusions of the serous coat 4-5 cm in length found along the teniae libera and omentalis; in subjects who are not undernourished they contain fatty tissue. Haustra coli, teniae coli, and appendices epiploicae serve as guiding points for distinguishing the large intestine from the small intestine during operation. Since the process of absorption is diminished in the large intestine (water is absorbed for the most part), its mucous membrane is devoid of villi and is, therefore, distinguished from the mucosa of the small intestine by a smooth surface. The circular folds in the large intestine are broken up to form separate crescent folds called the semilunar folds of the colon (plicae semilunares coli). They are formed not only by the mucous coat but also by all the other coats of the wall. The semilunar folds are functional adjustments dependent on the activity of the intestinal nervous and muscular systems. Only intestinal glands and solitary follicles but no Peyer's patches are present in the mucous membrane. The muscle coat is composed of two layers, an outer longitudinal and an inner circular layer. Only the inner circular layer is continuous; it is a constricting layer, which thickens when the hard faecal material has to be pushed distally. In contrast, the dilating longitudinal musculature (continuous in the small intestine) in the large intestine separates to form the three teniae described above because the pressure of the faecal material itself makes dilation of the lumen easier. The serous coat covers some parts of the large intestine completely and others partly (see below). Caecum (Gk typhlon, hence typhlitis, inflammation of the caecum) is the first segment of the large intestine from its origin to its junction with the small intestine. The caecum has the appearance of a sac with a vertical dimension of about 6 cm and a transverse dimension of 7.0-7.5 cm. It lies in the right iliac fossa directly above the lateral half of the inguinal ligament. The vermiform process (appendix vermiformis) arises from the medioposterior surface of the caecum, 2.5-3.5 cm below the iliocaecal junction. It varies greatly in length and position; its length is about 8.6 cm on the average, but in 2 per cent of cases it may be reduced to 3 cm. The vermiform process may be absent completely, but this is a rare occurrence. The mucous membrane of the vermiform process is relatively rich in lymphoid tissue in the form of folliculi lymphatici aggregati appendicis vermitormis; some authors relate this to its functional importance (the "intestinal tonsil" that retains and destroys pathogenic micro-organisms, which explains the high appendicitis incidence). The walls of the process consist of the same coats as those of the intestinal wall. The caecum and vermiform process are completely invested by the peritoneum. At the junction of the small and large intestines is the ileocaecal valve (valva ileocecalis). It is composed of two crescent folds at the base of which is a layer of circular muscles, the ileocaecal sphincter (sphincter ileocecalis). The ileocaecal valve and sphincter together form an adjustment regulating the passage of food from the small intestine, in which the reaction is alka line, into the large intestine, where the medium is again acid; they also prevent the backward flow of the contents and the neutralization of the chemical medium. The surface of the ileocaecal valve facing the small intestine is covered with villi, whereas the other surface has the features of the mucous membrane of the large intestine and bears no villi. The ascending colon (colon ascendens) is a direct continuation of the caecum and the boundary between them is the site where the small intestine empties into the colon (the ileocaecal valve in the lumen). From this junction it passes upward and slightly to the back and, on reaching the inferior surface of the liver, makes a bend to the left and forward to form the right, or hepatic flexure of the colon (flexura coli dextra, s. flexura hepatica coli) and is then continuous with the transverse colon. The transverse colon (colon transversum), the longest part of the colon (its length ranges from 25 to 30 cm, while that of the ascending part is about 12 cm and of the descending part approximately 10 cm), extends from the right flexure to the left flexure of the colon at the lower end of the spleen, where it is continuous with the descending colon. Between the two bends the transverse colon does not stretch strictly transversely but forms a shallow concave arch shifted slightly to the front; the left flexure is found at a higher level than the right flexure. The transverse colon is covered in front for a great length by the greater omentum. The descending colon (colon descendens) passes downward in the left side of the abdominal cavity from the left flexure of the colon in the left side of the abdominal cavity from the left flexure of the colon in the left hypochondrium and on the level of the iliac crest is continuous with the sigmoid (pelvic) colon. The sigmoid (pelvic) colon (colon sigmoideum) is a continuation of the descending colon and stretches to the beginning of the rectum. The shape and size of its loop is marked by considerable individual variations, depending on its length and the degree of filling. An empty sigmoid (pelvic) colon of average size is usually located for the most part in the cavity of the true pelvis and reaches the right wall or this cavity; here it curves, descends to the left, and is then continuous with the rectum. The relations of the parts of the colon to the peritoneum are as follows: the ascending colon in most cases is covered by the peritoneum in front and on the sides(mesoperitoneal) while its posterior surface is devoid of a serous coat. The transverse colon is completely invested by the peritoneum and has a long mesentery as a consequence of which it possesses considerable mobility(intraperitoneal). The relations of the descending colon to the peritoneum are approximately the same as those of the ascending colon. The sigmoid colon is covered by the peritoneum on all sides(intraperitoneal) and has a well developed mesentery and is, therefore easily mobile and forms an S-shaped curve, hence its name. The rectum is the last part of the large intestine and serves for the accumulation and evacuation of the faecal material. It begins at the level of the promontory and descends into the true pelvis in front of the sacrum to form two anteroposterior flexures: an upper sacral flexure (flexura sacralis) convex to the back in conformity with the sacral concavity and a lower perineal flexure (flexura perinealis) convex to the front in the region of the coccyx. As a result the rectum is S-shaped, wide in the middle and narrow at the ends. The upper part of the rectum corresponding to the sacral flexure is in the pelvic cavity and is called the pelvic part (pars pelvina): it widens in the direction of the perineal flexure to form the rectal ampulla (ampulla recti).. The terminal part of the rectum passing to the back and downward is called the anal part of the rectum, or the anal canal (pars analis recti s. canalis analis). It terminates as an orifice, the anus. Three parts are distinguished in the rectum according to its peritoneal relations: an upper part, which is covered with the peritoneum intraperitoneally and has a short mesentery, mesorectum; a middle part situated mesoperitoneally; and a lower part found extra peritoneally. The rectal wall is composed of three coats: mucous, submucous, and muscular. Due to a developed layer of submucosa, the mucous coat (tunica mucosa) gathers to form numerous longitudinal folds when the rectum is empty; these folds are easily smoothed out when the rectal walls are stretched. Only the most distal part of the rectum has eight to ten permanent longitudinal folds known as the anal columns (columnae anales). The depressions between them are called anal sinuses (sinus anales), which are particularly manifest in children. In addition to the longitudinal folds, transverse mucosal folds, horizontal folds of the rectum (plicae transversales recti) are present in the upper parts of the rectum. The muscular coat (tunica muscularis) consists of two layers; an inner circular and an outer longitudinal. The inner layer increases in thickness in the upper part of the perineal part to 5-6 cm and forms here the inner sphincter, the sphincter ani internus muscle (m. sphincter ani internus) 2-3 cm in height and terminating at the junction of the anal canal with the skin. (Directly under the skin is a ring of striated muscle fibres, the sphincter ani externus muscle [m. sphincter ani externus] which is among the muscles of the perineum.)

THE LARGE GLANDS OF THE DIGESTIVE SYSTEM

THE LIVER

The liver (hepar) is a large glandular organ weighing about 1500 g. The functions of the liver are diverse. It is primarily a large digestive gland secreting bile, which flows along the efferent duct into the duodenum. The liver is situated directly under the diaphragm in the right upper part of the abdominal cavity and only a relatively small portion of it protrudes to the left of the midline in an adult. Two surfaces and two borders are distinguished in the liver. The upper surface, or to be more precise, the anterosuperior surface (facies diaphragmatica) is convex in correspondence to the concavity of the diaphragm with which it is in contact. The lower surface (facies visceralis) faces downward and to the back and bears some depressions produced by the abdominal viscera with which it comes in contact. The upper and lower surfaces are separated by a sharp lower border (margo interior). The other border of the liver, the superoposterior border, is in contrast so blunt that it can be regarded as the posterior surface of the organ. Two lobes are distinguished in the liver, the right (lobus hepatis dexter) and the left (lobus hepatis sinister) lobes, which are separated on the diaphragmatic surface by the falciform ligament (ligamentum falciforme hepatis). In the free edge of the falciform ligament is a hard fibrous cord, the round ligament of the liver (ligamentum teres hepatis) that extends from the umbilicus and is the obliterated remains of the umbilical vein. The ligamentum teres curves around the inferior border of the liver, forms a notch here (incisura ligamenti teretis), and then fits on tpe visceral surface into the left longitudinal fissure, which is the boundary between the right and left lobes of the liver on this surface. The ligamentum teres occupies the anterior part of this fissure (sulcus venae umbilicalis); the posterior part contains the continuation of the ligamentum teres, a thin fibrous cord of the obliterated ductus venosus which functioned in the embryonal period; this part of the groove is known as fossa ductus venosi. The right hepatic lobe is separated on the visceral surface into two secondary lobes by two grooves, or depressions. One stretches parallel to the left longitudinal fissure and its anterior portion, in which the gall bladder (vesica fellea) is lodged, and is called the fossa for the gall bladder (fossa vesicae felleae); the posterior, deeper part of the groove contains the inferior vena cava and is called the groove for the vena cava (sulcus venae cavae). A deep transverse fissure connecting the posterior ends of the sulcus venae umbilicalis and fossa for the gall bladder is called the porta hepatis. Through the porta hepatis the hepatic artery, portal vein, and the attendant nerves enter the liver, while the lymphatic vessels and the bile carrying common hepatic duct ((ductus hepaticus communis), which is formed by the union of the right and left ducts) leave it. Part of the right lobe of the liver bounded posteriorly by the porta hepatis and laterally by grooves (the fossa of the gall bladder on the right and the sulcus of the vena cava on the left) is called the quadrate lobe (lobus guadratus). The area to the back of the porta hepatis and between the fossa ductus venosi (on the left) and the sulcus of the vena cava (on the right) constitutes the caudate lobe (lobus caudatus). The organs, which come in contact with the liver, form impressions on its surface, which are named according to the contacting organ. The liver is covered by the peritoneum for the most part except for an area on its posterior surface where it is in direct contact with the diaphragm. The gall bladder (vesica fellea s. biliaris, or cystic fellea) is pear-shaped. Its wide end extending slightly beyond the inferior border of the liver is called the fundus (fundus vesicae felleae). The opposite narrow end of the gall bladder is the neck (collum vesicae felleae); the middle part forms the body (corpus vesicae felleae). The neck is directly continuous witlt the cystic duct (ductus cysticus), which is about 3.5 cm in length. The cystic duct and the common hepatic duct join to form the common bile duct (ductus choledochus), the bile receptacle (Gk dochos receptacle). This duct is lodged between the two layers of the hepatoduodenalligament, with the portal vein and the common hepatic artery, pierces the medial wall of the descending part of the duodenum, and drains, together with the duct of the pancreas, by means of an orifice into a dilatation inside the greater duodenal papilla, called the ampulla of the bile duct (ampulla hepatopancreatica). The ductus choledochus is about 7 cm in length. The muscular coat (tunica muscularis) found under the serous coat consists of smooth fibres and an admixture of fibrous tissue. The mucous coat forms folds and contains many mucous glands. In the neck and in the cystic duct are folds arranged spirally and forming the spiral valve (plica spiralis).

THE PANCREAS

The pancreas is situated behind the stomach on the posterior abdominal wall in the epigastrium while its left part extends also to the left hypochondrium. Posteriorly it adjoins the vena cava inferior, the left renal vein, and the aorta. The pancreas has a head (caput pancreatis) bearing the uncinate process (processus uncinatus), a body (corpus pancreatis), and a tail (cauda pancreatis). The head of the gland is embraced by the duodenum and is situated on the level of the first and upper part of the second lumbar vertebra. At its junction with the body is the pancreatic notch (incisura pancreatis), lodging the superior mesenteric artery and vein, and sometimes a constriction in the form of a neck. The body is prismatic in shape and has three surfaces, anterior, posterior, and inferior. The anterior surface (facies anterior) is concave and comes in contact with the stomach. The posterior surface (facies posterior) is directed to the posterior abdominal wall. The inferior surface (facies inferior) faces downward and slightly to the front. The three surfaces are separated from one another by three edges, margo superior, anterior, and inferior. The gland is slightly raised from right to left as a cons equence of which the tail is situated higher than the head and reaches the inferior part of the spleen. The peritoneum covers the anterior and inferior surfaces of the pancreas, while the posterior surface is devoid of it. The pancreatic duct (ductus pancreaticus) receives numerous branches draining into it almost at a right angle. It joins the ductus choledochus and both open by means of a common orifice on the greater duodenal papilla. In additional to the main duct, there is usually an accessory pancreatic duct (ductus pancreaticus accessorius), which opens on the smaller duodenal papilla (about 2 cm above the greater duodenal papilla). Structure. According to structure, the pancreas is related to the group of acinar or acinar-tubular glands. Two components are distinguished in it: the main bulk of the gland is concerned with external secretion and excretes its secretions into the duodenum by way of the ducts; the smaller part of the gland consists of the islets of Langerhans (insulae pancreaticae) and is an endocrine structure secreting insulin (L insula island) into the blood; insulin regulates the blood sugar content. There are indications that the pancreas contributes to haemopoiesis and regulation of blood pressure.

THE URINARY ORGANS

The urinary organs (organa uropoietica) are, firstly, two glands (the kidneys whose excretion is the urine) and, secondly, organs concerned with the storage and excretion of the urine (the ureters, urinary bladder, and urethra). THE KIDNEY

The kidney (ren, Gk nephros) is a paired excretory organ producing the urine. The kidneys are situated on the posterior abdominal wall behind the peritoneum on either side of the vertebral column on the level of the last thoracic and upper two lumbar vertebrae. The right kidney is a little lower than the left, by 1.0-1.5 cm on the average (depending on the pressure exerted by the right lobe of the liver). The kidney is bean-shaped, its surface is smooth and dark red. In the kidney are distinguished the upper and lower inferior ends (poles) or extremities (extremitas superior and extremitas inferior), the lateral and the medial margins (margo lateral is and margo medialis), and the anterior and the posterior surfaces (facies anterior and facies posterior). The lateral margin is convex, while the medial margin has a concavity. The middle concave part of the medial margin contains the hilum of the kidney (hilus renalis) through which arteries and nerves enter the kidney and veins and the urethra leave it. The hilum opens into a narrow hollow extending into the renal substance; it is called the sinus of the kidney (sinus renalis). The capsules of the kidney. The kidney is invested in its own fibrous capsule (capsula fibrosa), a fine smooth lamina intimately attached to the renal substance. Normally it can be separated from the renal substance quite easily. The fibrous capsule consists of three layers: (1) an outer fibrous layer; (2) a middle muscular layer (an incomplete layer of smooth muscle fibres), and (3) an inner supraparenchymatous layer. Due to such structure the fibrous capsule accomplishes its functions: mechanical (fixation of the kidney and protection of its parenchyma) and contractile, facilitating filtration. Outside this fibrous capsule, particularly in the region of the hilum and on the posterior surface, is a layer of loose fatty tissue forming the fatty capsule (renal fat) (capsula adiposa) of the kidney; the anterior surface is quite often not covered by fat. Outside, each kidney is invested in a connective-tissue fascia (fascia renalis), which separates into two layers, one covering the anterior and the other the posterior surface. On the medial margin of the kidney, the layers do not unite but pass separately to the midline: the anterior layer passes in front of the renal vessels, aorta, and vena cava inferior to unite with its fellow layer of the contralateral side; the posterior layer passes in front of the vertebral bodies and is attached to them. A complex of the following structures is responsible for fixation of the kidney in place: (1) fascia renalis, which fuses with the renal capsules; (2) the muscular seat of the kidney formed by the psoas major and quadratus lumborum muscles; (3) the renal vessels, which prevent the kidney from moving away from the aorta and vena cava inferior, and, finally, (4) intra-abdominal pressure produced by contraction of the muscles of the abdominal wall. If this fixation apparatus is weak, the kidney may descend (mobile, or wandering kidney), which calls for its operative fixation with sutures. Normally, the long axes of both kidneys, being directed upward and medially, meet above the kidneys at an angle open downward. In nephroptosis, the kidneys being held in place on the midline by the vessels are displaced downward and medially, like on the periphery of a circle. As a result, the long axes of the kidneys intersect below them, forming an angle open upward. Structure. On a longitudinal section through the kidney it can be seen that it is composed of a cavity, the renal sinus, containing the calyces and the upper part of the renal pelvis, and of the renal substance proper adjoining the sinus on all sides except for the hilus. The cortex (cortex renis) and the medulla (medulla renis) are distinguished in the kidney. The cortex occupies the peripheral layer of the organ and is about 4 mm thick. The medulla is formed of conical structures called the renal pyramids (pyramides renales), or pyramids of Malpighi. The wide bases of the pyramids face the surface of the organ, the apices are directed toward the sinus. Two or more apices join to form rounded eminences termed the renal papillae (papillae renales); less frequently a single apex constitutes one separate papilla. There are a total of about 12 papillae on the average. Each papilla is dotted with small openings, papillary foramina (foramina papillaria), through which urine is discharged into the initial parts of the urinary tract (the calyces). The cortex penetrates between the pyramids and separates one from another; these parts of the cortex are known as the renal columns (columnae renales). The pyramids appear striated because the urinary tubules and the vessels are arranged in them in a straight direction. In an adult, the kidney is smooth on the outside but inside it remains separated into lobules, pyramids, though several pyramids fuse to form a single papilla (which explains why the number of papillae is less than the number of pyramids). The striae of the medulla continue also in the cortex but are less distinct here; they form the radiate part (pars radiata) of the cortex, whereas the part between them is the convoluted part (pars convoluta) (L convolvere to roll together). The kidney is a complex excretory organ. It contains tubules called urinary, or renal tubules (tubuli renales). The blind ends of these tubules surround, as double-walled capsules, tufts of capillary blood vessels. Each tuft, or glomerulus, lies in a deep bowl-like hollow of the capsule known as the glomerular capsule (capsula glomeruli), or the Bowman capsule; the space between the two layers of the capsule is its cavity and the beginning of the renal tubule. The glomerulus together with the capsule enclosing it forms the renal corpuscle (corpusculum renis), or the Malpighian corpuscle. The renal corpuscles are in the convoluted part of the cortex and can be seen in it with the naked eye as red dots. The renal corpuscle (the Bowman capsule, to be more precise) gives rise to the convoluted tubule (tubulus renalis contortus), which passes in the radiate part of the cortex. The tubule then descends into the pyramid, bends upon itself to form Henle's loop (ansa) (ansa nephroni) and returns into the cortex. The terminal segment of the renal tubule, the junctional segment, drains into the collecting tubule (tubulus renalis colligens), which receives several tubules and passes in a straight direction (tubulus renalis rectus) through the radiate part of the cortex and the pyramid. The straight tubules gradually fuse with one another to form 15 to 20 short papillary ductules (ductus papillares) opening by means of the papillary foramina in the cribriform area on the apex or the papilla. The structural unit or the kidney is the nephron. According to the findings or microscopic anatomy, it consists of the following parts. 1. Thu corpuscle of the kidney (corpusculum renis). 2. The renal corpuscle gives rise to the proximal segment consisting of the proximal convoluted tubule (the beginning of the segment), which is continuous with the proximal straight tubule. 3. The proximal straight segment is continuous with a thin loop, Henle's loop, which has two limbs, descending and ascending. 4. The ascending limb of Henle's loop is continuous with the distal segment consisting of a thick ascending limb in turn continuous with the distal convoluted tubule and then with the junctional tubule. The junctional tubule drains into the collecting tubule. These structural components of the nephron are arranged in the kidney parenchyma in the following order. The renal corpuscle, the convoluted and straight tubules of the proximal segment, the thick ascending limb of the nephron loop, and the distal convoluted and junctional tubules are situated in the cortex. The thin ascending and descending limbs of Henle's loop, the thick ascending limb of the nephron, and the collecting tubule are situated in the medulla. The nephron is concerned with the production of urine: ultrafiltration takes place in the glomeruli, reabsorption and secretion in the tubules.

THE RENAL PELVIS, CALYCES, AND URETER

Urine flowing from the papillary foramina passes on the way to the urinary bladder through the lesser calyces, the greater calyces, the renal pelvis, and the ureter. The lesser calyces (calyces renales minores), about eight or nine in number, enclose one, two, rarely three renal papillae with one end and drain into one of the major calyces with the other. There are usually two greater calyces (calyces renales majores): a superior calyx and an inferior calyx; they merge in the renal sinus to form a single renal pelvis (pelvis renalis) (Gk pyelos pelvis, hence pyelitis, inflammation of the renal pelvis), which leaves the kidney through the hilum behind the renal vessels and, curving downward, is continuous with the ureter directly below the renal hilus. The ureter is a tube about 30 cm long with a diameter of 4-7 mm. From the pelvis the ureter passes behind the peritoneum downward and medially into the true pelvis where it extends to the floor of the urinary bladder whose wall it pierces obliquely. In the ureter are distinguished an abdominal part (pars abdominalis), from its origin to the place where it curves over linea terminalis to enter the true pelvis, and a pelvic part (pars pelvina) located in the true pelvis. The ureteral lumen is not uniform in width and has constrictions: (1) near the site where the renal pelvis is continuous with the ureter; (2) at the junction of the abdominal and pelvic parts; (3) along the distance of the pelvic part, and (4) near the wall of the urinary bladder. Structure. The walls of the ureter, like the walls of the renal pelvis and the calyces, consist of three coats: an outer connective-tissue adventitious coat (tunica adventitia), an inner mucous coat (tunica mucosa) covered with transitional epithelium supplied with small mucous glands, and a middle, muscular coat (tunica muscularis). The muscular coat is formed of two layers (an inner longitudinal and an outer circular layers); these layers are not connected with the musculature of the bladder and prevent urine from flowing from the bladder back into the ureter. At the site where the ureter drains into the bladder is a third, outermost longitudinal muscular layer, which is intimately connected with the muscles of the bladder and contribute to expulsion of urine into the bladder.

THE URINARY BLADDER

The urinary bladder (vesica urinaria) is a receptacle for urine, which is excreted at regular intervals through the urethra. Its capacity is 500-700 ml on the average, but may vary considerably in individual cases. The shape of the bladder and its relations to the adjoining organs greatly change depending on the extent of its filling. An empty bladder lies completely in the cavity of the true pelvis behind the pubic symphysis; posteriorly it is separated from the rectum by the seminal vesicles and the terminal parts of ductus deferens in males and by the vagina and uterus in females. When the bladder is filled with urine, its upper part changes in shape and size and rises above the pubis and, if greatly distended, reaches to the level of the umbilicus. A filled bladder is egg-shaped and the lower, wider and attached part, the base (fundus vesicae) is directed at the rectum or vagina; gradually narrowing, it is continuous with the urethra by means of a neck (cervix vesicae); the sharpened apex of the bladder (apex vesicae) is related to the inferior part of the anterior abdominal wall. The middle part between the apex and the base is known as the body of the bladder (corpus vesicae). The bladder has an anterior, posterior, and lateral surfaces. The anterior surface lies on the pubic symphysis, from which it is separated by loose areolar tissue filling the prevesical, or retropubic space (spatium prevesicale s. retropubicum). The superior surface of the bladder is related to the intestinal loops in males and to the anterior surface of the uterus in females. Posteriorly, the peritoneum is reflected from the superoposterior surface of the bladder onto the anterior surface of the rectum (to form the retrovesical pouch, excavatio retrovesicalis) in males and onto the anterior surface of the uterus (to form the uterovesical pouch, excavatio vesicouterina) in females. Besides the serous coat (tunica serosa) covering the posterior wall and apex of the bladder, the constituents of its wall are the muscular coat (tunica muscularis) formed of smooth muscle fibres, the submucous coat (tela submucosa) and the mucous coat (tunica mucosa). Three intertwining strata are distinguished in the muscular coat: (1) external (stratum externum) composed of longitudinal fibres; (2) middle (stratum medium) of circular or transverse fibres; (3) internal (stratum internum) formed of longitudinal and transverse fibres. The middle layer is developed most, especially in the region of the internal orifice of the urethra (ostium urethrae internum) where it forms the sphincter vesicae muscle (m. sphincter vesicae). Around each ureteral orifice is also a sphincter-like structure of strong circular fIbres of the internal muscular stratum. The inner surface of the bladder is lined with the mucous coat, which forms folds in an empty bladder due to the presence of a rather well developed submucous coat. These folds disappear when the bladder is distended. An opening is visible on the inner surface of the inferior part of the bladder, this is the internal orifice of the urethra. Directly behind it is a smooth triangular area called the trigone of the bladder (trigonum vesicae), its mucous coat adheres to the muscular coat and never forms folds. The apex of the trigone faces the internal urethral orifice; at the corners of the base of the trigone are the orifices of the ureters (ostia ureteres).

THE GENITAL ORGANS

Male genital (or reproductive) organs (organa genitalia masculina) and female genital organs (organa genitalia feminina) are distinguished.

THE MALE GENITAL ORGANS

The male genital, or reproductive organs are: the testes together with their coverings., the ductus deferens with the seminal vesicles, the prostata, Cowper's (bulbourethral) glands, and the penis formed of cavernous bodies. The male urethra, which has a mixed character of an urogenital tube, will also be described here.

THE TESTES

The testes are a pair of oval bodies, slightly flattened on the sides. They are situated in the scrotum and measure 4 cm in length and 3 cm in thickness. Their weight ranges from 15 to 25 g. Two surfaces, medial and lateral (facies medialis and facies lateralis), two borders, anterior and posterior (margo anterior and margo posterior), and two poles, upper and lower (extremitas superior and extremitas inferior), are distinguished in the testis. In normal position of the testis in the scrotum, its upper pole is directed upward, forward, and laterally as a consequence of which the lower pole is directed not only downward but also to the back and medially. The left testis usually descends to a lower level than the right testis. The posterior border of the testis is approached by the spermatic cord (funiculus spermaticus) and the epididymis; the epididymis is situated on the posterior border. The epididymis is a narrow, long body in which are distinguished an upper, rather thick part, the head (caput epididymidis) and a lower, more pointed part, the tail (cauda epididymidis); the intervening part is the body (corpus epididymidis). In the region of the body, between the anterior concave surface of the epididymis and the testis, is a pocket, sinus of the epididymis (sinus epididymidis) lined with a serous membrane and opened laterally. Structure of the testis. The testis is invested by a dense fibrous whitish coat called the tunica albuginea covering directly the substance, or parenchyma of the testis. On the posterior border the fibrous tissue of the coat penetrates the glandular tissue of the testis for a short distance as an incomplete vertical septum or thickening; this is the mediastinum testis from which fibrous septa (septula testis) radiate and are attached by their outer ends to the inner surface of tunica albuginea and thus divide the whole parenchyma into lobes (lobuli testis). The total number of lobes may reach 250-300. The apices of the lobes are directed at the mediastinum, the bases at the tunica albuginea. The epididymis is also covered by tunica albuginea which, however, is thinner. The parenchyma of the testis is composed of seminiferous tubules, in which two parts are distinguished: the convoluted seminiferous tubules (tubuli seminiferi contorti) and the straight seminiferous tubules (tubuli seminiferi recti). Two-three and more tubules are contained in one lobule. In the lobe they are convoluted but approaching the mediastinum they unite and become narrower to form short straight seminiferous tubules directly at the mediastinum. The straight seminiferous tubules open into a network of channels, rete testis, in the thickness of the mediastinum. The network gives origin to 12 to 15 minute tubules, the efferent ductules of the testis (ductuli efferentes testis), passing to the head of the epididymis. On leaving the testis the efferent ductules become convoluted and form a series of conical lobules of the epididymis (lobuli s. coni epididymidis). The efferent ductules drain into a single canal of the epididymis (ductus epididymidis), which follows a very tortuous course and is continuous with the ductus deferens. A straightened out duct of the epididymis is 3 to 4 m long. The efferent ductules, the lobules of the epididymis, and the initial portion of the canal of the epididymis form together the head of the epididymis. Ductuli aberrantes are encountered on the epididymis. The secretion of the male reproductive organs, the semen, is produced only in the convoluted seminiferous tubules. The straight seminiferous tubules and the channels of the rete testis belong to the deferent tract. Only a very small amount of the fluid component of the semen is produced in the testes (though it is exactly from this standpoint that they can be regarded as glands of external secretion). It is for the most part the secretion of the accessory glands of the reproductive apparatus, which open into the deferent tract.

DUCTUS DEFERENS

The ductus deferens (vas deferens) is a direct continuation of the duct of the epididymis but its walls are much thicker. It is separated from the testis by vessels (the testicular artery and vein), ascends, and becomes a component of the spermatic cord in which it is situated behind the vessels and is easily palpated because of its firm walls. As a component of the spermatic cord it ascends vertically to the external (superficial) inguinal ring. Having passed in the inguinal canal obliquely upward and laterally, it leaves the testicular vessels (which pass into the lumbar region) and extends downward and to the back on the lateral pelvic wall where it is covered by peritoneum. On reaching the urinary bladder it curves toward its base and reaches the prostata. The distal part of the ductus is conspicuously dilated, and this part is termed the ampulla of the vas deferens (ampulla ductus deferentis). The length of the ductus varies from 40 to 45 cm, the diameter is 2.5 mm on the average, and the width of its lumen measures only 0.2-0.5 mm. The wall of the ductus is composed of three layers: an outer adventitious coat of fibrous tissue (tunica adventitia), a middle muscular coat (tunica muscularis) and an inner mucous coat (tunica mucosa).

THE SEMINAL VESICLES

The seminal vesicles (vesiculae seminales) are situated lateral of both ductus deferentes between the base of the bladder and the rectum. Each vesicle is about 5 cm in length and the greatest width is about 2.5-3.0 cm. Only the upper, wider and rounded part of the vesicle is covered with peritoneum. Each seminal vesicle is actually a greatly coiled tube whose length when stretched out reaches 12 cm. The lower. pointed end of the vesicle is continuous with a narrow duct of seminal vesicle (ductus excretorius), which unites with ductus deferens of the same side at a right angle to form a common ejaculatory duct (ductus ejaculatorius). The ejaculatory duct is a very slender tube, which, arising from the site of union of the ductus deferens and the excretory duct of the seminal vesicle, passes through the thickness of the prostate gland and drains into the prostatic part of the urethra by a narrow opening at the base of the seminal colliculus. The ejaculatory duct is about 2 cm in length. The wall of the seminal vesicle is formed of coats similar to those of the ductus deferens. The seminal vesicles do not contain stores of semen; they are secretory organs, which mix the fluid they produce with the secretions of the testes.

THE SPERMATIC CORD AND COATS OF THE TESTIS

The testes are situated in the scrotum as if suspended there on the spermatic cords. The spermatic cord (funiculus spermaticus) consists of the ductus deferens, the testicular arteries and veins and the arteries and veins of the vas deferens, lymphatic vessels, and nerves. At the deep ring of the inguinal canal these components separate and the spermatic cord, therefore, extends as a whole structure only between the posterior margin of the testis and the deep inguinal ring.

THE MALE URETHRA

The male urethra (urethra masculina) is a tube approximately 18 cm in length extending from the bladder to the external urethral orifice (ostium urethrae externum) on the glans penis. The urethra serves not only for the passage of urine but also for the passage of the semen entering it through the ejaculatory duct. The urethra passes through different structures according to which three parts are distinguished in it: prostatic, membranous, and spongy. 1. The prostatic part (pars prostatica) is nearest to the bladder and passes through the prostata. It is about 2.5 cm in length. This part, its middle portion in particular, is the widest and most distensible segment of the urethra. On its posterior wall it bears a small median eminence, the seminal colliculus (colliculus seminalis).On the apex of the colliculus is a small, slit-like opening leading into a small blind pouch in the substance of the prostata; it is called the prostatic utricle (utriculus prostaticus), or the male uterus (uterus masculinus). The last name indicates that this structure is derived from the fused lower ends of the paramesonephric (Mueller's) ducts from which the female uterus and vagina develop. On either side of the entry into the prostatic utricle are small openings of the ejaculatory ducts (one on the right and one on the left side). On both sides, lateral of the seminal colliculus, are numerous openings of the prostatic glandules. On the circumference of the prostatic part of the urethra is a ring of smooth muscle fibres that compose part of the prostatic muscular tissue (substantia muscularis) and reinforce the sphincter of the bladder (sphincter vesicae) (smooth muscle, involuntary). 2. The membranous part (pars membranacea) is a segment of the urethra between the apex of the prostata and the bulb of the penis; it is about 1 cm in length. It is, therefore, the shortest and at the same time the narrowest of the three parts. It passes through the urogenital diaphragm. The membranous part is surrounded by striated muscle bundles, the voluntary sphincter urethrae muscle. 3. The spongy part (pars spongiosa), about 15 cm in length, is surrounded by the tissue of the corpus spongiosum penis. The canal is somewhat dilated in the bulbus, then for the distance to the glans it is uniform in calibre, but in the glans it is again dilated for about 1 cm to form the fossa terminalis (fossa navicularis urethrae).

THE BULBOURETHRAL GLANDS

The bulbourethral (Cowper's) glands (glandulae bulbourethrales s. Cowperi) are a pair of bodies the size of a pea lodged in the thickness of the urogenital diaphragm above the posterior end of the bulb of the penis and behind the membranous part of the urethra. The duct, about 3-4 cm in length, opens into the spongy part of the urethra in the region of the bulbus. The bulbourethral glands produce a viscous secretion, which protects the urethral walls from irritation by the urine. In old age, the glands become smaller. According to the latest data, they take part in internal secretion.

THE PROSTATE

The prostate (prostata) (Gk prostates to put in front) is, for a smaller part, a glandular and, for a greater part, a muscular organ that embraces the initial part of the male urethra. As a gland it secretes an important component of the semen, which stimulates the spermatozoa and, therefore, develops by the time of puberty. As a muscle, it is an involuntary sphincter of the urethra that prevents, in particular, the flow of urine during ejaculation. As a result the urine and the semen do not mix. Before puberty the prostate is exclusively a muscular organ but by the time of puberty (17 years of age) it becomes a gland. The prostate resembles a chestnut in shape and size. A base (basis prostatae) directed toward the bladder and an apex adjoining the urogenital diaphragm are distinguished in it. The anterior convex surface of the gland (facies anterior) faces the pubis symphysis.The posterior surface (facies posterior) is related to the rectum, that is why it can be palpated in a living subject on the anterior wall of the rectum with a finger introduced into it. The urethra passes through the prostate from the base to the apex of the gland on the midline, nearer to the anterior than to the posterior surface. The ejaculatory ducts enter the gland through the posterior surface, run in the tissue downward, medially, and foreward, and open into the prostatic part of the urethra. The wedge- shaped portion of the gland between both ejaculatory ducts and the posterior surface of the urethra is the median lobe, or isthmus of the prostate (lobus medius s. isthmus prostatae}. The remaining greater part is formed by the right and left lobes (lobi dexter and sinister), which are, however, not demarcated from one another superficially. The median lobe is of considerable surgical interest because it is enlarged in hypertrophy of the prostate and may cause serious disorders of urination. The prostate is surrounded by fascial layers, portions of the pelvic fascia, which form a sheath. Inner to the fascial coat is the prostatic capsule (capsula prostatica) formed of smooth muscular and connective tissue. The tissue of the prostate is composed of glands, glandular substance of the prostate (substantia glandularis) buried mainly in muscular tissue; its lobules are formed of fine, slightly branched tubules draining into the prostatic ducts (ductuli prostatici), about 20 to 30 in number, which open on the posterior wall of the prostatic part of the urethra on the sides of the seminal colliculus. The part of the prostate in front of the urethra passing through it consists almost exclusively of muscular tissue. Semen-conveying ducts indicated in the successive order straight seminiferous tubules, rete testis, efferent ductules of the testis, duct of the epididymis, ductus deferens, ejaculatory duct, prostatic part and the other parts of the urethra.

THE FEMALE GENITAL ORGANS

The female genital organs (organa genitalia feminina) are as follows; (1) the internal genital organs situated in the pelvis, i.e. the ovaries, oviducts (uterine tubes), uterus, and vagina, and (2) organs visible from the exterior (pudendum femininum s. vulva), i.e. the labia majora and minora, the clitoris, and the hymen.

THE OVARY

The ovary (ovarium), a paired organ is the female sex gland similar to the male testis. It is a flat oval body 2.5 cm in length, 1.5 cm in width, and 1.0 cm thick. The cortex (cortex) and the medulla (medulla ovarii) are distinguished in the ovary. Two ends are distinguished in it: the upper, rounded end directed at the uterine tube is called the tubal end (extremitas tubaria); the opposite, lower and more pointed end, the uterine end (extremitas uterina) is connected with the uterus by the ligament of the ovary (ligamentum ovarii proprium). The two surfaces, lateral and medial (facies loteralis and facies medialis), are separated one from the other by borders: the free posterior border (margo liber) is concave while the other, anterior mesovarian border (margo mesovaricus), is straight and is attached to the mesentery. This border is termed the hilum of the ovary (hilus ovarii) because vessels and nerves enter the ovary through it. The ovary has a short mesentery, called the mesovarium, which is a peritoneal fold by means of which its anterior border is attached to the posterior layer of the broad ligament of the uterus. To the tubal end of the ovary are attached the ovarian fimbria (fimbria ovarica), which is the largest of the fimbriae surrounding the abdominal end of the tube, and a triangular peritoneal fold, the suspensory ligament of the ovary (lig. suspensorium ovarii) descending to the ovary from the pelvic terminal line and containing the ovarian vessels and nerves. The ovary is filled with small vesicles called the vesicular ovarian (Graafian) follicles (folliculi ovarici vesiculosi s. Graafi). Each follicle contains a developing female sex cell, the oocyte (or egg-cell). The follicles are lodged in the stroma (stroma ovarii), in which the vessels and nerves pass. Depending on the developmental stage, the follicles vary in size, from microscopic to 6 mm in diameter. When the ripe follicle ruptures (ovulation) and the oocyte is discharged from it, its walls collapse and its cavity is filled with blood and yellowish cells. As a result the yellow body (corpus luteum) is formed. The oocyte develops into a mature cell, after ovulation, in the uterine tube. When pregnancy occurs, the corpus luteum increases in size and transforms into a large structure about 1 cm in diameter, corpus luteum verum s. graviditatis, whose traces may persist for years; if ovulation of the oocyte that has left the follicle does not occur, the corpus luteum atrophies, loses its yellow colour and is now called the white body (corpus albicans). The corpus albicans disappears completely with time. A single follicle usually ripens in 28 days. The ovary is not covered with the peritoneum, which is reduced here, but is instead covered with embryonic epithelium. Due to this the oocyte is discharged from the ruptured Graafian vesicle directly onto the surface of the ovary and then into the oviduct.

THE UTERINE TUBE

The uterine, or Fallopian tube (tuba uterina [Fallopii] s. salpinx) is a paired duct (oviduct) conveying the oocytes from the surface of the ovary, onto which they are discharged during ovulation, to the cavity of the uterus. Each tube is invested in a peritoneal fold, which is the upper portion of the broad ligament of the uterus and is termed the mesentery of the tube, mesosalpinx. The tube is 10-12 cm in length, on the average, the right tube usually being a little longer than the left one. The following parts are distinguished in the tube: (1) uterine (pars uterina), the part embedded in the wall of the uterus; (2) isthmus, the nearest to the uterus uniformly constricted part (inner third of the tube) with a diameter of about 2-3 mm; (3) ampulla, the part continuous with the isthmus and gradually increasing in diameter (the ampulla accounts for approximately half of the length of the tube); (4) infundibulum, a continuation of the ampulla and, in complIance with its name, is a funnel- shaped expansion of the tube with numerous irregularly shaped processes, called fimbriae tubae, on its margins. One of these fimbriae, the largest usually, stretches in the peritoneal fold and reaches the ovary and is termed fimbriae ovarica. At the apex of the infundibulum is around opening into the abdominal cavity (ostium abdominale tubae), through which the oocyte discharged from the ovary enters the ampulla of the tube. The opposite opening of the tube, through which it opens into the cavity of the uterus, is called the uterine opening (ostium uterinum tubae). Structure of the wall of the tube. Immediately under the peritoneum, or serous coat (tunica serosa) is a connective-tissue coat (tunica subserosa) containing vessels and nerves. Under the last-named is a muscular coat (tunica muscularis) consisting of two layers of smooth muscle fibres, an outer longitudinal and an inner circular layer; the circular layer is especially well developed in the part close to the uterus. The mucous coat (tunica mucosa) forms numerous longitudinal folds (plicae tubariae) and is covered with ciliated epithelium whose cilia drive the contents of the tube in the direction of the uterus. The mucous coat is continuous with the mucous coat of the uterus on one end and on the other end is in relation with the serous coat of the abdominal cavity through the abdominal opening. As a result the tube opens into the abdominal cavity, which in females, as distinct from that in males, is not a closed serous sac.

THE UTERUS

The uterus, or womb (Gk metra s. hystera) is an unpaired hollow muscular organ situated in the cavity of the pelvis between the urinary bladder in front and the rectum behind. The oocyte entering the uterine cavity through the uterine tubes undergoes further development here if fertilized and remains in the uterus until delivery of the mature foetus. In addition to this generative function, the uterus is also concerned with menstrual activity. A fully developed virgin uterus is pear-shaped and flattened from front to back. A fundus, body, and neck are distinguished in it. The fundus of the uterus (fundus uteri) is the upper part projecting above the line joining the points of entrance of the tubes into the uterus. The body (corpus uteri) is triangular, gradually narrowing toward the neck. The neck (cervix uteri) is a continuation of the body but is rounder and narrower. The external end of the cervix projects into the upper part of the vagina and is calleci the vaginal part (portio vaginalis). The upper segment of the neck with which the body is continuous is called the supravaginal part (portio supravaginalis). The anterior and posterior surfaces are separated from each other by the right and left borders (margo uteri dextra and margo uteri sinistra). Due to the considerable thickness of the walls, the cavity of the uterus (cavum uteri) is small in comparison with the size of the organ. On a frontal section, the uterine cavity has the shape of a triangle with the base directed at the fundus and the apex facing the neck. The tubes open into the angles of the fundus, while at the apex of the triangle the uterine cavity is continuous with the cavity, or canal of the neck (canalis cervicis uteri). The junction of the body with the neck is a narrowed part, and is called the isthmus (isthmus uteri). The cervical canal communicates with the cavity of the vagina by means of the uterine opening external os uteri (ostium uteri). The os uteri is round or transversely oval in nulliparas but has the appearance of a transverse slit with healed tears on the margins in women who have given birth to children. The cervical canal in nulliparas is spindle-shaped. The ostium uteri, or mouth of the womb, is bounded by two lips, anterior and posterior (labium anterius and labium posterius). The posterior lip is thinner and projects downward less than the thicker anterior lip. It also appears longer because the vagina is attached to it at a higher level than to the anterior lip. The mucous membrane is smooth and is devoid of folds in the cavity of the uterine body but forms palmate folds, arbor vitae (plicae palmatae) in the cervical canal, which consist of an anterior and a posterior longitudinal ridges and a series of lateral rugae directed laterally and upward. The uterine wall is composed of three main layers. 1. The outer layer, the serous coat (tunica serosa), or perimetrium, is the visceral peritoneum, which firmly adheres to the uterus. (In practice it is important to distinguish the perimetrium, i.e. the visceral peritoneum, from the parametrium, i.e. the fatty tissue on the anterior surface and along the sides of the cervix uteri, between the peritoneal layers forming the broad ligament of the uterus). 2. The middle layer, the muscular coat (tunica muscularis), or myometrium, is the main part of the uterine wall. It consists of smooth muscle fibres interlacing one with another in different directions. Three layers can be distinguished in it: an inner longitudinal, a middle circular, the thickest of the three layers (large venous plexus passes in it as a consequence of which it is called stratum vasculosum), and an outer longitudinal layer lying under the peritoneum (stratum supravasculosum). The separation into layers is most pronounced in the neck; the amount of connective tissue with an admixture of elastic fibres is considerably greater here than in the body of the uterus, that is why the cervix is in general distinguished by greater firmness. 3. The inner layer, the mucous coat (tunica mucosa), or endometrium, is covered with ciliated epithelium and has no folds. It is supplied with simple tubular glands, glandulae uterinae, which extend to the muscular coat. In addition to the tubular glands, mucous glands, glandulae cervicales, are present in the thicker mucous membrane of the cervix. The average length of a mature, non-pregnant uterus ranges from 6.0 to 7.5 cm of which about 2.5 cm are accounted for by the length of the cervix. At birth the neck is longer than the body of the uterus, but with the onset of puberty the last-named grows intensively. The pregnant uterus changes rapidly in size and shape. By the eighth month it becomes 18-20 cm in length, acquires a rounded oval shape and with growth moves apart the layers of the broad ligament. Some of the muscle fibres not only multiply in number but also increase in size. After delivery the uterus gradually diminishes in size, though in quite a short time, and regains its former state but is a little larger in dimensions. The enlarged muscle fibres undergo fatty degeneration. The peritoneum covers the uterus anteriorly to the junction of the body with the neck and is then reflected on the urinary bladder. The peritoneal pouch formed as the result is called the vesicouterine pouch (excavatio vesicouterina). From the posterior uterine surface the peritoneum passes to the posterior wall of the vagina, which it covers for a small distance and is then reflected on the rectum. The deep peritoneal pouch between the rectum at the back and the uterus and vagina in front is called the rectouterine (Douglas') pouch (excavatio rectouterina s. cavum Douglasi. The peritoneal layers covering the anterior and posterior surfaces of the uterus come together at its borders and extend to the lateral walls of the pelvis as the broad ligaments of the uterus (ligamenta lata uteri), which are also (below the mesosalpinx) its mesentery, the mesometrium. In the free edge of the broad ligament is enclosed the uterine tube, on the anterior and posterior surfaces are two eminences formed by the round ligament of the uterus (lig. teres uteri) and the ligament of the ovary (lig. ovarium proprium). The ovary is attached to the posterior surface of the broad ligament by means of a short mesenterium, the mesovarium. On the sides of the cervix and upper area of the vagina, in contrast, the layers separate and an aggregation of loose fatty tissue containing blood vessels lies between them. This cellular tissue is called the parametrium. The round ligaments (ligamenta teres uteri) arise one on each side, from the superior angles of the uterus and ascend anteriorly and laterally toward the deep inguinal ring. After passing through the inguinal canal the round ligament reaches the pubic symphysis where its fibres blend with the connective tissue of the mons pubis and the labium majus.

THE SCIENCE OF THE NERVOUS SYSTEM (NEUROLOGY)

GENERAL DATA One of the most important characteristics of living substances is their capacity to respond to stimuli. Every living organism receives stimuli from its environment and responds to such stimuli by corresponding reactions which link the organism to the environment. Metabolic processes within the organism itself, in turn, create a number of stimuli to which the organism must also react. In higher multicellular organisms the area receiving the stimulus and the reacting organ are connected by the nervous system. Branching into all the organs and tissues, the nervous system binds and integrates all parts of the organism into a single, unified whole. The basic anatomical element of the nervous system is the nerve cell which, together with all the processes arising from it, is called the neuron. A long axial cylindrical process, called the axon or neurite, arises from the body of the cell in one direction. Short branched processes called dendrites lead in the other direction. Nervous impulse inside the neuron flows from the dendrites to the cell body and from there to the axon; the axons convey the nervous impulse away from the cell body. The conduct of the nerve impulse from one neuron to another is accomplished by means of specially built end apparatuses or synapses (Gk synaptein to join). Axosomatic connections of neurons in which the branches of one neuron approach the cell body of another neuron, can also be distinguished, as can axodendritic connections in which contact is accomplished by the dendrites of nerve cells. The nervous system is composed of a complex of neurons which come into contact with one another but never grow together. Consequently, the nervous impulse that arises in one part of the body is conveyed along the processes of nerve cells from one neuron to a second, from there to a third, and so on. A clear example of the connection established between organs through the neurons is the reflex arc which forms the basis of the reflex, the simplest and at the same time most fundamental reaction of the nervous system. The simple reflex arc consists of at least two neurons, one of which connects with a sensory surface (the skin, for instance) and the other, which, with its axon, ends in a muscle (or a gland). When the sensory surface is stimulated, the nervous impulse passes centripetally along the neuron connected to it to the reflex centre where the synapse of both neurons is located. Here the nervous impulse is transferred to the other neuron and directed centrifugally to the muscle or gland. As a result the muscle contracts or the secretion of the gland changes. Quite often third internuncial neuron, which serves as a transmitting station from the sensory route to the motor route, is included in the simple reflex arc. Besides the simple (three-member) reflex arc there are complex multineuronal reflex arcs passing through different levels of the brain, including the cerebral cortex. Thus, the elements of the nervous system may be classified as one of three kinds according to function. 1. Receptors transform the energy of the external stimulus into a nerve process; the receptors are connected with afferent (centripetal or receptor) neurons, which transmit the triggered excitation (nerve impulse) toward the centre. 2. Conductors are internuncial or connecting neurons which accomplish the contact, i.e. the transfer of the nerve impulse from the centripetal to the centrifugal neuron and the transformation of the impulse received by the centre into an external reaction. 3. Efferent (centrifugal) neurons implement response reactions (motor or secretory) by conducting the nervous impulse from the centre to the effector (the producer of the effect or the action) at the periphery i.e. to the working organ (muscle, gland). This is why this neuron is also called the effector neuron. The receptors are stimulated by three sensory surfaces, or receptor fields, of the organism: (1) the external skin surface of the body (exteroceptive field) through the sense organs which are genetically related to the skin and receive stimuli from the environment; (2) the internal surfaces of the body (interoceptive field) stimulated mostly by chemical substances entering the internal cavities; and (3) the thickness of the walls of the body itself (proprioceptive fields) where the bones, muscles, and other organs are laid out and produce stimuli received by special receptors. The receptors from such fields are connected with afferent neurons which reach the centre and transfer there to various efferent conductors by a very complicated system of conductors. These efferent conductors produce various effects in conjunction with the working organs. The unified human nervous system is conditionally divided into two parts corresponding to the two principal parts of the organism-vegetative and animal: (1) the vegetative nervous system innervates the internal organs. the endocrine system, and the smooth muscles of the skin, heart, and vessels. i.e. the organs of vegetative life which create the internal media of the organism; (2) the animal nervous system controls the striated musculature of the skeleton and certain internal organs (tongue, larynx, pharynx) and primarily innervates the organs of animal life. The animal nervous system is also inaptly called the somatic system, meaning soma, i.e. the body itself. For the most part it controls the functions connecting the organism with the environment, provides the sensitivity of the organism (through the sense organs), and the movements of the muscles of the skeleton. In addition to this structural classification the nervous system can be classified topographically into central and peripheral systems. The central nervous system consists of the spinal cord and brain made up of grey and white matter; the peripheral system includes all other components, i.e. the nerve roots, ganglia, plexuses, nerves, and peripheral nerve endings. Both the central and peripheral parts of the nervous system contain elements of its animal and vegetative components, thus uniting the nervous system as a whole. Its most highly developed section, which controls all the processes in the body, both animal and vegetative, is the cortex of the brain.

THE CENTRAL NERVOUS SYSTEM STRUCTURE OF THE SPINAL CORD

The spinal cord (medulla spinalis) is lodged in the vertebral canal and in adults is a long cylindrical cord slightly flattened from front to back (45 cm in length in males and 41-42 cm in females); above (cranially) it is continuous with the medulla oblongata and below (caudally) it terminates as a conic tip called conus medullaris at the level of the second lumbar vertebra. The conus medullaris is continuous caudally with a thread-like structure called filum terminale. The spinal cord has two enlargements along its length which correspond to the roots of the nerves of the upper and lower limbs: the upper (cervical) enlargement is called intumescentia cervicalis, the lower one (lumbar), intumescentia lumbalis. The lumbar enlargement is more voluminous, but the cervical one is more highly differentiated due to the more complex innervation of the hand as the organ of labour. Two fissures, a deep one, anterior median fissure (fissura mediana anterior), and a superficial posterior median sulcus (sulcus medianus posterior),divide the spinal cord into two symmetrical halves, right and left. Each half, in turn, has a slightly pronounced longitudinal sulcus running on the line of entry of the posterior roots; it is called sulcus dorsolateralis s. posterolateralis and the longitudinal sulcus running on the line of the anterior roots is called sulcus anterolateralis. The dorsolateral sulcus and the site of emergence of the anterior roots from the cord subdivide each half into three longitudinal columns, or funiculi: anterior, lateral, and posterior. A small intermediate sulcus, sulcus intermedius posterior, divides the cervical and superothoracic segments of the posterior funiculus into two fasciculi: fasciculus gracilis (column or fasciculus of Goll) and fasciculus cuneatus (column or fasciculus of Burdach). The roots of the spinal nerves emerge from the spinal cord on either side to form two longitudinal rows. The anterior root (radix ventralis s. anterior) consists of axons of the motor (efferent) neurons whose cell bodies are situated in the spinal cord; the posterior root (radix dorsalis s. posterior) lodged in the dorsolateral sulcus contains the processes of the sensory (afferent) neurons whose bodies lie in the spinal (intervertebral) ganglia. For some distance from the spinal cord the motor root adjoins the sensory root to form the trunk of the spinal nerve, which neurologists term funiculus. The funiculus is usually very short because it soon emerges from the intervertebral foramen after which the spinal nerve separates into its main branches. In the intervertebral foramen close to the junction of both roots, the posterior root has a swelling, the spinal or intervertebral ganglion (ganglion spinale s. intervertebrale), containing the false unipolar nerve cells (afferent neurons) with a single process which then separates into two branches, a central branch passing in the posterior root into the spinal cord, and a peripheral branch continuous with the spinal nerve. Thus there are no synapses in the spinal ganglia because the cell bodies of only the afferent neurons are present here. The site of emergence of the nerve roots does not correspond to the level of the intervertebral foramina because the spinal cord is shorter than the vertebral canal. To enter the foramina, the roots are directed not only laterally of the cord, but also downward and the lower the root arises from the cord the steeper it descends. In the lumbar part of the cord the nerve roots descend to the corresponding intervertebral foramina parallel to the filum terminale and invest it and the conus medullaris like a thick sheaf which is called cauda equina.

INTERNAL STRUCTURE OF THE SPINAL CORD

The spinal cord consists of grey matter containing nerve cells and of white matter composed of medullated nerve fibres. The grey matter (substantia grisea) is surrounded entirely by the white matter. The grey matter forms two vertical columns in the right and left halves of the cord. In the middle of it is the central canal of the spinal cord (canalis centralis) stretching throughout the length of the cord and containing the cerebrospinal fluid. It communicates with the fourth ventricle of the brain above and terminates as a small dilatation, the terminal ventricle (ventriculus terminalis), below in the region of the conus medullaris. In each column of grey matter two columns are distinguished, columna anterior and columna posterior. On transverse section of the spinal cord they are seen as a dilated anterior horn (cornu anterius) and a pointed posterior horn (cornu posterius). As a result the general pattern or the grey matter forms the letter “H” against the background of the white matter. The grey matter consists of nerve cells grouped to form nuclei whose position corresponds, on the whole, to the segmental structure of the spinal cord and its primary three-member reflex arc. The first, sensory neuron of this arc is situated in the spinal ganglia, the peripheral process of the neuron passes as a component of nerves to the organs and tissues and comes in contact there with the receptors while the central process pierces the spinal cord as part of the posterior sensitive roots through the dorsolateral sulcus and comes in contact in the cord with the cells of the posterior horns.These are the somatic sensory nuclei. The following are most defined among them: the nucleus of the base or the posterior horn, the thoracic nucleus (nucleus thoracicus) known also as the Stilling-Clarke column, which is most pronounced in the thoracic segments of the cord; a gelatinous matter (substantia gelatinosa) lying at the apex of the horn; and the nuclei proprii. Cells embedded in the posterior horn form the second neurons which give rise to axons passing to the brain; the cells of the substantia gelatinosa and cells diffusely scattered in the grey matter (called the fascicular cells) are responsible for connection with the third neurons lodged in the anterior horns of the same segment. The processes of these cells which extend from the posterior to the anterior horns are situated, naturally, close to the grey matter, along its periphery, and form a narrow band of white matter surrounding it intimately on all sides. These are the main fasciculi of the spinal cord, the fasciculi proprii. The axons of the other fasicular cells separate to form the ascending and descending branches which terminate on the cells of the anterior horns of several segments immediately proximal and distal to the given segment. As a result a stimulus sent from a definite body region can not only be transmitted to the spinal cord segment corresponding to this region, but can also spread to other segments. Consequently, simple reflex can involve a whole group of muscles into the response and thus cause a complex coordinated movement which, however, has the character of an unconditioned reflex. The anterior horns contain the third, motor, neurons whose axons on emergence from the spinal cord form the anterior, motor roots. These cells form the nuclei of the efferent somatic nerves innervating the skeletal muskulature and called the somatico- motor nuclei. The anterior and posterior horns in each half of the spinal cord are connected to each other by the intermediate zone of the grey matter which is particularly pronounced in the thoracic and lumbar segments of the spinal cord on the distance between the first thoracic and second to third lumbar vertebral segments and projects as the lateral horn (cornu laterale). As a consequence, a transverse section of the grey matter in these segments has the pattern of a butterfly. The lateral horns contain cells which innervate the vegetative organs and which are grouped into a nucleus called the intermediolateral nucleus (nucleus intermediolateralis) described for the first time by Yakubovich. The axons of the cells forming this nucleus emerge from the spinal cord as components of the anterior roots.The other cells which are grouped into a nucleus called the intermediomedial nucleus (nucleus intermediomedialis). The white matter (substantia alba) of the spinal cord is composed of nerve processes forming three systems of nerve fibres. 1. Short bundles of associated fibres connecting areas of the spinal cord at different levels (afferent and internuncial neurons). 2. Long centripetal (sensory, afferent) fibres. 3. Long centrifugal (motor, efferent) fibres. The first system (of short fibres) is related to the apparatus proper of the spinal cord, the other two systems (of long fibres) constitute the conducting apparatus of two-way connections with the brain. A nerve segment is a transverse segment of the spinal cord and the right and left spinal nerves connected with it. It consists of a horizontal layer of the white and grey matter (the posterior, anterior, and lateral horns) containing neurons whose processes stretch in one paired (right and left) spinal nerve and its roots. Thirty-one segments are distinguished in the spinal cord, which are topographically divided into eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal segments. The white matter is also contained in the white commissure (commissura alba) formed due to crossing of the fibres in front of the central intermediate substance; posteriorly the white commissure is absent. The posterior funiculi contain fibres of the posterior spinal roots which form two systems. 1. The medially situated fasciculus gracilis of Goll. 2. The laterally situated fasciculus cuneatus of Burdach. The fasciculi gracilis and cuneatus conduct from the corresponding parts of the body to the brain cortex voluntary (conscious) proprioceptive (muscle and joint sense) and skin (the sense of stereognosis, i.e. the recognition of objects by touch) sensitivity related to determining the position of the body in space, as well as tactile sense. The lateral funiculi contain the following fasciculi. A. Ascending. To the metencephalon: (1) the posterior spinocerebellar, or Flechsig's tract (tractus spinocerebellaris posterior) located on the periphery of the posterior part of the lateral funiculus; (2) the anterior spinocerebellar, or Gowers' tract (tractus spinocerebellaris anterior) situated ventral to the posterior tract. Both spinocerebellar tracts conduct involuntary (unconscious) proprioceptive impulses (involuntary coordination of movements). To the mesencephalon: (3) the spinotectal tract (tractus spinotectalis) adjoining the medial side and anterior part of the anterior spinocerebellar tract. To the diencephalon: (4) the lateral spinothalamic tract (tractus spinothalamicus lateralis) adjoins the anterior spinocerebellar tract directly behind the spinotectal pathway. It conducts temperature stimuli in the dorsal part and pain stimuli in the ventral part; (5) the anterior, or ventral spinothalamic tract (tractus spinothalamicus anterior s. ventralis) is similar to the lateral tract but lies in front of it and is the pathway conducting impulses of touch (tactile sensitivity). Some authors claim that this tract is situated in the anterior funiculus. B. Descending. From the cerebral cortex: (1) the lateral pyramidal, or corticospinal tract (tractus corticospinalis, s. pyramidalis anterior). It is a voluntary efferent motor tract. From the mesencephalon: (2) the rubrospinal, or Monakow's tract (tractus rubrospinalis). This is an involuntary efferent motor pathway. From the metencephalon: (3) the olivospinal, or Bechterew-Helweg, tract (tractus olivospinalis) lies ventral to the tract of Gowers, close to the anterior funiculus. The anterior funiculi contain descending tracts. From the cerebral cortex: (1) the anterior pyramidal, or corticospinal tract (tractus corticospinalis s. pyramidalis anterior). Together with the lateral pyramidal tract it constitutes the common pyramidal system. From the mesencephalon: (2) the tectospinal tract (tractus tectospinalis) lies medial to the pyramidal tract and bounds the anterior median fissure. It is concerned with protective reflex movements in visual and auditory stimulations and is therefore the visual and auditory reflex tract. Some fasciculi pass to the anterior horns of the spinal cord from different nuclei of the medulla oblongata which are concerned with balance and coordination of movements. These are as follows: (3) from the nuclei of the vestibular nerve, the anterior vestibulospinal tract (tractus vestibulospinalis anterior) lying at the junction of the anterior and lateral funiculi; (4) from the reticular formation, the reticulospinal tract (tractus reticulospinalis) situated in the middle part of the anterior funiculus; (5) the fasciculi proprii which adjoin the grey matter and are related to the spinal-cord apparatus.

THE MENINGES OF THE SPINAL CORD

The spinal cord is invested in three connective-tissue membranes (meninges). These are: the external, dura mater, or pachymeninx; the next, the arachnoid mater, or arachnoidea; and the innermost, the pia mater. In distinction from the thick dura mater, the last two delicate meninges are also called the leptomeninges. Cranially all three are continuous with similar meninges of the brain. The spinal dura mater (dura mater spinalis) forms the outer sac-like sheath of the cord. It is not related intimately with the walls of the vertebral canal which are covered with their own periosteum the endorrhachis). The last named is also called the external layer of the dura mater. Between the endorrhachis and the dura mater is the extradural space (cavitas epidurale) containing fat and venous plexus called the internal vertebral plexus. 2. The spinal arachnoid mater (arachnoidea spinalis) adjoins the internal surface of the dura mater as a thin transparent avascular layer but is separated from it by a slit-like subdural space (cavum subdurale) pierced with thin trabeculae. Between the arachnoidea and the pia mater immediately covering the spinal cord is the subarachnoid space (cavum subarachnoideale) in which the spinal cord and nerve roots lie free surrounded by a great amount of cerebrospinal fluid (liquor cerebrospinalis). This space is especially enlarged in the lower part of the arachnoid sac where it surrounds the cauda equina of the spinal cord; this is the cisterna terminalis. The fluid contained in the subarachnoid space communicates continuously with the fluid filling the subarachnoid spaces of the brain and its ventricles. 3. The spinal pia mater (pia mater spinalis) is covered on the surface with endothelium and immediately invests the spinal cord; it contains between its two layers vessels together with which it enters the sulci and medullary matter of the spinal cord and forms perivascular lymphatic spaces.

THE BRAIN DEVELOPMENT (EMBRYOGENESIS) OF THE BRAIN

At the early stage of development the neural tube is subdivided into two parts corresponding to the brain and the spinal cord. The anterior, expanded part, the rudiment of the brain, is divided by constrictions into three primary cerebral vesicles situated one next to the other: anterior, or forebrain, the prosencephalon; middle, or midbrain, the mesencephalon; posterior, or hindbrain, the rhombencephalon. This stage of three vesicles transforms during subsequent differentiation into five vesicles which give rise to the five main parts of the brain. The posterior brain vesicle, the rhombencephalon, has two portions. The posterior portion, the myelencephalon, transforms finally into the medulla oblongata, while the anterior portion, called the metencephalon, develops into the pons Varolii on the ventral side and into the cerebellum on the dorsal side. The common cavity of the rhombencephalon, rhomboid-shaped on frontal section, forms the fourth ventricle communicating with the central canal of the spinal cord. The mesencephalon ventrally gives rise to the cerebral peduncles, dorsally to the tectal lamina. The cavity of the mesencephalon is converted into a narrow canal, the aqueduct, communicating with the fourth ventricle. The differentiation and change in shape of the prosencephalon are more marked. It is subdivided into a posterior part, the diencephalon, and an anterior part, the telencephalon. The lateral walls of the diencephalon thicken to form the thalami. The tuber cinereum, the infundibulum, and the posterior (neural) lobe of the hypophysis cerebri are drawn on the ventral side. Still further to the back, in the region of the diencephalon, the paired mammillary bodies (corpora mamillaria) are laid down. The cavity of the diencephalon forms the third ventricle. The telencephalon is divided into a smaller, median part (telencephalon medium) and two large, lateral parts, the vesicles of the cerebral hemispheres (hemispherium dextrum and sinistrum) which grow intensively in man and when fully developed are much larger than all the other parts of the brain. The cavity of the median telencephalon, being a continuation of the cavity of the diencephalon (the third ventricle), communicates on its sides with the cavities of the hemispheric vesicles by means of interventricular apertures; in a developed brain the cavities of the hemispheric vesicles are termed the lateral ventricles.

THE PARTS OF THE BRAIN

As it is indicated above, according to the embryonic development, the brain is separated into parts situated in the following order from the caudal end: (1) the rhombencephalon, or the hindbrain consisting, in turn, of (a) the myelencephalon, or the medulla oblongata, and (b) the metencephalon, the hindbrain proper; (2) the mesencephalon, or the midbrain; (3) the prosencephalon, or the forebrain in which are distinguished (a) the diencephalon or between- brain and (b) the telencephalon, or endbrain. All the parts named, with the exception of the cerebellum and telencephalon, form the brain stem. Besides these parts, the isthmus rhombencephali, lying between the rhombencephalon and the mesencephalon, is distinguished.

THE RHOMBENCEPHALON

THE MEDULLA OBLONGATA, MYELENCEPHALON

The medulla oblongata (myelencephalon) is a direct continuation of the spinal cord into the brain stem and is part of the rhombencephalon, or the hindbrain. It combines the structural properties of the spinal cord and the initial part of the brain, hence the name myelencephalon (Gk myelos marrow, egkephalos brain). The medulla is shaped like a bulb (bulbus cerebri, s. bulbus medullae spinalis, from which the term bulbar disorders is derived); its upper expanded end borders upon the pons varolii while its inferior boundary is the site of emergence of the roots of the first pair of cervical nerves or the level of the foramen magnum. On the midline of the anterior (ventral) surface of the medulla stretches the anterior median fissure (fissura mediana anterior) which is a continuation of the anterior median fissure of the spinal cord. On either side of it is a longitudinal elevation called the pyramids of the medulla oblongata (pyramides medullae oblongatae); both pyramids are continuous with the anterior funiculi of the spinal cord. Some of the bundles of nerve fibres composing the pyramid decussate in the depth of the anterior median fissure with the bundles of the contralateral pyramid to form the decussation of the pyramids (decussatio pyramidum) and then descend in the lateral funiculus of the contralateral side of the spinal cord; this is the lateral corticospinal, or pyramidal tract (tractus corticospinalis s. pyramidalis lateralis). Those bundles which do not decussate descend in the anterior funiculus on their own side and form the anterior corticospinal or pyramidal tract (tractus corticospinalis s. pyramidalis anterior). Lateral to the pyramid is an oval swelling, the olive (oliva), separated from it by a fissure, the anterolateral sulcus. On the posterior (dorsal) surface of the medulla is the posterior median fissure (sulcus medianus posterior), a continuation of the posterior median fissure of the spinal cord. Lateral to it are the posterior funiculi bounded laterally on either side by a poorly defined posterolateral sulcus. Above, the posterior funiculi diverge laterally and pass to the cerebellum to be components of its inferior peduncles (pedunculi cerebellares inferiores) bounding the rhomboid fossa below. Each posterior funiculus is separated by an intermediate fissure into a medial, fasciculus gracilis and a lateral, fasciculus cuneatus. At the inferior angle of the rhomboid fossa both funiculi acquire an elevation known as tuberculum nuclei gracilis and tuberculum nuclei cuneati. These elevations are formed by grey matter nuclei, called the gracile nucleus (nucleus gracilis) and the cuneate nucleus (nucleus cuneatus). The ascending fibres of the posterior roots of the spinal cord, which pass in the posterior funiculi, terminate in these nuclei (fasciculi of Gall and Burdach). The lateral surface of the medulla oblongata between the anterolateral and posterolateral fissures corresponds to the lateral funiculus (lateral white column). The eleventh, tenth, and ninth pairs of cranial nerves emerge from the posterolateral sulcus behind the olive. The lower part of the rhomboid fossa is part of the medulla oblongata.

INTERNAL STRUCTURE OF MEDULLA OBLONGATA

The medulla oblongata originated in association with the development of the organs of statics and acoustics and the branchial apparatus concerned with respiration and circulation. That is why nuclei of grey matter related to the balancing and coordination of movements and to the control of metabolism are embedded in it. 1. The olivary nucleus (nucleus olivaris) has the appearance of a curved lamina of grey matter open medially (the hilus); it causes the external bulging of the olive. The olivary nucleus is connected with the cerebellar dentate nucleus and is the intermediate nucleus of balance which is most pronounced in man, whose erect posture needs the most perfect apparatus of balance. (An accessory medial olivary nucleus is also encountered.) 2. The reticular formation (formatio reticularis) is formed of interlaced nerve fibres and nerve cells lying between them. The reticular formation of the medulla oblongata is connected with the spinal cord. A bundle of descending fibres, the reticulospinal tract (tractus reticulospinalis) passes from it to the spinal cord. 3. The nuclei of the lower four pairs of the cranial nerves (twelfth to ninth) concerned with innervation of the derivatives of the branchial apparatus and the viscera. 4. The vitally important respiratory and circulatory centres connected with the nuclei of the vagus nerve. Injury to the medulla oblongata may therefore cause death. The white matter of the medulla oblongata contains long and short tracts. The long tracts are as follows: the descending pyramidal tracts passing by transit into the anterior funiculi of the spinal cord and partly decussating in the region of the pyramids. In addition, from the nuclei of the posterior funiculi (the gracile and cuneate nuclei) arise the second neurones of the ascending sensory tracts passing from the medulla oblongata to the thalamus. The fibres of this bundle form a medial loop, the medial lemniscus (lemniscus medialis) which decussates in the medulla to form the decussatio lemniscorum and then extends further in the form of a bundle of fibres lying dorsal to the pyramids. Thus, in the medulla oblongata there are two decussations of the long conduction tracts, namely a ventral, motor pyramidal decussation and a dorsal, sensory decussation of the lemniscus. The short tracts include the bundles of nerve fibres connecting some nuclei of the grey matter to one another, as well as bundles connecting the nuclei of the medulla oblongata with the adjacent parts of the brain. Among these, the olivocerebellar tract (tractus olivocerebellaris) and the medial longitudinal bundle (fasciculus longitudinalis medialis) stretching dorsal to the interolivary layer deserve mention. Sensory (afferent) tracts The posterior funiculus - one conductive tract passes 1. The spinocortical tract - sensory, conscious, proprioceptive tract. The tract gives an information about the position of the body. This tract consists of two halves: medial or Goll’s ( fasciculus gracilis), which conveys impulses from the inferior 19 segments of the body, and lateral or Burdach’s (fasciculus cuneatus), conveys impulses from the superior 12 segments. The I neuron is the false unipolar nerve cell of the intervertebral ganglion, which dendrite reaches the muscles, joints or ligaments, and the axon by the posterior root enters the spinal cord and through the posterior funiculus ascends untill the medulla oblongata, where is located the II neuron in the gracilis and cuneatus nuclei. Part of the axons of the II neuron as internal arcuate fibers (fibrae arcuatae internae) cross over in the medulla oblongata and form the decussatio lemniscorum, from which the medial lemniscus starts. As a part of the lemniscus medialis the fibers reach the lateral nuclei of the thalamus ( the III neuron). The axons of the III neuron form the thalamo-cortical tract, which passes through the internal capsule (the posterior peduncle) and terminates in the superficial layers (3-4 layers) of the anterocentral, posterocentral gyri of the cortex and superior parietal lobe. The other part of the axons of the II neurons’ of the gracilis and cuneatus nuclei as external arcuate fibers (fibrae arcuatae externae) through the inferior cerebellar peduncles enter the cerebellum and terminate in the cortex of the vermis (the III neuron).

The lateral funiculus – 4 conductive tracts pass 1. The posterior spinocerebellar tract (Flechsig’s tract) – unconscious, proprioceptive tract. The I neuron is the false unipolar nerve cell of the intervertebral ganglion. Its axon through the posterior root enters the grey matter of the spinal cord and terminates on the dorsal nucleus of the posterior horn (the II neuron). The axons of the II neuron pass to the lateral funiculus of the same side and ascend in it, through the inferior cerebellar peduncles enter the cerebellum and terminate in the cortex of the vermis (the III neuron). 2. The anterior spinocerebellar tract (Gover’s tract)– unconscious, proprioceptive tract. The I neuron is the false unipolar nerve cell of the intervertebral ganglion. Its axon through the posterior root enters the grey matter of the spinal cord and terminates on the intermediomedial nucleus( the II neuron), which axons cross over (segmental crossing), pass to the lateral funiculus of the opposite side and in it ascend to the mesencephalon , where they cross again and through the superior cerebellar peduncles enter the cerebellum and terminate in the cortex of the vermis (the III neuron).Despite the fact that the tract crosses twice, it remains a straight one, because the cerebllum is responsible for the same side. 3. The spinothalamic anterior and lateral tracts – conscious, exteroceptive tracts.The anterior one conducts pressure and tactile sensitivity (occupies the anterior funicle), and the lateral one conducts temperature and pain stimuli (occupies the lateral funicle). The I neuron is the false unipolar nerve cell of the intervertebral ganglion. Its dendrite goes to the skin and neurite through the posterior root enters the spinal cord and terminates on the proper nucleus (the II neuron), which axons cross over (segmental crossing), pass to the lateral funiculus of the opposite side and through the brain stem enter the medial lemniscus, then terminate on the lateral nuclei of the thalamus ( the III neuron). From there as a thalamo-cortical tract it reaches the cortex of the brain and terminates in the posterocentral gyrus and superior parietal lobe. 4. The spinotectal tract – unconscious, exteroceptive tract. It conducts sensitivity from the subcutaneus tissue. The I neuron is the false unipolar nerve cell of the intervertebral ganglion. The II neuron is the proper nucleus of the posterior horn. Its axons cross over to the opposite side and as a part of the lateral funiculus ascend, enter the medial lemniscus and terminate on the proper nuclei of the tectum (lamina quadrigemina).

Motor (efferent) tracts 1. The corticospinal tract (the pyramidal tract) – conscious, motor tract. The tract starts from the Betz’s pyramidal cells of the V-VI deep layers of the precentral gyrus of the cortex (the I neuron). Their axons form the radiate crown( corona radiata ), then gather to form a trunk,which passes through the internal capsule(the posterior peduncle), then through the cerebral stem descends untill the boundary of the medulla oblongata and the spinal cord. Here its medial part crosses (pyramidal decussation) and passes to the lateral funiculus and as a lateral corticospinal tract terminates on the motor nuclei of the anterior horn(the II neuron). Non crossed part descends through the anterior funiculus and is called anterior corticospinal tract and crosses over segment by segment and terminates on the motor nuclei of the anterior horn (the II neuron). The tract is responsible for the conscious movements of the body down from the head. 2. The corticonuclear tract – conscious, motor tract.The tract starts from the Betz’s pyramidal cells of the V-VI deep layers of the precentral gyrus of the cortex (the I neuron). The axons of these neurons form the radiate crown (corona radiata), pass through the knee (genu) of the internal capsule and cross over to the opposite side and terminate on the motor nuclei of the cranial nerves (the II neuron). This tract is responsible for the conscious movements of the head. 3. The rubrospinal tract – unconscious, motor tract, belongs to the extrapyramidal system. It starts from the red nuclei of the mesencephalon (the I neuron).Axons cross over to the opposite side and form Forel’s decussation, then descend through the lateral funiculus and terminate on the motor nuclei of the anterior horn(the II neuron). This tract is responsible for tonus of the striated muscles and regulates its activity, coordination of the movements. It’s a myostatic tract. 4. The tectospinal tract – visual-auditory reflector, motor tract. It starts from the proper nuclei of the lamina quadrigemina in the mesencephalon(the I neuron). The axons cross over to the opposite side, form Mejnert’s decussation, through the anterior funiculus descend and terminate on the motor nuclei of the anterior horn(the II neuron). 5. The olivospinal tract – unconscious, motor tract. It connects the olive nucleus(the I neuron) to the spinal cord. It doesn’t cross over, enters the anterior funiculus and terminates on the motor nuclei of the anterior horn(the II neuron). 6. The vestibulospinal tract – unconscious, reflector tract. It starts from the vestibular nuclei(the 8th nerve) in the medulla oblongata(the I neuron), enters the anterior funiculus, without crossing it terminates on the motor nuclei of the anterior horn (the II neuron). This tract serves for coordination of the body.

THE METENCEPHALON

The metencephalon consists of two parts: a ventral part – the pons Varolii, and a dorsal part – the cerebellum.

THE PONS

The pons (varolii) is seen on the base of the brain as a thick white rounded ridge bordering caudally upon the superior end of the medulla oblongata and cranially upon the cerebral peduncles. The lateral boundary of the pons is a line drawn through the roots of the trigeminal and facial nerves (the trigeminofacial line). Lateral to this line are the middle cerebellar peduncles (pedunculi cerebellaris medii) sinking on both sides into the cerebellum. The dorsal surface of the pons is not visible because it is hidden by the cerebellum; it forms the upper part of the rhomboid fossa (the floor or the fourth ventricle). The ventral surface of the pons is of a fibrous character with the fibres passing transversely for the most part and in the direction of the middle cerebellar peduncles. A shallow basilar sulcus (sulcus basilaris) lodging the basilar artery stretches on the midline of the ventral surface.

Internal Structure of the Pons

It can be seen on transverse sections that the pons consists of a larger, lower, or ventral part (pars ventralis pontis) and a smaller, dorsal part (pars dorsalis pontis). The boundary between them is formed by a thick layer of transverse fibres, the trapezoid body (corpus trapezoideum) whose fibres are related to the auditory tract. The dorsal nucleus of the corpus trapezoideum, or the superior olivary nucleus (nucleus olivaris superior), also related to the auditory tract, is situated in the region of the trapezoid body. The nucleus is given the latter name because it has a dentate shape similar to that of the olivary nucleus in the medulla oblongata. Pars ventralis contains longitudinal and transverse fibres between which are scattered the grey- matter pontine nuclei (nuclei pontis). The longitudinal fibres belong to the pyramidal tracts, the cerebropontine fibres (fibrae corticopontinae), which are connected to the pontine nuclei from which arise transverse fibres passing to the cerebellar cortex, the pontocerebellar tract (tractus pontocerebellaris). This whole system of conduction tracts links the cortex of the cerebral hemispheres with the cortex of the cerebellar hemispheres through the pons. The more is the cerebral cortex developed, the more are developed the pons and the cerebellum. Pars dorsalis contains the reticular formation of the pons (formatio reticularis pontis) which is a continuation of the similar part of the medulla oblongata. Above the reticular formation is the floor of the rhomboid fossa lined with the ependyma, below the floor lie the nuclei of the cranial nerves (eighth to fifth pairs). THE CEREBELLUM

The cerebellum is a derivative of the hindbrain which developed in connection with the receptors of statics. It is therefore concerned directly with the coordination of movements and is an organ of the body's adaptation to overcoming the main properties of its mass, i.e. gravity and inertia. It is also considered one of the highest centres of the vegetative (sympathetic) nervous system (L.A. Orbeli and his school). The cerebellum is situated under the frontal lobes of the cerebral hemispheres, dorsal to the pons and medulla oblongata, and is lodged in the posterior cranial fossa. The large lateral parts, the hemispheres (hemispheria cerebelli) and a narrow, middle part lying between them, the vermis, are distinguished in it. On the anterior border of the cerebellum is the anterior notch embracing the adjoining part of the brain stem. The posterior border has a narrower posterior notch separating the hemispheres one from the other. The surface of the cerebellum is covered with a layer of grey matter constituting its cortex and forms narrow cerebellar folia of the cerebellum (folia cerebelli) separated one from another by fissures (fissurae cerebelli). The deepest, horizontal fissure (fissura horizontalis cerebelli) passes on the posterior border of the cerebellum and separates the upper surface of the hemispheres (facies superior) from the lower surface (facies inferior). The horizontal and other large fissures divide the entire surface of the cerebellum into lobules (lobuli cerebelli). Among these it is necessary to point out the most isolated, small lobule, the flocculus, lying on the inferior surface of each hemisphere near the middle cerebellar peduncle, and the nodulus, a part of the vermis connected with the flocculus.

Internal Structure of the Cerebellum

Paired nuclei of grey matter are embedded in the white matter of both halves of the cerebellum. On either side of the midline, where the fastigium projects into the cerebellum, is the extreme medial nucleus fastigii. Lateral to it lie small islets of the nucleus globosus, and still further lateral, the emboliformis nucleus. Finally in the centre of the hemisphere is the dentate nucleus (nucleus dentatus) which has the appearance of a grey curved lamina resembling the olivary nucleus. The resemblance of the cerebellar dentate nucleus to the crenate nucleus of the olive is not a chance phenomenon; both nuclei are connected with the conduction tracts, the fibrae olivocerebellares, and each folium of one nucleus is similar to the folium of the other nucleus. Thus, both nuclei contribute together to the accomplishment of balance. The white matter of the cerebellum has on section the appearance of small leaves of a plant which correspond to each folium covered on the periphery by a cortex of grey matter. As a result the general pattern of the white and grey matter on section resembles a tree and is called arbor vitae cerebelli (this name, the tree of life, however, reflects merely the external appearance since damage to the cerebellum is not a direct threat to life). The white matter of the cerebellum is composed of different types of nerve fibres. Some of them link the folia and lobules, others pass from the cortex to the nuclei embedded within the cerebellum, still others connect the cerebellum with the adjoining parts of the brain. The last-mentioned fibres are components of the three pairs of cerebellar peduncles. 1. The inferior cerebellar peduncles (pedunculi cerebellares inferiores) (running to the medulla oblongata) convey to the cerebellum the tractus spinocerebellaris (Flechsig's) posterior, the fibrae arcuatae externae from the nuclei of the posterior funiculi of the medulla oblongata, and the fibrae olivocerebellares from the olive. All these fibres end in the cortex of the vermis and hemispheres. In addition, fibres pass in these peduncles from the nuclei of the vestibular nerve and end in the nucleus fastigii. These fibres convey to the cerebellum impulses from the vestibular apparatus and the proprioceptive field; as a result it becomes a nucleus of proprioceptive sensation concerned with automatic correction of the motor activity of the other parts of the brain. Descending tracts also pass in the opposite direction in the inferior peduncles; from the nucleus fastigii to the lateral vestibular nucleus and from this nucleus to the anterior horns of the spinal cord (the vestibulospinal tract). The cerebellum produces an effect on the spinal cord through this tract. 2. The middle cerebellar peduncles (pedunculi cerebellares medii) (running to the pons) contain fibres passing from the pontine nuclei to the cerebellar cortex. The pontocerebellar tracts, the conducting pathways to the cerebellar cortex, arise in the pontine nuclei and are a continuation of the corticopontine fibres which terminate in the nuclei of the pons after decussation. These pathways link the cerebral cortex with the cerebellar cortex, which explains the fact that the more developed is the cortex of the brain, the more developed are the pons and the cerebellar hemispheres; this is encountered in man. 3. The superior peduncles (pedunculi cerebellares superiores) (running to the tectal lamina) consist of nerve fibres stretching in both directions: (1) to the cerebellum, the anterior spinocerebellar, or Gowers' tract (tractus spinocerebellaris anterior), and (2) from the cerebellar dentate nucleus to the tectum, the cerebellotegmental tract (tractus cerebellotegmentalis) which after decussation terminates in the red nucleus and thalamus. The cerebellum receives impulses from the spinal cord through the first tract and sends impulses to the extrapyramidal system along the second tract and thus it itself has an influence on the spinal cord through this system.

THE FOURTH VENTRICLE

The fourth ventricle (ventriculus quartus) is a remnant of the cavity of the posterior cervical vesicle and is therefore the common cavity of all parts of the hindbrain composing the rhombencephalon (the medulla oblongata, cerebellum, pons, and isthmus). It is shaped like a tent in which a floor and roof are distinguished. The floor, or base, of the ventricle is shaped like a rhombus which seems to be pressed into the posterior surface of the medulla and pons. It is consequently termed the rhomboid fossa (fossa rhomboidea). The central canal of the spinal cord opens into the posteroinferior angle of the rhomboid fossa, while in the anterosuperior angle the fossa communicates with the aqueduct. The roof of the fourth ventricle (tegmen ventriculi quarti) is tent-like (fastigium) (BNA) and is composed of two medullary veli: the velum medullare superius stretched between the superior cerebellar peduncles, and the velum medullare inferius, a paired structure adjacent to the floccular peduncles. Part of the roof between the veli consists of cerebellar matter. The inferior medullary velum is supplemented by a layer of the pia mater, tela chortoidea ventriculi quarti, covered on the inside by a layer of epithelium, lamina choriotdea epithelialis, a rudiment of the posterior wall of the posterior cerebral vesicle (the choroid plexus of the fourth ventricle is connected with it). Tela chorioidea completely closes the cavity of the ventricle at first, but in the process of development three openings form in it: a median aperture (apertura mediana ventriculi quarti), or Magendie's foramen in the region of the inferior angle of the rhomboid fossa, and two smaller, lateral apertures in the region of the lateral recesses of the ventricle (aperturae laterales ventriculi quarti); the lateral apertures are also known as Luschka's foramina. By means of these apertures the fourth ventricle communicates with the subarachnoid space of the brain as a result of which the cerebrospinal fluid from the cerebral ventricles enters the intermeningeal spaces. Cerebrospinal fluid accumulating in the ventricles of the brain in constriction or obliteration of these apertures in inflammation of the meninges (meningitis) cannot drain into the subarachnoid space and hydrocephalus develops.

THE RHOMBOID FOSSA

The rhomboid fossa (fossa rhomboidea) has four sides, two superior and two inferior, according to its shape. The superior sides are bounded by the two superior cerebellar peduncles, the inferior sides are bounded by the two inferior cerebellar peduncles. On the midline of the rhombus, from the superior to the inferior angle, stretches the sulcus medianus which divides the rhomboid fossa into right and left halves. On either side of the sulcus is an elevation, the eminentia medialis, formed by an aggregation of grey matter. The eminentia medialis gradually narrows downward and is continuous with the hypoglossal triangle (trigonum nervi hypoglossi) under which is the nucleus of the hypoglossal nerve. Lateral to the lower part of this triangle is a smaller triangle noticeable because of its grey colour; this is the vagal triangle (trigonum n. vagi) in which the vegetative nucleus of the vagus nerve (nucleus dorsalis nervi vagi) is embedded. Above, the eminentia medialis has a swelling, the facial colliculus, produced by the root of the facial nerve and projection of the nucleus of the abducent nerve. In the region of the lateral angles, on either side, is the vestibular area (area vestibularis); the nuclei of the eighth pair of nerves lie here. Some of the fibres emerging from them pass across the rhomboid fossa from the lateral angles to the median sulcus in the form of horizontal bands, striae medullares ventriculi quarti. These striae divide the rhomboid fossa into a superior and inferior halves and correspond to the junction of the medulla oblongata and the pons.

THE MESENCEPHALON

The midbrain, or mesencephalon develops in the process of phylogenesis under the predominant influence of the visual receptor and its most important structures are, therefore, concerned with the innervation of the eye. The centres of hearing also formed here and, together with the centres of vision, developed later to form four rounded projections – the quadrigeminal, or tectal lamina. As a result the mesencephalon contains the following structures: (1) subcortical centres of vision and nuclei of nerves innervating the muscles of the eye; (2) subcortical auditory centres; (3) all ascending and descending conducting pathways connecting the cerebral cortex with the spinal cord and passing through the mesenephalon; (4) bundles of white matter connecting the mesencephalon with the other parts of the central nervous system. In accordance with this, the mesencephalon, which in man is the smallest and structurally simplest part of the brain, has two main portions: the roof, or tectum, in which the subcortical auditory and visual centres are located, and the cerebral peduncles in which the conducting tracts mainly pass. 1. The dorsal part, the roof of the mesencephalon, or the tectal lamina (tectum mesencephali s. lamina tecti) is concealed under the posterior end of the corpus callosum and is subdivided by two transecting grooves, transverse and longitudinal, into four white elevations (bodies or colliculi) arranged in pairs. The two superior colliculi are the subcortical visual centres; both inferior colliculi are the subcortical auditory centres. The pineal body is lodged in the shallow sulcus between the superior colliculi. Each colliculus is continuous with a brachium of the midbrain (brachium colliculi) directed laterally, forward, and upward to the diencephalon. The superior brachium (brachium colliculi superioris) passes under the thalamic pulvinar to the lateral geniculate body (corpus geniculatum laterale). The inferior brachium (brachium colliculi inferioris) passing along the superior margin of the trigonum lemnisci to the lateral mesencephalic sulcus sinks under the medial geniculate body (corpus geniculatum mediale). The geniculate bodies mentioned are related to the diencephalon. 2. The ventral part, the cerebral peduncles (pedunculi cerebri), contains all the conducting pathways stretching to the prosencephalon. The cerebral peduncles are two thick semicylindrical white cords of a clearly detectable longitudinal fibrous structure 'With slightly spirally curved fibres. They stretch from the superior margin of the pons upward and laterally, diverging at an angle of almost 80 degrees, and then sink into the tissue of the cerebral hemispheres. At the entry into the hemispheres the optic tract (tractus optici) crosses the peduncle. 3. The cavity of the mesencephalon, a remnant of the primary cavity of the middle cerebral vesicle, is called the cerebral aqueduct, or aqueduct of Sylvius (aqueductus cerebri s. Sylvii). It is a narrow canal, 1.5-2.0 cm in length, lined with ependyma, and connects the fourth ventricle with the third ventricle.. The internal structure of the mesencephalon. The following three main parts of the brain stem are distinguished on a transverse section of the mesencephalon: (1) the tectal lamina (lamina tecti) from which project the quadrigeminal bodies; (2) the tegmentum which is the superior part of the cerebral peduncles; (3) the ventral part of the cerebral peduncles or the crus of the cerebrum (crus cerebri). In lower vertebrates, the two superior colliculi are the main site of termination of the optic nerve and are the visual centre. With the transposition of the visual centres into the prosencephalon in mammals and in man, the persisting connection of the optic nerve to the superior colliculus is important only for reflexes. The fibres of the lateral lemniscus terminate in the nucleus of the inferior colliculus and in the medial geniculate body. The tectal lamina has a two-way connection with the spinal cord through the spinotectal tract and the tectobulbar and tectospinal tracts. After decussating in the tegmentum (Meynert's dorsal decussation) the tectospinal tracts pass to the muscular nuclei located in the medulla oblongata and the spinal cord. This is the visual-auditory reflex tract which is mentioned above in the description of the spinal cord. The tectal lamina can therefore be regarded as a reflex centre for different types of movements occurring mainly due to the effect of visual and auditory stimuli. Under the ventral wall of the aqueduct in the tegmentum, are embedded the nuclei of two cranial motor nerves, the oculomotor nerve (third pair) (n. oculomotorius) on the level of the superior colliculi and the trochlear nerve (fourth pair) (n. trochlearis) on the level of the inferior colliculi. The nucleus of the oculomotor nerve consists of several parts according to innervation of several muscles of the eyeball. Medial and to the back of it is another small, also paired, vegetative accessory nucleus (nucleus accessorius), or Yakubovich's nucleus (who described it in 1857 before it was described by Westphal and Edinger after whom it was incorrectly named), and an unpaired median nucleus. Yakubovich's nucleus and the unpaired median nucleus innervate the smooth muscles of the eye, namely, the ciliary muscle and the sphincter of the pupil. This part of the oculomotor nerve is related to the parasympathetic system. As it is pointed out above, two parts are distinguished in the cerebral peduncles, a ventral part, the crus cerebri, and the tegmentum. The borderline between them is formed by substantia nigra, named so because of its dark colour due to the black pigment melanin contained in its nerve cells. The substantia nigra extends for the whole length of the cerebral peduncle from the pons to the diencephalon; in function it is related to the extrapyramidal system. The crus of the cerebrum, located ventral to the substantia nigra, contains longitudinal nerve fibres which descend from the hemispheric cortex to all parts of the central nervous system located below it (the corticopontine, corticonuclear, corticospinal, and other tracts). The tegmentum, which is situated dorsal to the substantia nigra, contains for the most part different ascending fibres and nuclei of grey matter, the most important among which is the red nucleus (nucleus ruber). This elongated, sausage-shaped structure extends in the tegmentum of the midbrain from the hypothalamic region of the diencephalon to the inferior colliculi where it gives rise to an important descending rubrospinal tract (tractus rubrospinalis) connecting the red nucleus with the anterior horns of the spinal cord. On emerging from the red nucleus the rubrospinal tract decussates with the collateral tract in the ventral part of the median suture to form the ventral tegmental decussation, or Forel's decussation. The red nucleus is a very important coordinating centre of the extrapyramidal system and is connected to all its other parts. It receives fibres from the cerebellum which pass to it in the superior cerebellar peduncles after their decussation under the tectal lamina, ventral to the cerebral aqueduct, as well as from the pallidum, the most distally located and the oldest of all cerebral ganglia which are components of the extrapyramidal system. Owing to these connections, the cerebellum and the extrapyramidal system affect the whole skeletal musculature through the red nucleus and the rubrospinal tract arising from it and induce unconscious automatic movements. In addition to descending longitudinal fibres, the tegmentum contains ascending fibres forming in the midbrain continuations of the medial and lateral lenmisci. All sensory tracts except for the visual, auditori and the olfactory, ascend in these lemnisci to the cerebrum.

Lemniscus medialis (L.M) The medial lemniscus is an ascending, central conscious sensory tract, which contains proprioceptive, exteroceptive and interoceptive(taste) tracts. Medial lemniscus is formed in the medulla oblongata after the crossing (decusatio lemniscorum) of the internal arcuate fibers, which are the axons of the gracillis and cuneatus nuclei. In this bandle the proprioceptive tracts are the continuation of the spinocortical tract (gracillis and cuneatus bandels) and exteroceptive tracts are the continuation of the spinothalamic tracts. The taste fibers start from the opposite nucleus tractus solitarius, where the VII, IX and X nerves are terminated. In the pons lemnsicus receives the fibers of the V nerve’s sensory nucleus, which carries pain sensitivity from the face region, the tongue, the teeth, etc. The medial lemniscus ascending terminates in the lateral nucleus of the thalamus. From there the axons of the last neuron reach the cortex, where terminates in the corresponding centers. The lemniscus medialis doesn’t contain auditory, visual and olfactory sensitivity.

Lemniscus lateralis The lateral lemniscus is the part of the central auditory tract. This tract consists of 3 neurons. The I neuron is located in the snail (cochlea) as a ganglion spiralis. The dendrites come from the auditory receptors (Cortii organ), the neurites exit the temporal bone as a cochlear nerve, terminate in the pons in the ventral and dorsal cochlear nuclei (the II neuron). The axons of the II neuron cross over, form the trapezoid body, which continuation outside the pons is called lateral lemniscus. The last one terminates in the inferior colliculi and medial geniculate bodies (the III neuron), which are subcortical auditory centers.The neurites of the III neuron pass through the internal capsule and reach the auditory cortical center, which is located in the superior temporal gyrus (Heshle’s gyrus).

THE PROSENCEPHALON

The forebrain (prosencephalon) develops in association with the olfactory receptor and is at first (in aquatic animals) a purely olfactory part of the brain, i.e. the rhinencephalon. With the change of animals from life in water to life in the atmosphere, the role of the olfactory receptor grows. With the help of this receptor it becomes possible to perceive the presence in the air of chemical substances which make the animal aware from a far distance of prey, danger, and other vitally important natural phenomena; this is a distance receptor.

THE DIENCEPHALON

The diencephalon lies under the corpus callosum and fornix and its sides merge with the hemispheres of the telencephalon. According to the function and development of the prosencephalon, two main parts are distinguished in the diencephalon: (1) the dorsal (phylogenetically younger) part, the thalamencephalon, the centre of the afferent pathways, and (2) the ventral (phylogenetically older) part, the hypothalamus, the higher vegetative centre. The third ventricle is the diencephalic cavity.

THE THALAMENCEPHALON

The thalamencephalon consists, in turn, of three parts: the thalamus, the epithalamus, and the metathalamus. A. The thalamus is a large paired mass of grey matter in the lateral walls of the diencephalon on either side of the third ventricle. It is egg-shaped with the anterior end tapered to form the anterior tubercle (tuberculum antertus); the posterior end is expanded and thickened and forms the pulvinar. The separation into the anterior end and the pulvinar corresponds to the functional division of the thalamus into centres of the afferent tracts (the anterior end) and the centre of vision (the posterior end). The medial surface of the thalamus is demarcated from the dorsal surface by a band of white matter, the stria medullaris thalami. The medial surfaces of both thalami are joined to each other by the interthalamic adhesion (adhesio interthalamica) of grey matter which is located almost in the middle. Some of the afferent sensory tracts running from the lower parts of the central nervous system terminate in its nuclei; the medial lemniscus ends in the lateral nucleus. The thalamus is therefore a subcortical centre of all types of sensitivity. From it the sensory tracts pass partly into the subcortical ganglia (as a consequence of which the thalamus is a sensory centre of the extrapyramidal system) and partly directly into the cortex (the thalamocortical tract). B. The epithalamus. The striae medullares of bath thalami pass to the back (caudally) and form on either side a triangular area (trigonum habenulae). This trigonum gives rise to a rein-like structure, the habenula, which, together with the habenula of the contralateral side, joins with the pineal body (corpus pineale s. epiphysis cerebri). In front of the pineal body both habenulae are united by means of the habenular commissure (commissura habenularum). The pineal body itself, resembling a pine cone in shape (hence its name: L pineus pine cone), is related to the endocrine glands in structure and function. Bulging posteriorly into the mesencephalic region, it is lodged in the groove between the superior colliculi of the tectal lamina, forming as it were a fifth colliculus. C. The metathalmus. Behind the thalamus are two small elevations, the lateral and medial geniculate bodies. The medial geniculate body (corpus geniculatum mediale) is smaller but more pronounced. It lies in front of the inferior brachium under the thalamic pulvinar from which it is separated by a clearly defined sulcus. The fibres of the lateral lemniscus terminate in it, as a consequence of which it is a subcortical auditory centre together with the inferior colliculi. The lateral geniculate body (corpus geniculatum laterale) is a large flat elevation on the inferolateral surface of the pulvinar. Most of the lateral part of the optic tract ends in it (the other part of the tract terminates in the pulvinar). As a result, together with the pulvinar and the superior colliculi, the lateral geniculate body is a subcortical visual centre. The nuclei of both geniculate bodies are connected with the cortical ends of the corresponding analysers through the central tracts.

THE HYPOTHALAMUS

The term hypothalamus, in the broad sense, embraces structures located ventrally under the floor of the third ventricle, in front of the posterior perforated substance. Two parts are distinguished in the hypothalamus: the anterior, optic part ,under which term are united the tuber cinereum with the infundibulum and hypophysis cerebri as well as the optic chiasma with the optic tract, and the posterior, olfactory part, consisting of the mamillary bodies and the subthalamic region. A. The tuber cinereum located in front of the mamillary bodies is an unpaired hollow projection in the floor of the third ventricle and consists of a thin plate of grey matter. The apex of the tuber is tapered to form a narrow hollow infundibulum on whose blind end is the hypophysis cerebri, or the pituitary gland, lodged in a depression on the sella turcica. The tuber cinereum contains nuclei of grey matter which are higher vegetative centres causing an effect, in particular, on metabolism and thermoregulation. B. The optic chiasma (chiasma opticum) lies in front of the tuber cinereum and is formed by the crossing of the optic nerves. C. The mamillary bodies (corpora mamillaria) are two small, white elevations of an irregular spherical shape lying symmetrically on either side of the midline in front of the posterior perforated substance. The mamillary bodies are functionally related to the subcortical olfactory centres. The cavity of diencephalons is the third ventricle (ventriculus tertius),wich lies on the midline; on a frontal section of the brain it has the appearance of a slit-like cavity. The columnae fornicis together with the anterior ends of the thalami form the boundaries of the interventricular foramina, or foramina of Monro, through which the third ventricle communicates with the lateral ventricles situated in the cerebral hemispheres. The aqueduct communicates the third ventricle with the forth ventricle.

THE TELENCEPHALON

The endbrain (telencephalon) consists of two cerebral hemispheres (hemispheria cerebri). Each hemisphere is composed of the pallium, the rhinencephalon, and the basal ganglia. The lateral ventricles (ventriculi laterales) are remnants of the primary cavities of both vesicles of the endbrain. In the process of evolution the endbrain develops most rapidly and intensively among all parts of the central nervous system; in man it becomes the largest part of the brain and acquires the shape of two large hemispheres, the right and left (hemispherum dextrum and sinistrum). At the bottom of the longitudinal fissure of the cerebrum both hemispheres are joined by a thick horizontal plate, the corpus callosum, which is composed of nerve fibres crossing from one hemisphere to the other. The corpus callosum is composed of the following portions: an anterior end curving downward (genu corporis callosi), a middle part (truncus corporis callosi), and a posterior thickened end (splenium corporis callosi). All these parts are clearly defined on a longitudinal section made through the brain between both hemispheres. The genu of the corpus callosum curves downward and tapers to form a beak-like structure, the rostrum corporis callosi, continuous with a thin lamina rostralis continuous, in turn, with the lamina terminalis. Under the corpus callosum is the fornix which is composed of two arched white strands. These are united in the middle part, the body of the fornix (corpus fornicis), but diverge anteriorly and posteriorly to form the columns of the fornix (columnae fornicis) in front and the crura of the fornix (crura fornicis) behind. The crura pass to the back and descend into the inferior horns of the lateral ventricles and are then continuous with the fimbria of the hippocampus (fimbria hippocampi). Between the crura of the fornix, under the splenium of the corpus callosum, stretch transverse bundles of nerve fibres forming the commissure of the fornix (commissura fornicis). The anterior ends of the fornix end in the mamillary bodies. In front of the columns is the anterior commissure (commissura anterior), a white cross-beam of nerve fibres. A thin vertical plate of brain tissue, the septum pellucidum (septum pellucidum), is stretched between the anterior part of the fornix and the genu of the corpus callosum; it has a small cavity, cavum septi pellucidi, within.

THE PALLIUM

Three surfaces are distinguished in each hemisphere, superolateral, medial, and inferior and three ends, or poles, the anterior pole (polus frontalis), the posterior pole (polus occipitalis), and the temporal pole (polus temporalis) corresponding to the projection of the inferior surface and separated from it by the lateral cerebral, or Sylvian, sulcus. The surface of the hemisphere (the pallium) is formed by a regular layer of grey matter 1.3-4.5 mm thick containing nerve cells. This layer, also called the cerebral cortex (cortex cerebri), The deep and constantly present sulci are used as guides to divide each hemisphere into large areas called lobes (lobi). These are, in turn, separated into small lobes and gyri. Each hemisphere has five lobes: frontal (lobus frontalis), parietal (lobus parietalis), temporal (lobus temporalis), occipital (lobus occipitalis), and a small lobe, the insula, hidden in the depth of the lateral sulcus. The superolateral surface of the hemisphere is separated into lobes by three sulci: lateral, central, and the upper end of the parieto-occipital sulcus which, passing on the medial surface of the hemisphere, forms a notch on its superior border. The lateral sulcus (sulcus cerebri lateralis s. Sylvii) arises on the basal surface of the hemisphere. The central, or Rolando's, sulcus (sulcus centralis s. Rolandi) arises on the superior border of the hemisphere, slightly to the back of its middle part, and passes forward and downward. The area of the hemisphere in front of the central sulcus is related to the frontal lobe; part of the brain surface to the back of the central sulcus composes the parietal lobe. The posterior boundary of the parietal lobe is formed by the end of the above- mentioned parieto-occipital sulcus (sulcus parietooccipitalis) located on the medial surface of the hemisphere. Each lobe consists of a number of gyri, which are called lobules at places, bounded by the sulci on the brain surface. The frontal lobe. In the posterior part of the outer surface of this lobe is the precentral sulcus (sulcus precentralis) stretching almost parallel to the central sulcus. It gives rise to two longitudinal sulci, the superior and inferior frontal sulci (sulcus frontalis superior and sulcus frontalis inferior). As a result the frontal lobe is divided into four gyri, one vertical and three horizontal. The vertical, precentral gyrus (gyrus precentralis) is between the central and precentral sulci. The horizontal gyri of the frontal lobe are as follows: (1) the superior frontal gyrus (gyrus frontalis superior) passes above the superior frontal sulcus parallel to the superior border of the hemisphere and onto its medial surface; (2) the middle frontal gyrus (gyrus frontalis medius) stretches between the superior and inferior frontal sulci; and (3) the inferior frontal gyrus (gyrus frontalis inferior) located between the inferior frontal and lateral sulci. The parietal lobe. On this lobe, almost parallel to the central sulcus, is the postcentral sulcus (sulcus postcentralis) which usually blends with the intraparietal sulcus (sulcus intraparietalis) passing horizontally. According to the position of these sulci, the parietal lobe is separated into one vertical and two horizontal gyri. The vertical, postcentral gyrus (gyrus postcentralis) passes behind the central sulcus in the same direction as the precentral gyrus from which it is separated by the central sulcus. Above the intraparietal sulcus is the superior parietal gyrus, or lobule (lobulus parietalis superior) which extends also to the medial surface of the hemisphere. Below the intraparietal sulcus lies the inferior parietal lobule (lobulus parietalis inferior). The anterior part of the inferior parietal lobule bending round the lateral sulcus is called the gyrus supramarginalis; the middle part curving round the superior temporal sulcus is known as the gyrus angularis. The temporal lobe. The lateral surface of this lobe carries three longitudinal gyri separated from one another by the superior and inferior temporal sulci (sulcus temporalis superior and sulcus temporal is inferior). The superior temporal gyrus (gyrus temporal is superior) is between the lateral and the superior temporal sulci. Its superior surface hidden deep in the lateral sulcus has three short transverse temporal gyri (gyri temporales transversi), or Heschl's convolutions. Between the superior and inferior temporal sulci stretches the middle temporal gyrus (gyrus temporal is medius). Below the last-named, and separated from it by the inferior temporal sulcus, is the inferior temporal gyrus (gyrus temporalis inferior) forming the junction between the lateral and inferior surfaces of the temporal lobe. The occipital lobe. The sulci on the lateral surface of this lobe are variable and inconstant. The insula. This lobule becomes visible when the margins of the lateral sulcus which overhang it are drawn apart or removed. These margins are related to the frontal, parietal, and temporal lobes and are termed the operculum. The surface of the insula is covered by several short gyri. The medial surface of the hemisphere. This surface carries the callosal sulcus (sulcus corporis callosi) which passes directly above the corpus callosum and is continuous posteriorly with the deep hippocampal sulcus (sulcus hippocampi) running forward and downward. Parallel to and above the callosal sulcus extends the sulcus cinguli which arises in front under the rostrum of the corpus callosum and passes to the back, its posterior end terminating on the superior border of the hemisphere. A small area above the sulcus cinguli, bounded by its posterior end at the back and by a small paracentral sulcus (sulcus paracentralis) in front, is called the paracentral lobule (lobulus paracentralis). To the back of the paracentral lobule is a quadrangular surface, the precuneus, bounded anteriorly by the end of the sulcus cinguli, inferiorly by the small subparietal sulcus (sulcus subparietalis).Behind it is a sharply demarcated area of cortex related to the frontal lobe; this Is the cuneus, bounded anteriorly by the parieto-occipital and posteriorly by the calcarine sulci (sulcus calcarinus) which converge at an angle. Inferiorly and posteriorly; the cuneus is in contact with the medial occipitotemporal gyrus. Between the sulcus cinguli and the callosal sulcus stretches the gyrus cinguli which by means of the isthmus is continuous with the parahippocampal gyrus terminating as a hook-like structure, the uncus. The gyrus cinguli, isthmus, and hippocampal gyrus constitute the gyrus fornicatus describing almost a complete circle open only below and in front. The gyrus fornicatus is not related to any of the lobes of the cortex.

THE LATERAL VENTRICLES

Two lateral ventricles (ventriculi laterales) are situated below the level of the corpus callosum on either side of the midline in the hemispheres as remnants of the primary cavities of both vesicles of the endbrain. The cavity of each lateral ventricle corresponds to the shape of the hemisphere: it begins in the frontal lobe as the anterior horn (cornu anterius) curving downward and laterally; then its central part (pars centralis) stretches through the parietal lobe and at the level of the posterior margin of the corpus callosum turns downward and passes forward in the temporal lobe as the inferior horn (cornu inferius). The descending part of the ventricle gives off a projection to the back, into the occipital lobe – this is the posterior horn (cornu posterius).

THE BASAL (CENTRAL) GANGLIA (NUCLEI) OF THE HEMISPHERES

In addition to the grey cortex on the surface of the hemisphere, masses of grey matter are present in the depth of its tissue. These are called basal, central, or subcortical nuclei, in brief the “subcortex”. As distinct from the cortex which has the structure of screen centres, the subcortical nuclei possess the structure of nuclear centres. Three conglomerates of subcortical nuclei are distinguished: corpus striatum, claustrum, and the amygdaloid nucleus. 1. Corpus striatum consists of two parts, the caudate and lentiform nuclei, which are incompletely separated one from the other. A. The caudate nucleus (nucleus caudatus) lies above and medial to the lentiform nucleus and is separated from it by a layer of white matter called the internal capsule (capsula interna). The thickened anterior part of the nucleus, its head (caput nuclei caudati), while its attenuated parts, the body and tail (corpus and cauda nuclei caudati), stretch to the back. Medially the caudate nucleus adjoins the thalamus. These two nuclei on the ventral side, there are fine bands of grey matter joining them dorsally. These bands, alternating with the white bundles of the internal capsule, are responsible for the name corpus striatum. B. The lentiform nucleus (nucleus lentiformis) lies lateral to the caudate nucleus and the thalamus and is separated from them by the internal capsule. On a horizontal section through the hemisphere the medial surface of the lentiform nucleus facing the internal capsule has the shape of an angle whose apex is directed centrally, the anterior side is parallel to the caudate nucleus, and the posterior side is parallel to the thalamus. Anteriorly and ventrally the lentiform nucleus is fused with the head of the caudate nucleus. Two parallel white layers divide the lentiform nucleus into three segments, one lateral grey segment called the putamen (L paring) and two medial lighter coloured segments united under the term globus pallidus. The globus pallidus has a distinctive macroscopic appearance and also differs from the other parts of the corpus striatum histologically. It is phylogenetically older (palaeostriatum) than the putamen of caudate nucleus (neostriatum). In view of all these features, the globus pallidus is now distinguished as a specific morphological structure, the pallidum, while the term striatum is used as a designation only for the putamen and the caudate nucleus. As a consequence, the term “lentiform nucleus” loses its initial meaning and should be used only in a purely topographical sense; the caudate and lentiform nuclei are now called the striapallidal system instead of corpus striatum. This system is the principal part of the extrapyramidal system (see below) and, besides, it is the higher centre of control of vegetative functions concerned with thermoregulation and carbohydrate metabolism and predominates over similar vegetative centres in the hypothalamus. 2. The claustrum (L barrier) is a thin sheet of grey matter in the region of the insula between it and the putamen. Certain authors do not relate the claustrum to the basal ganglia but consider it to be a deep layer of the insular cortex which became separated with the development of the capsula extrema. According to another point of view the claustrum is an absolutely independent structure related to the basal ganglia in origin. 3. The amygdaloid nucleus (corpus amygdaloideum), or the epistriatum, lies under the putamen in the anterior end of the temporal lobe. It does not reach the temporal pole, but lies in front of the apex of the inferior horn of the lateral ventricle. Morphologically, the amygdaloid nucleus is a postero- ventral continuation of the claustrum. It is evidently a subcortical olfactory centre where a bundle of fibres passing from the olfactory lobe and the anterior perforated substance terminates.

THE WHITE MATTER OF THE HEMISPHERES

The white matter occupies the whole space between the grey matter of the cerebral cortex and the basal ganglia. It consists of a great number of nerve fibres stretching in different directions and forming the conduction pathways of the telencephalon. The following three systems of nerve fibres are distinguished: (1) association fibres, (2) commissural fibres, and (3) projection fibres. A. The association fibres connect different cortical areas of one hemisphere. Short and long fibres are distinguished. The short fibres (fibrae arcuatae cerebri) connect neighbouring gyri by means of arched bundles. The long fibres connect cortical areas located at a greater distance from one another. There are several bundles of these fibres. The cingulum, a girdleshaped bundle of fibres passing in the gyrus fornicatus, connects different; areas of the gyrus cinguli cortex one with the other and also with the adjacent gyri of the medial surface of the hemisphere. The frontal lobe is connected with the inferior parietal lobule, the occipital lobe, and the posterior part of the temporal lobe by the superior longitudinal bundle (fasciculus longitudinalis superior). The inferior longitudinal bundle (fasciculus longitudinalis inferior) connects the temporal and occipital lobes. Finally, the uncinate bundle (fasciculus uncinatus) connects the orbital surface of the frontal lobe with the temporal pole. B. The commissural fibres, components of the cerebral commissures, connect symmetrical parts of both hemispheres. The largest cerebral commissure, the corpus callosum, joins parts of both hemispheres. Two cerebral commissures, the anterior and the hypocampal commissures (commissura anterior and commissura fornicis), are much smaller and related to the rhinencephalon. The anterior commissure connects the olfactory lobes and both hippocampal gyri, the fornix commissure connects the hippocampi. C. The projection fibres connect the cerebral cortex partly with the thalamus and the geniculate bodies and partly with the distally located parts of the nervous system, the spinal cord among others. Some of these fibres conduct stimuli centripetally, i.e. towards the cortex, others, on the contrary, centrifugally. The projection fibres in the white matter of the hemisphere nearer to the cortex form the corona radiata after which their main part converges to be continuous with the internal capsule. The internal capsule is a layer of white matter between the lentiform nucleus on one side and the caudate nucleus and thalamus on the other. On a horizontal section it is curved to form an angle open laterally. As a result the following parts are distinguished in the internal capsule: the anterior limb (crus anterius capsulae internae) between the caudate nucleus and the anterior half of the internal surface of the lentiform nucleus, the posterior limb (crus posterius) between the thalamus and the posterior half of the lentiform nucleus, and, finally, the genu capsulae internae at the bend formed at the junction of both parts of the internal capsule. According to length, the projection fibres can be grouped into the following systems (listed beginning with the longest). The cerebrospinal (pyramidal) tract (tractus corticospinalis s. pyramidalis) conducts voluntary motor impulses to the muscles of the trunk and limbs. Having arisen from the cortical pyramidal cells of the middle and superior parts of the anterior central gyrus and paracentral lobule, the fibres of the pyramidal tract pass in the corona radiata and then through the internal capsule in which they occupy the anterior two thirds of its posterior part, the fibres for the upper limb passing in front of those for the lower limb. After that, they extend through the crus of the cerebrum, through the pons, and into the medulla oblongata. 2. The corticonuclear tract (tractus corticonuclearis) is the conducting pathway to the nuclei of the cranial nerves. The fibres arise from the cortical pyramidal cells of the inferior part of the anterior central gyrus, pass through the genu of the internal capsule and through the cerebral peduncle, then enter the pons, and passing to the opposite side terminate there in the motor nuclei and form a decussation. A few of the fibres terminate without decussating. Since all motor fibres are gathered on a small area in the internal capsule (in its genu and anterior two thirds of the posterior limb) injury to them here results in unilateral paralysis (hemiplegia) of the opposite side of the body. 3. The cerebropontine tract (tractus corticopontinus) passes from the cerebral cortex to the nuclei of the pons. The fibres of the tract arise from the cortex of the frontal lobe (frontopontine tract, tractus frontopontinus), occipital lobe (occipitopontine tract, tractus occipitopontinus), temporal lobe (temporopontine tract, tractus temporopontinus), and parietal lobe (parietopontine tract, tractus parietopontinus). Fibres passing from the pontine nuclei in the middle cerebellar peduncles to the cerebellum are a continuation of these tracts. Through them the cerebral cortex produces an inhibiting and regulating effect on the activity of the cerebellum. 4. The thalamocortical and corticothalamic fasciculi (fasciculi thalamocorticalis and corticothalamici) pass from the thalamus to the cortex and in the opposite direction, from the cortex to the thalamus. Among fibres stretching from the thalamus, the tegmental fasciculus deserves mention. It is the terminal part of a sensory tract passing to the centre of skin sense in the posterior central gyrus. Arising from the lateral thalamic nucleus the fibres of this tract pass through the posterior part of the internal capsule behind the pyramidal tract. This site was termed the “sensory decussation” because other sensory tracts also pass here, the optic radiation (radiatio optica) stretching from the lateral geniculate body and thalamic pulvinar to the visual centre in the cortex of the occipital lobe, then the auditory radiation (radiatio acustica) stretching from the medial geniculate body and inferior quadrigeminal body to Heschl's gyri in the temporal lobe where the auditory centre is located. The visual and auditory fasciculi occupy the extreme posterior position in the posterior part of the internal capsule.

MORPHOLOGICAL BASIS OF DYNAMIC LOCALIZATION OF FUNCTIONS IN THE CEREBRAL CORTEX (CENTRES OF THE CEREBRAL CORTEX)

Knowledge of the location of functions in the cerebral cortex is of great theoretical importance because it gives an idea of the nervous regulation of all processes in the body and its adaptation to the environment. It is also of great practical importance for identifying the sites of lesions in the cerebral hemispheres. Our idea of the location of functions in the cerebral cortex is primarily linked with the concept of the cortical centre. According to Pavlov, the centre is the cerebral end of an analyser. We shall deal first of all with the cortical ends of the internal analysers. 1. The nucleus of the motor analyzer lies in the anterior central gyrus. Here in the third layer of the cortex terminates the spino-cortical tract. In its deep layers (the fifth and partly the sixth) lie the giant pyramidal cells of Betz which are efferent neurons and from these layers starts the cortico-spinal or pyramidal tract. 2. Sensory center lies in the cortex of the posterior central gyrus. Here terminates the thalamo- cortical tracts which carries ( conducts) the exteroception. 3. Auditory centre lies in the posterior part of the superior temporal gyrus in the gyri of Heshli. 4. Visual centre lies in the occipital lobe on the cortex of the calcarine sulcus. 5. The nucleus of the olfactory analyser lies on the medial surface of the brain in Within the limits of the base the uncus and partly the hippocampus.

THE PERIPHERAL PART OF THE NERVOUS SYSTEM

ANIMAL, OR SOMATIC, NERVES

The nerve trunks are divided according to the place where they branch off from the central nervous system: the spinal nerves (nn. spinales) branch off from the spinal cord while the cranial nerves (nn. craniales) arise from the brain.

THE CRANIAL NERVES (NN. CRANIALES)

There are twelve pairs of cranial nerves: I, n. olfactorius; II, n. opticus; III, n. oculomotorius; IV, n. trochlearis; V, n. trigeminus; VI, n. abducens; VII, n. facialis; VIII, n. vestibulocochlearis; IX, n. glossopharyngeus; X, n. vagus; XI, n. accessorius; XII, n. hypoglossus.

THE HYPOGLOSSAL (12th) NERVE

The hypoglossal nerve is a muscle nerve containing efferent (motor) fibres to the muscles of the tongue and efferent (proprioceptive) fibres from the receptors of these muscles. It also contains sympathetic fibres from the superior cervical sympathetic ganglion; it has connections with the lingual nerve, with the inferior ganglion of the vagus nerves, and with the first and second cervical nerves. The only somatic-motor nucleus of a nerve laid down in the medulla oblongata, in the region of the trigonum of the hypoglossal nerve of the rhomboid fossa, descends through the medulla oblongata to the first-second cervical segment; it is a component of the reticular formation system. Appearing on the base of the brain between the pyramid and olive by several roots, the nerve then passes through the hypoglossal canal of the occipital bone, canalis (nervi) hypoglossi, descends along the lateral side of the a. carotis interna, runs under the posterior belly of the ill. digastricus and proceeds in the form of an arch, with convexity down, along the lateral surface of the m. hyoglossus. Here the arch of the hypoglossal nerve is limited by Pirogoff’s triangle at the top. In a high position of the arch of the hypoglossal nerve Pirogoff’s triangle has a larger area, and vice versa. At the anterior edge of the m. hyoglossus the hypoglossal nerve separates into its terminal branches, which enter the muscles of the tongue. Some of the fibres of the hypoglossal nerve become part of the branches of the facial nerve running to the orbicularis oris muscle; when the nucleus of the nerve is damaged, the function of the muscle becomes impaired. One of the branches of the nerve, radix superior, descends to join the radix inferior of the cervical plexus so that they both form the ansa hypoglossi (ansa cervicalis), which innervates the muscles located under the hyoid bone and m. geniohyoideus.

THE TRIGEMINAL (5th) NERVE

The trigeminal nerve (n. trigeminus) develops in association with the first visceral arch (mandibular) and is mixed. Its sensory fibres supply the skin of the face and the anterior part of the head, bordering posteriorly with the skin area where the posterior branches of the cervical nerves and the branches of the cervical plexus are distributed. The cutaneous branches (posterior) of the second cervical nerve reach into the territory of the trigeminal nerve, thus creating a border zone of mixed innervation 3-4 cm in width. The trigeminal nerve is also a conductor of sensitivity from the receptors of the mucous membranes of the mouth, nose, ear and conjunctiva of the eye, except those parts of these organs which act as specific receptors of the senses (innervated by the first, second, seventh, eighth and ninth pairs). As a nerve of the first visceral arch, the trigeminal nerve innervates the muscles of mastication developing from it and the muscles of the floor of the mouth; it also contains the afferent (proprioceptive) fibres arising from the receptors of these muscles; the afferent fibres end in the mesencephalic nucleus (nucleus tractus mesencephalici n. trigemini). Moreover, the branches of the nerve contain secretory (vegetative) fibres to the glands located in the cavities of the face. Since the trigeminal nerve is a mixed nerve, it has tour nuclei of which two sensory and one motor nuclei are in the metencephalon, while the sensory (proprioceptive nucleus) is in the mesencephalon. The processes of cells contained in the motor nucleus (nucleus motorius) emerge from the pons on the line separating the pons from the middle cerebellar peduncle and connecting the place of emergence of the trigeminal and facial nerve (linea trigeminofacialis), forming the motor root of the nerve, radix motoria. Next to it the sensory root, radix sensoria, enters the brain matter. Both roots comprise the trunk of the trigeminal nerve, which on emergence from the brain penetrates under the dura mater of the middle cranial fossa and is distributed onto the superior surface of the pyramid of the temporal bone at its apex where the trigeminal impression (impressio trigemini) is located. Here the dura mater separates to form a small cavity for it, cavum trigeminale. In this cavity the sensory root has a large semilunar (or Gasser's) ganglion, ganglion trigeminale (s. semilunare Gasseri). The central processes of the cells of this ganglion comprise the sensory root and run to the sensory nuclei: superior sensory nucleus (nucleus sensorius principalis n. trigemini), spinal nucleus (nucleus tractus spinalis n. trigemini) and mesencephalic nucleus (nucleus tractus mesencephalicis n. trigemini), while the peripheral processes are part of the three main branches (divisions) of the trigeminal nerve emerging from the convex edge of the ganglion. These branches are as follows: the first, or ophthalmic nerve (n. ophthalmicus), the second, maxillary nerve (n. maxillaris), and the third, mandibular nerve (n. mandibularis). The motor root of the trigeminal nerve takes no part in forming the ganglion. It runs freely under the latter and then joins the third branch. Each of the three branches of the trigeminal nerve sends a thin branch to the dura mater. In the region of the ramification of each of the three branches of the trigeminal nerve there are several small nerve ganglia which belong to the vegetative nervous system but which are usually described relative to the trigeminal nerve. These vegetative (parasympathetic) ganglia developed from cells, which settled during embryogenesis along the course of the trigeminal nerve branches; this explains their lifetime connection, namely, with n. ophthalmicus through ganglion ciliare, with n. maxillaris through g. pterygopalatinum, with n. mandibularis through g. oticum, and with n. lingualis (from the third branch) through g. submandibulare.

The First Branch (Division) of the Trigeminal Nerve

The ophthalmic nerve (n. ophthalmicus) passes out of the cranial cavity into the orbit through the fissura orbitalis superior, but prior to its entrance it divides again into three branches: n. frontalis, n. lacrimalis and n. nasociliaris. 1. The frontal nerve (n. frontalis) passes directly forward under the roof of the orbit through the incisura (or foramen) supraorbitalis into the skin of the forehead, where it is known as the supra-orbital nerve (n. supra-orbitalis). On its way it gives rise to branches supplied to the skin of the upper eyelid and medial angle of the eye. 2. The lacrimal nerve (n. lacrimalis) passes to the lacrimal gland and on passing through it ends in the skin and conjunctiva of the lateral angle of the eye. Prior to entering the gland the lacrimal nerve joins with the zygomatic nerve (from the second branch of the.trigeminal nerve). Through this “anastomosis” the lacrimal nerve receives secretory fibres for the lacrimal gland and also supplies it with sensory fibres. 3. The nasociliary nerve (n. nasociliaris) innervates the anterior part of the nasal cavity (nn. ethmoidales anterior and posterior), eyeball (nn. ciliares longi), the skin of the medial angle of the eye, conjunctiva and the lacrimal sac (n. infratrochlearis). It also gives rise to the connective branch to the ciliary ganglion. The ophthalmic nerve is responsible for the sensory (proprioceptive) innervation of the ocular muscles by means of communications with the third, fourth and sixth nerves. The ciliary ganglion (ganglion ciliare) has the shape of an oblong lump about 1.5 mm long; it lies in the posterior part of the orbit on the lateral side of the optic nerve. In this ganglion, which belongs to the vegetative nervous system, there is an interruption of the parasympathetic fibres running from the Yakubovich nucleus as part of the oculomotor nerve to the smooth muscles of the eye. From three to six short ciliary nerves arise from the anterior end of the ganglion which pierce the sclera of the eyeball around the optic nerve and pass inside the eye. The parasympathetic fibres mentioned above pass through these nerves (after their interruption in the ganglion) to the m. sphincter pupillae and m. ciliaris.

The Second Branch of the Trigeminal Nerve

The maxillary nerve (n. maxillaris) emerges from the cranial cavity through the foramen rotundum into the pterygopalatine fossa; here it is directly continuous with the infra-orbital nerve (n. infraorbitalis) which passes through the inferior orbital fissure into the orbital sulcus and canal on the inferior orbital wall and then emerges through the infraorbital foramen onto the face where it ramifies into a bundle of branches. These branches join partly with the branches of the facial nerve and innervate the skin of the lower eyelid, the lateral surface of the nose and the upper lip. The following branches arise from the maxillary nerve and its continua ion the infra-orbital nerve. 1. The zygomatic nerve (no zygomaticus) to the skin of the cheek and the anterior part of the temporal region. It anastomoses with the lacrimal nerve (from the first branch of the trigeminal nerve). 2. The superior dental nerves (nn. alveolares superiores) form a plexus in the thickness of the maxilla – the superior dental plexus (plexus dentalis superior) from which superior dental branches arise to the upper teeth and the superior gingival branches to the gums. 3. The ganglionic branches (nn. pterygopalatini), several (two-three) short branches connecting the maxillary nerve with the sphenopalatine ganglion. The sphenopalatine ganglion (ganglion pterygopalatinum) is located in the pterygopalatine fossa medially and down from the maxillary nerve. In this ganglion, which relates to the vegetative nervous system, the parasympathetic fibres are interrupted; they run from the vegetative nucleus of the sensory root of the facial nerve (n. intermedius) to the lacrimal gland and to the mucous glands of the nose and palate as part of the nerve itself and further as the greater superficial petrosal nerve (a branch of the facial nerve). The sphenopalatine ganglion gives off the following (secretory) branches. (1) Nasal branches (rami nasales posteriores) pass through the sphenopalatine foramen to the mucosal glands of the nose; the largest of these, the long sphenopalatine nerve (n. nasopalatinus) passes through the incisive canal to the mucous glands of the hard palate; (2) the palatine nerves (nn. palatini) descend along the greater palatine canal and, after passing through the greater and lesser palatine foramina, innervate the mucosal glands of the hard and soft palates. In addition to the secretory fibres the nerves arising from the sphenopalatine ganglion contain also sensory nerves (from the second branch of the trigeminal nerve) and sympathetic fibres. Thus, the fibres of the sensory root (the parasympathetic part of the facial nerve), which pass along the greater superficial petrosal nerve, through the sphenopalatine ganglion, innervate the glands of the nasal cavity and palate and the lacrimal gland. The last pathway runs from the sphenopalatine ganglion through the ganglionic branches of the maxillary nerve (nn. pterygopalatini) into the zygomatic nerve, and from it through an anastomosis into the lacrimal nerve.

The Third Branch of the Trigeminal Nerve

The mandibular nerve (n. mandibularis) contains, in addition to the sensory root, the whole motor root of the trigeminal nerve. The motor root arises from the motor nucleus and passes to the muscles originating from the maxillary arch. As a result the mandibular nerve innervates the muscles attached to the mandible, the skin covering it and other derivatives of the maxillary arch. On emerging from the cranium through the foramen ovale, it divides into two groups of branches. A. Muscle branches. To the muscles of the same name; nerve to the masseter (n. massetericus), deep temporal nerves (nn. temporales profundi), nerves to the medial and lateral pterygoid muscles (nn. pterygoidei medialis and lateralis), nerve to the tensor tympani muscle (n. tensoris tympani), nerve to the tensor palati muscle (n. tensoris veli palatini), mylohyoid nerve (n. mylohyoideus); the latter arises from the inferior dental nerve (n. alveolaris inferior), a branch of the mandibular nerve and also innervates the anterior belly of the digastric muscle. B. Sensory branches. 1. The buccal nerve (n. buccalis) to the mucosa of the cheek. 2. The lingual nerve (n. lingualis) descends along the medial side of the m. pterygoideus medialis and lies under the mucous membrane of the floor of the oral cavity. After giving off the sublingual nerve to the mucosa of the floor of the mouth it innervates the mucosa of the anterior two thirds of the back of the tongue. At the place where the lingual nerve passes between both pterygoid muscles, it is joined by a small thin branch of the facial nerve, chorda tympani, which emerges from the squamotympanic fissure. It contains parasympathetic secretory fibres that arise from the superior salivary nucleus of the sensory root of the facial nerve and pass to the hypoglossal and submaxillary salivary glands. It also contains gustatory fibres from the first two thirds of the tongue. The fibres of the lingual nerve itself distributed in the tongue are conductors of general sensitivity (sense of touch, pain, and temperature). 3. The inferior dental nerve (n. alveolar is inferior), together with the artery of the same name, passes through the foramen mandibulae into the mandibular canal where after forming the inferior dental plexus it gives off branches to all the lower teeth. At the front end of mandibular canal the inferior dental nerve gives off a thick branch, the mental nerve (n. mentalis), which emerges through the foramen mentale and spreads in the skin of the chin and the lower lip. The inferior dental nerve is a sensory nerve with a small addition of motor fibres which leave it at the foramen mandibulae as part of the mylohyoid nerve (see above). 4. The auriculotemporal nerve (n. auriculotemporalis) penetrates into the upper part of the parotid gland and, turning upward, passes to the temporal region accompanying the superficial temporal artery. Along the way the nerve gives off secretory branches to the parotid and salivary glands (whose origin is discussed below) and sensory branches to the temporomandibular articulation, to the skin of the anterior part of the concha of the auricle and the external acoustic meatus. The terminal branches of the auriculotemporal nerve supply the skin of the temple. In the region of the third branch of the trigeminal nerve there are two ganglia belonging to the vegetative system by means of which the salivary glands are innervated. One of them, the otic ganglion (ganglion oticum), is a small round body located under the foramen ovale on the medial side of the mandibular nerve. It receives parasympathetic secretory fibres in the composition of the lesser superficial petrosal nerve which is a continuation of the tympanic nerve originating from the glossopharyngeal nerve. These fibres are interrupted in the ganglion and pass to the parotid gland by means of the auriculotemporal nerve, with which the otic ganglion is joined. Another small ganglion, the submandibular ganglion (ganglion submandibulare), is located at the anterior edge of the m. pterygoideus medialis, above the submandibular salivary gland, under the lingual nerve. The ganglion is connected with the lingual nerve by branches. By means of these branches the fibres of chorda tympani pass to the ganglion where they terminate; their continuation become the fibres arising from the submandibular ganglion, which innervate the submaxillary and sublingual salivary glands.

THE FACIAL (7th) NERVE

The facial nerve (n. facialis, s. intermedio-facialis) is a mixed nerve. As a nerve of the second visceral arch it innervates the muscles developing from it, namely, all the facial-expression and part of the sublingual muscles. It also contains the efferent (motor) fibres emerging from its motor nucleus that pass to these muscles, and the afferent (proprioceptive) fibres arising from their receptors. It includes gustatory (afferent) and secretory (efferent) fibres belonging to the n. intermedius (see below). Corresponding to its components, the facial nerve has three nuclei located in the pons varolii: the motor nucleus (nucleus [motorius] nervi facialis), the sensory nucleus (nucleus tractus solitarii), and the secretory nucleus, superior salivatory nucleus (nucleus salivatorius superior). The last two nuclei belong to the nervus intermedius. The facial nerve emerges to the surface of the brain laterally along the posterior edge of the pons on the linea trigeminofacialis, next to the auditory nerve (n. vestibulocochlearis). Then, together with the latter nerve, the facial nerve penetrates the porus acusticus internus and enters the canal for the facial nerve (canalis facialis Fallopii). In the canal the nerve first runs horizontally to the outside; then, in the region of the hiatus it turns at a right angle to the back and, still lying horizontally, passes along the inner wall of the tympanic cavity in its upper part. It still lies in the bone canal and is separated from the tympanic cavity by a bony plate. Outside of the tympanic cavity the facial nerve again curves and descends vertically, emerging from the cranium through the foramen stylomastoideum. In the place where the nerve turns back to form an angle (geniculum), its sensory (gustatory) segment forms a small nervous ganglion (ganglion geniculi). On emerging from the foramen stylomastoideum the facial nerve enters the thickness of the parotid gland and separates into its terminal branches. In the canal of the temporal bone the facial nerve gives rise to the following branches. 1. The greater superficial petrosal nerve (n. petrosus major) (secretory nerve) originates in the region of the genu and emerges through the foramen of the greater superficial petrosal nerve. Then it passes along the groove of the same name on the anterior surface of the pyramid of the temporal bone, sulcus n. petrosi majoris, and passes into the pterygoid canal together with the sympathetic nerve, deep petrosal nerve (n. petrosus profundus) forming with it a common nerve of the pterygoid canal and reaches the sphenopalatine ganglion. The nerve is interrupted in the ganglion and as rami nasales posteriores and the palatine nerves (nn. palatini) passes to the mucosal glands of the nose arid palate; part of the fibres of the zygomatic nerve (from the maxillary nerve) through connections with the lacrimal nerve reach the lacrimal gland. 2. The nerve to the stapedius muscle (n. stapedius) (muscular) innervates the stapedius muscle. 3. The chorda tympani (mixed branch) separates from the facial nerve in the lower part of the facial canal, enters the tympanic cavity, fits there onto the medial surface of the tympanic membrane and then leaves through the fissura petrotympanica. On emerging from the fissure it descends forward and joins the lingual nerve. The sensory (gustatory) part of the chordae tympani (peripheral processes of cells contained in the ganglion of the facial nerve) as a component of the lingual nerve runs to the mucosa of the tongue supplying its anterior two thirds with gustatory fibres. The secretory part approaches the submandibular ganglion and after the interruption it supplies the submavdibular and hypoglossal salivary glands with secretory fibres. After leaving the foramen stylomastoideum the several muscle branches separate from the facial nerve and supply the muscles of expression.

THE AUDITORY (8th) NERVE

The auditory nerve (n. vestibulocochlearis s. statoacusticus, BNA), an afferent nerve which separated away from the facial nerve, contains somatic-sensory fibres running from the organ of hearing and balance. It consists of two parts – the vestibular nerve (pars vestibularis) and the cochlear nerve (pars cochlearis), which differ in their functions: the vestibular nerve conducts impulses from the static apparatus laid out in the vestibule and the semicircular canals of the labyrinth of the internal ear, while the cochlear nerve conducts acoustic impulses from the organ of Corti in the cochlea which receives acoustic stimuli. Since these are all sensory nerves, each of them has its own nerve ganglion containing bipolar nerve cells. The ganglion of the vestibular nerve called the vestibular ganglion (ganglion vestibulare) lies on the floor of the internal acoustic meatus, while the ganglion of the cochlear nerve, spiral ganglion of the cochlea (ganglion spirale) is located in the cochlea. The peripheral processes of the bipolar cells of the ganglia terminate . in the receptors of the above mentioned parts of the labyrinth; this will be discussed in detail in the chapter on sensory organs (see “The Organ of Hearing and Balance”). The central processes that emerge from the internal ear through the porus acusticus internus pass as components of the corresponding part of the nerve to the brain. They enter it lateral to the facial nerve reaching their nuclei: the vestibular nerve – four nuclei and the cochlear nerve two nuclei.

THE GLOSSOPHARYNGEAL (9th) NERVE

The glossopharyngeal nerve (n. glossopharyngeus) is a nerve of the third visceral arch, which in the process of development separated from the tenth pair of nerves, the vagus nerve. It consists of three types of fibres: (1) afferent (sensory) fibres running from the receptors of the pharynx, the tympanic cavity, mucosa of the tongue (the posterior third), tonsils and palatal arches; (2) efferent (motor) fibres innervating one of the muscles of the pharynx (m. stylopharyngeus); (3) efferent (secretory) fibres, parasympathetic for the parotid gland. Corresponding to its components it has three nuclei: the nucleus tractus solitarii, to which come the central processes of the cells of two afferent ganglia: ganglion superius and inferius (see below). The vegetative (secretory) parasympathetic nucleus, the inferior salivary nucleus (nucleus salivatorius inferior) consists of cells distributed in the reticular formation around the third nucleus, motor, which is common with the vagus nerve, nucleus ambiguus. The glossopharyngeal nerve with its roots originates from the medulla oblongata behind the olive, above the vagus, and, together with the latter, leaves the cranium through the foramen jugulare. Within the limits of the latter the sensory part of the nerve forms a ganglion, ganglion superius, and on emerging from the foramen it forms another ganglion, ganglion inferius, which lies on the inferior surface of the pyramid of the temporal bone. The nerve descends first between the internal jugular vein and the internal carotid artery, and then bends around and behind the m. stylopharyngeus and, along the lateral side of this muscle, approaches the root of the tongue in a slanting arch where it divides into its terminal branches.

The branches of the glossopharyngeal nerve

1. The tympanic nerve (n. tympanicus) branches away from the ganglion inferius and penetrates the tympanic cavity (cavum tympani) where it forms a plexus, plexus tympanicus, which receives branches from the sympathetic plexus of the internal carotid artery. This plexus innervates the mucous membrane of the tympanic cavity and the auditory tube. On leaving the tympanic cavity through the superior wall as the lesser superficial petrosal nerve (n. petrosus minor), the nerve passes in a sulcus of the same name (sulcus n. petrosi minoris) over the anterior surface of the pyramid of the temporal bone and reaches the otic ganglion. Through this nerve the otic ganglion receives parasympathetic secretory fibres coming from the inferior salivatory nucleus for the parotid gland. After interruption within the ganglion the secretory fibres reach the gland as part of the auriculotemporal nerve from the third branch of the trigeminal nerve (see “The Third Branch of the Trigeminal Nerve”). 2. Branch to the stylopharyngeus (ramus m. stylopharyngei) to the muscle of the same name. 3. Tonsilar branches (rami tonsillares) to the mucosa of the faucial tonsils and arches. 4. Pharyngeal branches (rami pharyngei) to the pharyngeal plexus (plexus pharyngeus). 5. Lingual branches (rami linguales), the terminal branches of the glossopharyngeal nerve, to the mucosa of the posterior third of the tongue supplying it with sensory fibres, among which the gustatory fibres also pass to the papillae vallatae.

THE VAGUS (10th) NERVE

The vagus nerve (n. vagus) developed from the fourth and subsequent visceral arches. It was given such a name because it is the longest of the cranial nerves. With its branches the vagus nerve supplies the respiratory organs, a considerable part of the digestive tract (up to the colon sigmoideum) and also gives off branches to the heart which receives fibres that slow down the heart beat. The vagus nerve consists of three types of fibres. 1. Afferent (sensory) fibres emerging from the receptors of internal organs and vessels described above, as well as from a certain part of the dura mater and the external auditory meatus with the concha auriculae to the sensory nucleus, nucleus tractus solitarii. 2. Efferent (motor) fibres for striated muscles of the pharynx, the soft palate and the larynx and the afferent (proprioceptive) fibres arising from the receptors of these muscles. These muscles receive fibres from the motor nucleus (nucleus ambiguus). 3. Efferent (parasympathetic) fibres originating in the vegetative nucleus – the dorsal nucleus of the vagus nerve (nucleus dorsalis n. vagi). They run to the striated muscles of the heart (slowing down the heart beat) and to the smooth muscles of the vessels (dilating vessels). Moreover, the cardiac branches of the vagus nerve include the n. depressor which is a sensory nerve for the heart itself and the initial segment of the aorta and is concerned with the reflex control of blood pressure. The parasympathetic fibres also innervate the trachea and lungs (they constrict the bronchi), oesophagus, stomach, and intestine up to the colon sigmoideum (intensify peristalsis), the glands situated in these organs and the glands of the abdominal cavity: liver, pancreas (secretory fibres), and kidneys. The parasympathetic portion of the vagus nerve is very big and as a consequence it is predominantly a vegetative nerve, very important for the organisms's vital functions. Fibres of all kinds connected with the three main nuclei of the vagus nerve arise from the medulla oblongata. They pass into its sulcus lateralis posterior below the glossopharyngeal nerve in ten to fifteen roots, which form the thick trunk of the nerve. This trunk together with the glossopharyngeal and accessory nerves leaves the cranium through the foramen jugulare. In the jugular orifice the sensory part of the nerve forms a small ganglion (ganglion superius) and on leaving this orifice it forms another fusiform swelling, the ganglion interius. Both ganglia contain pseudounipolar cells whose peripheral processes are components of the sensory branches running to these ganglia from receptors of the internal organs and vessels (ganglion inferius) and the external auditory meatus (ganglion superius); the central processes group together in a singular bundle which ends in the sensory nucleus (nucleus tractus solitarii). On leaving the cranial cavity, the vagus nerve descends to the neck behind the vessels into a groove, first between the internal jugular vein and internal carotid artery, and then lower between the same vein and the common carotid artery; it lies in the same sheath as the vessels mentioned above. Further the vagus nerve enters through the superior thoracic aperture into the thoracic cavity where its right trunk lies in front of the subclavian artery, while the left trunk extends on the anterior surface of the aortic arch. Descending further, both vagus nerves by-pass the root of the lung on both sides dorsally, and then accompany the oesophagus, forming plexuses on its walls; the left nerve runs along the anterior surface and the fight nerve along the posterior surface. Together with the oesophagus both vagus nerves pass through the oesophageal hiatus into the abdominal cavity where they form plexuses on the stomach walls. The trunks of the vagus nerves in the uterine period are arranged symmetrically along the sides of the oesophagus. After the stomach turns from left to right the left vagus migrates forward and the right vagus backward and, as a result, the left vagus branches out on the anterior surface, while the right vagus on the posterior surface. In its initial portion the vagus nerve joins the glossopharyngeal nerve, the accessory nerve, the hypoglossal nerve and the superior ganglion of the sympathetic trunk. The vagus nerve gives rise to the following branches. A. In the cranial part (between the beginning of the nerve and the inferior ganglion): 1. The meningeal branch (ramus meningeus) to the dura mater of the posterior cranial fossa. 2. The auricular branch (ramus auricularis), to the posterior wall of the external auditory meatus and to part of the skin of the concha auriculae. This is the only cutaneous branch from the cranial nerves that bears no relation to the trigeminal nerve. B. In the cervical part: 1. Pharyngeal branches (rami pharyngei) together with branches of the glossopharyngeal nerve and sympathetic trunk form a plexus (plexus pharyngeus). The pharyngeal branches of the vagus nerve supply the constrictor muscles of the pharynx, the muscles of palatal arches and soft palate (with the exception of tensor veli palatini). The pharyngeal plexus also gives rise to sensory fibres running to the mucosa of the pharynx. 2. The superior laryngeal nerve (n. laryngeus superior) supplies sensory fibres to the laryngeal mucosa above the level of the rima glottidis, part of the root of the tongue and the epiglottis, and sends motor fibres to part of the laryngeal muscles and the lower constrictor muscle of the pharynx. 3. The upper cardiac branches (rami cardiaci cervicales superiores) often emerge from the superior laryngeal nerve and are distributed in the cardiac plexus. N. depressor is a component of the branches. C. In the thoracic part: 1. The recurrent laryngeal nerve (n. laryngeus recurrens) branches off where the vagus nerve lies in front of the aortic arch (on the left) and in front of the subclavian artery (on the right). On the right side this nerve Curves around the subclavian artery from below and from the back, and on the left side it curves around the aortic arch also from the back and from below. After this it ascends in the groove between the oesophagus and the trachea supplying them with numerous branches, oesophageal branches (rami esophagei) and tracheal branches (rami tracheales). The end of the nerve known , as laryngeal branches (n. laryngeus interior) innervates some of the laryngeal muscles, the mucosa of the larynx below the vocal chords, an area of the mucous membrane of the root of the tongue near the epiglottis, and also the trachea, throat and oesophagus, the thyroid gland and thymus, the lymph nodes of the neck, heart and mediastinum. It is connected with the neighbouring nerves, sympathetic ganglia and perivascular plexuses. 2. Cardiac branches (lower) (ramus cardiaci cervicales inferiores) usually consist of two branches which arise from the recurrent laryngeal nerve and the thoracic portion of the vagus nerve and pass to the cardiac plexus. 3. Pulmonary and tracheal branches (rami bronchiales and tracheales) together with the branches of the sympathetic trunk form the pulmonary plexus on the walls of the bronchi. The smooth muscles and glands of the trachea and bronchi are innervated by the branches of this plexus and, moreover, it also contains sensory fibres for the trachea, bronchi and lungs. 4. Thoracic cardiac branches (rami cardiaci thoracici) (NA). 5. Oesophageal branches (rami esophagei) run to the wall of the oesophagus. 6. Small branches to the thoracic duct. D. In the abdominal part: The plexuses of the vagus nerves running along the oesophagus continue onto the stomach forming well- defined vagal trunks (trunci vagales) (anterior and posterior). Each vagal trunk is a complex of nerve conductors not only of the parasympathetic, but also of the sympathetic and afferent animal nervous systems, and contains fibres of the vagus nerves. The continuation of the left vagus nerve which descends from the anterior side of the oesophagus to the anterior wall of the stomach forms a plexus (plexus gastricus anterior) located mainly along the small curvature from which anterior gastric branches (rami gastrici anteriores) mixed with sympathetic branches arise to the wall of the stomach (to the muscles, glands and mucous membrane). Some small branches pass to the liver through the lesser omentum. The right n. vagus also forms a plexus on the posterior wall of the stomach (plexus gastricus posterior) in the region of the small curvature. This plexus gives off posterior gastric branches; besides, the greater part of its fibres, coeliac branches (rami celiaci) follow the tract of a. gastrica sinistra to the coeliac ganglion, and from there along the vascular branches together with the sympathetic plexuses to the liver, spleen, pancreas, kidneys, small and large intestine up to the sigmoid colon. In cases of unilateral or partial damage to the tenth nerve the disorders concern mainly its animal functions. Disorders of visceral innervation may be comparatively mildly manifested. This is explained first by the fact that in innervation of the internal organs there may be zones of overlapping, and, second, that the peripheral segments of the vagus nerve contain nerve cells, i.e. vegetative neurons, which playa role in the automatic control of the function of internal organs.

THE ACCESSORY (11th) NERVE lThe accessory nerve (n. accessorius) develops in association with the last visceral arches; it is a muscle nerve, contains efferent (motor) and afferent (proprioceptive) fibres and has two motor nuclei lodged in the medulla oblongata and the spinal cord. According to these nuclei, the cerebral and spinal portions are distinguished. The cerebral portion arises from the medulla oblongata immediately below the vagus nerve. The spinal portion of the accessory nerve forms between the anterior and posterior roots of the spinal nerves (C2-C5) and partly from the anterior roots of the three superior cervical nerves. It ascends as a small nervous trunk and joins the cerebral portion. Since the accessory nerve is a part that separated from the vagus nerve, it emerges together with the vagus from the cranial cavity through the foramen jugulare, and preserves connection with it by means of the internal branch, the accessory branch to the vagus nerve (ranws internus). Another branch, external, of the accessory nerve, the branch to the sternocleidomastoid muscle (ramus externus) innervates the trapezius muscle and sternocleidomastoid muscles that separated from it. The cerebral portion of the accessory nerve as a component of the recurrent laryngeal nerve innervates the muscles of the larynx. The spinal portion of the accessory nerve participates in the motor innervation of the pharynx reaching its muscles as part of the vagus nerve from which the accessor nerve separated incompletely.

THE OCULOMOTOR (3rd) NERVE

The oculomotor nerve (n. oculomotorius), developmentally the motor root of the first preauricular myotome, is a muscle nerve. It contains: (1) efferent (motor) fibres arising from its motor nucleus and running to most of the extrinsic muscles of the eyeball and (2) parasympathetic fibres running from the accessory nucleus to the intrinsic ocular muscles (m. sphincter pupillae and m. ciliaris). The oculomotor nerve emerges from the brain along the medial edge of the peduncle of the brain and passes to the fissura orbitalis superior through which it enters the orbit. Here it divides into two branches. 1. The superior branch (ramus superior) that supplies the superior rectus and the levator palpebrae superius muscles. 2. The inferior branch (ramus inferior) supplies the inferior rectus, medial rectus and inferior oblique muscles. The inferior branch gives rise to a nerve root that passes to the ciliary ganglion. The motor root of the ciliary ganglion (radix oculomotoria) carries parasympathetic fibres for the sphincter of the pupil and to the ciliary muscle. Since the oculomotor nerve is located at the base of the brain in the interpeduncular cistern, it is abundantly washed over by cerebrospinal fluid. This is why, in inflammation of the meninges (meningitis), this nerve is the first to be affected, particularly its external fibres that innervate the levator palpebrae superius muscle (m. levator palpebrae superior).

THE TROCHLEAR (4th) NERVE

The trochlear nerve (n. trochlearis) is developmentally the motor root of the second preauricular myotome. It is a muscle nerve containing somatic-motor nuclei from which arise efferent (motor) fibres to the superior oblique muscle of the orbit. Emerging from the dorsal side of the superior medullary velum, it curves laterally around the peduncle of the brain, enters the orbit through the superior orbital fissure and supplies the superior oblique muscle.

THE ABDUCENT (6th) NERVE

The abducent nerve (n. abducens), the motor root of the third preauricular myotome, is a muscle nerve with a somatic-motur nucleus lodged in the pons. The nerve contains efferent (motor) fibres running to the external rectus muscle of the eye. Emerging from the brain at the posterior edge of the pons it enters the orbit through the superior orbital fissure and supplies the lateral rectum muscle.

THE OLFACTORY (1st) NERVES

The olfactory nerves (nn. oljactorii [BNA]) develop from the olfactory brain, which originated in association with the olfactory receptor. They contain visceral-sensory fibres which run from the organs of reception of chemical stimuli. Since the nerves are outgrowths of the prosencephalon, they have no ganglion; they are an accumulation of thin nerve filaments, fila olfactoria, about fifteen to twenty in number. which are the central processes of olfactory cells located in the olfactory region of the mucous membrane of the nose. Fila olfactoria pass through the openings of the lamina cribrosa in the superior wall of the nasal cavity and terminate in the olfactory bulb, which continues in the olfactory tract and the olfactory pyramid. The further course of the olfactory tract is described in the section “The Conducting Pathways of Olfaction”.

THE OPTIC (2nd) NERVE

The optic nerve (n. opticus) in the process of embryogenesis grows as a peduncle of the optic cup from the diencephalon. According to Sepp, in the process of phylogenesis the optic nerve is connected with the mesencephalon, which originates in association with the receptor of light. This explains its strong ties with these portions of the brain. It is a conductor of light stimuli and contains somatic-sensory fibres. As a derivative of the brain it has no ganglion like the first pair of cranial nerves, while the afferent fibres, which are part of it are a continuation of the axons of multipolar nerve cells of the retina. On leaving the posterior periphery of the eyeball the optic nerve runs from the orbit through the optic canal, and, on entering the cranial cavity together with a similar nerve from the opposite side, forms the optic chiasma located in the optic groove (sulcus chiasmatis) of the sphenoid bone (the chiasma is partial because only the medial fibres of the nerve cross). The optic tract (tractus opticus) is a continuation of the visual tract after the chiasma. It terminates in the lateral geniculate body (corpus geniculatum laterale), pulvinar thalami and in the superior tubercle of lamina quadrigemina (described in greater detail in the chapter “Organ of Vision”).

THE ORGAN OF HEARING

The organ of hearing and gravitation is located in the temporal bone and is divided into three parts: the external, middle and internal ear. The first two serve exclusively for conducting sound vibrations, and the third, in addition to this, contains sound-sensory and static apparatuses which are the peripheral parts of the auditory and statokinetic analyzers.

THE EXTERNAL EAR

The external ear consists of the auricle and the external auditory meatus. The auricle (auricula) commonly called the ear, is formed of elastic cartilage covered with skin. This cartilage determines the external shape of the auricle and its projections: the free curved margin called the helix, the anthelix, located parallel to it, the anterior prominence, the tragus, and the antitragus situated behind it. Downward the ear terminates as the lobule which has no cartilage; this is a characteristic progressive developmental sign for man. In the depression on the lateral surface of the. auricle (the concha auriculae), behind the tragus, is the external auditory meatus around which the remainder of the rudimentary muscles has been preserved. They are of no functional significance. Since the auricle of man is immobile, some authors consider it to be a rudimentary formation, but others disagree with this point of view because the cartilaginous skeleton of the human ear is well defined. The external auditory meatus (meatus acusticus externus) consists of two parts: cartilaginous and bony. The cartilaginous auditory meatus is a continuation of the auricular cartilage in the form of a groove open upward and to the back. Its internal end is joined by means of connective tissue with the edge of the tympanic part of the temporal bone. The cartilaginous auditory meatus constitutes two thirds of the whole external auditory meatus. The bony auditory meatus which constitutes two thirds of the entire length of the auditory meatus opens to the exterior by means of the porus acusticus externus on the periphery of which runs a circular bony tympanic groove (sulcus tympanicus). The direction of the whole auditory meatus is frontal in general but it does not advance in a straight line, it winds in the form of letter “S” both horizontally and vertically. Because of the curves of the auditory meatus. the deeply situated tympanic membrane can only be seen by pulling the auricle backward, outward and upward. The skin that covers the auricle continues into the external auditory meatus. In the cartilaginous part of the meatus the skin is very rich both in sebaceous glands and in a particular kind of glands, the ceruminous glands (glandulae ceruminosae), which produce a yellowish secretion, cerumen (ear wax). In this part there are also short hairs growing in the skin which prevent tiny particles from getting into the organ. In the bony part of the duct the skin thins out considerably and extends without interruption onto the external surface of the tympanic membrane which closes the medial end of the meatus.

THE TYMPANIC MEMBRANE

The tympanic membrane or ear drum (membrana tympani) is located at the junctions of the external and middle ears. Its edge fits into the sulcus tympanicus at the end of the external auditory meatus as into a frame. The tympanic membrane is secured in the sulcus tympanicus by a fibrocartilaginous ring (anulus fibrocartilagineus). The membrane is inclined because of the oblique position of the medial end of the auditory meatus, but in newborns it is almost horizontal. The tympanic membrane in an adult is oval in shape and measures 11 mm in length and 9 mm in breadth. It is a thin semitransparent sheet in which the centre, called the umbo (umbo membranae tympani) is drawn in like a shallow funnel. Its external surface is covered by a thinned-out continuation of the skin covering the auditory meatus (stratum cutaneum), the internal surface by the mucous lining of the tympanic cavity (stratum mucosum). The substance of the membrane itself between the two layers consists of fibrous connective tissue, the fibres of which in the peripheral part of the membrane run in a radial direction and in the central part in a circular direction. In the upper part the tympanic membrane contains no fibrous fibres and consists only of the skin and mucous layers and a thin stratum of loose tissue between them; this part of the tympanic membrane is softer and less tightly stretched; it has therefore been named the flaccid part (pars flaccida) in contrast to the remaining tightly stretched tense part (pars tensa).

THE MIDDLE EAR

The middle ear consists of the tympanic cavity and the auditory tube through which it communicates with the nasopharynx. The tympanic cavity (cavitas tympani) is situated in the base of the pyramid of the temporal bone between the external auditory meatus and the labyrinth (internal ear). It contains a chain of three small ossicles transmitting sound vibrations from the tympanic membrane to the labyrinth. The tympanic cavity is very small (volume of about 1 cm3) and resembles a tambourine propped up on its side and greatly inclined toward the external auditory meatus. Six walls are distinguished in the tympanic cavity. 1. The lateral, or membranous, wall (paries membranaceus) of the tympanic cavity is formed by the tympanic membrane and the bony plate of the external auditory meatus. The upper dome-like, expanded part of the tympanic cavity, the epitympanic recess (recessus epitympanicus), contains two auditory ossicles: the head of the malleus and the anvil. In disease the pathological changes in the middle ear are most evident in the epitympfnic recess. 2. The medial wall of the tympanic cavity belongs to the labyrinth and is therefore called the labyrinthine wall (paries labyrinthicus). It has two openings: a round one, the fenestra cochleae opening into the cochlea and closed with the secondary tympanic membrane (membrana tympani secundaria), and an oval fenestra vestibuli opening into the vestibulum labyrintii. The base of the third auditory ossicle, the stapes, is inserted in this opening. The fenestra cochlea is the most vulnerable spot in the bony wall of the internal ear. 3. The posterior, or mastoid, wall of the tympanic cavity (parties mastoideus) has an eminence, the pyramid of the tympanum (eminentia pyramidalis), containing the stapedius muscle. The epitympanic recess is continuous posteriorly with the tympanic antrum (antrum mastoideum) into which the mastoid dir cells (cellulae mastoideae) open. The tympanic antrum is a small cavity protruding toward the mastoid process from whose external surface it is separated by a layer of bone bordering with the posterior wall of the auditory meatus immediately behind the suprameatal spine where the antrum is usually cut open in suppuration of the mastoid process. 4. The anterior, or carotid, wall of the tympanic cavity (paries caroticus) is called so because it is closely adjoined by the internal carotid artery separated from the cavity of the middle ear only by a thin bony plate. In the upper part of this wall is the tympantc opening of the pharyngotympanic tube (ostium tympanicum tubae auditivae) which in newborns and infants gapes; this explains the frequent penetration of infection from the nasopharynx into the cavity of the middle ear and further into the skull. 5. The roof, or tegmental wall of the tympanic cavity (paries tegmentalis) corresponds on the anterior surface of the pyramid to the tegmen tympani and separates the tympanic cavity from the cranial cavity. 6. The floor, or jugular wall of the tympanic cavity (paries jugularis) laces the base of the skull in close proximity to the jugular fossa. The three tiny auditory ossicles in the tympanic cavity are called the malleus, incus, and stapes, the Latin for hammer, anvil and stirrup, respectively, which they resemble in shape. 1. The malleus has a rounded head (caput mallei) which by means of a neck (collum mallei) is joined to the handle (manubrium mallei). 2. The incus has a body (corpus incudis) and two diverging processes, a short (crus breve), and a long process (crus longum). The short process projects backward and abuts upon the fossa. The long process runs parallel to the handle of the malleus, medially and posteriorly of it and has a small oval thickening on its end, the lenticular process (processus lenticularis), which articulates with the stapes. 3. The stapes justifies its name in shape and consists of a small head (caput stapedis), carrying an articulating surface for the lenticular process of the incus and two limbs: an anterior, less curved limb (crus anterius), and a posterior more curved limb (crus posterius). The limbs are attached to an oval base (basis stapedis) fitted into the fenestra vestibuli. In places where the auditory ossicles articulate with one another, two true joints of limited mobility are formed: the incudomalleolar joint (articulatio incudomallearis) and the incudostapedial joint (articulatio incudostapedia). The base of the stapes is joined with the edges of fenestra vestibuli by means of connective tissue to form the tympanostapedial syndesmosis (syndesmosis tympanostapedia). The auditory ossicles are attached, moreover, by several separate ligaments. On the whole, all three ossicles form a more or less mobile chain running across the tympanic cavity from the tympanic membrane to the labyrinth. The mobility of the ossicles becomes gradually reduced from malleus to stapes, as the result of which the organ of Corti located in the internal ear is protected from excessive concussions and harsh sounds. The chain of ossicles performs two functions: (1) the conduction of sound through the bones and (2) the mechanical transmission of sound vibrations to the fenestra cochlea. The latter function is accomplished by two small muscles connected with the auditory ossicles and located in the tympanic cavity; they regulate the movement of the chain of ossicles. One of them, the tensor tympani muscle, lies in the canal for the tensor tympany (semicanalis m. tensoris) constituting the upper part of the musculotubal canal of the temporal bone; its tendon is fastened to the handle of the malleus near the neck. This muscle pulls the handle of the malleus medially, thus tensing the tympanic membrane. At the same time all the system of ossicles moves medially and the stapes presses into the fenestra cochlea. The muscle is innervated from the third division of the trigeminal nerve by a small branch of the nerve supplied to the tensor tympani muscle. The other muscle, the stapedius muscle, is lodged in the pyramid of the tympanum and fastened to the posterior limb of the stapes at the head. In function this muscle is an antagonist of the preceding one and accomplishes a reverse movement of the ossicle in the middle ear in the direction of the fenestra cochlea. The stapedius muscle is innervated from the facial nerve, which, passing nearby, sends small branch to the muscle. In general, the muscles of the middle ear perform a varietyof functions: (1) maintain the normal tonus of the tympanic membrane and the chain of auditory ossicles; (2) protect the internal ear from excessive sound stimuli and (3) accommodate the sound-conducting apparatus to sounds of different intensity and pitch. The basic principle of the work of the middle ear on the whole consists in conducting sound from the tympanic cavity to the fenestra cochlea. The auditory, or Eustachian, or pharyngotympanic tube (tuba auditiva [Eustachii]) which lends the name “eustachitis” to inflammation of the tube, lets the air pass from the pharynx into the tympanic cavity, thus equalizing the pressure in this cavity with the atmospheric pressure, which is essential for the proper conduction to the labyrinth of the vibrations of the tympanic membrane. The auditory tube consists of osseous and cartilaginous parts which are joined with each other. At the site of their junction, called the isthmus of the tube (isthmus tubae), the canal of the tube is narrowest. The bony part of the tube, beginning with its tympanic opening (ostium tympanicum tubae auditivae), occupies the large inferior portion of the muscular-tube canal (semicanalis tubae auditivae) of the temporal bone. The cartilaginous part, which is a continuation of the bony part, is formed of elastic cartilage. The tube widens downward and terminates on the lateral wall of the nasopharynx as the pharyngeal opening (ostium pharyngeum tubae auditivae); the edge of the cartilage pressing into the pharynx forms the tubae elevation (torus tubarius). The mucosa lining the auditory tube is covered by ciliated epithelium and contains mucous glands (glandulae tubariae mucosae) and lymphatic follicles which accumulate in large amounts at the pharyngeal ostium to form the tube tonsil (tonsilla tubaria). Fibres of the tensor palati muscle arise from the cartilaginous part of the tube and, consequently, when this muscle contracts in swallowing, the lumen of the tube can expand, which is conducive to the passage of air into the tympanic cavity.

THE INTERNAL EAR

The internal ear, or the labyrinth, is located in the depth of the pyramid of the temporal bone between the tympanic cavity and the internal auditory meatus, through which the auditory nerve emerges from the labyrinth. A bony and membranous labyrinth is distinguished with the latter enclosed in the former. The bony labyrinth (labyrinthus osseus) comprises a number of very small intercommunicating cavities, whose walls are of compact bone. Three parts are distinguished in the labyrinth: the vestibule, semicircular canals, and the cochlea. The cochlea lies in front of, medially to, and somewhat below the vestibule; the semicircular canals are situated behind, laterally to and above the vestibule. 1. The vestibule (vestibulum) which forms the middle part of the labyrinth is a small, approximately oval- shaped cavity, communicating in back through five openings with the semicircular canals. In front it communicates through a wider opening with the canal of the cochlea. On the lateral vestibular wall facing the tympanic cavity is the opening mentioned above, the fenestra vestibuli, which is occupied by the base of the stapes. Another opening, fenestra cochleae, closed by the secondary tympanic membrane is located at the beginning of the cochlea. The vestibular crest (crista vestibuli) passing on the inner surface of the medial vestibular wall divides this cavity in two, of which the posterior connected with the semicircular canals is called the elliptical recess (recessus ellipticus) and the anterior, nearest the cochlea, is called the spherical recess (recessus sphericus). The aqueduct of the vestibule begins in the elliptical recess as a small opening (apertura interna aqueductus vestibuli), passes through the bony substance of the pyramid, and terminates on its posterior surface. Under the posterior end of the crest on the floor of the vestibule is a small depression called the cochlear recess (recessus cochlearis). 2. The semicircular canals (canales semicirculares ossei) are three arch-like bony passages situated in three mutually perpendicular planes. The anterior semicircular canal (canalis semicircularis anterior) is directed vertically at right angles to the axis of the pyramid of the temporal bone; the posterior semicircular canal (canalis semicircularis posterior), which is also vertical, is situated nearly parallel to the posterior surface of the pyramid, while the lateral canal (canalis semicircularis lateralis) lies horizontally, protruding toward the tympanic cavity. Each canal has two limbs which open into the vestibule by five apertures only, however, because the neighbouring ends of the anterior and posterior canals join to form a common limb termed the crus commune. One of the limbs of each canal before joining the vestibule forms a dilatation called an ampulla. An ampullated limb is called crus ampullare, and a non-ampulated limb is termed crus simplex. 3. The cochlea is as a spiral bony canal (canalis spiral is cochleae) which, beginning from the vestibule, winds up like the shell of a snail into two and a half coils. The bony pillar around which the coils wind lies horizontally and is called the modiolus. An osseous spiral lamina (lamina spiralis ossea) projects from the modiolus into the cavity of the canal along the entire length of its coils. This lamina together with the cochlear duct divides the cavity of the cochlea into two sections: the scala vestibuli which communicates with the vestibule, and the scala tympani which opens on the skeletized bone into the tympanic cavity through the fenestra cochlea. Near this fenestra in the scala tympani is a very small inner orifice of the aqueduct ol the cochlea (aqueductus cochleae), whose external opening (apertura externa canaliculi cochleae) lies on the inferior surface of the pyramid of the temporal bone. The membranous labyrinth (labirynthus membranaceus) lies inside the bony labyrinth and repeats its configurations more or less exactly. It contains the peripheral parts of the statokinetic and auditory analyzers. Its walls are formed of a thin semitransparent connective tissue membrane. The membranous labyrinth is filled with a transparent fluid called the endolymph. Since the membranous labyrinth is somewhat smaller than the bony labyrinth, a space is left between the walls of the two; this is the petilymphaticspace (spatium perilymphaticum) filled with perilymph. Two parts of the membranous labyrinth are located in the vestibule of the bony labyrinth: the utricle (utriculus) and the saccule (sacculus). The utricle has the shape of a closed tube and occupies the elliptic recess of the vestibule and communicates posteriorly with three membranous semicircular ducts (ductus- semicirculares) which lie in the same kind of bony canals exactly repeating their shape. This it why it is necessary to distinguish the anterior, posterior and lateral membranous ducts (ductus semicircularis anterior, posterior and lateralis) with their corresponding ampulles: ampulla membranacea anterior, posterior and lateralis. The saccule, a pear-shaped sac, lies in the spherical recess of the vestibule and is joined with the utricle and with the long narrow endolymphatic duct which passes through the aqueduct of the vestibule and ends as a small blind dilatation termed the endolymphatic sac (saccus endolymphaticus) under the dura mater on the posterior surface of the pyramid of the temporal bone. The small canal joining the endolymphatic duct with the utricle and saccule is called the utricosaccular duct (ductus utriculosaccularis). With its harrowed lower end which is continuous with the narrow ductus reuniens, the saccule joins with the vestibular end of the duct of the cochlea. Both vestibular saccules are surrounded by the perilymphatic space. In the region of the semicircular canals the membranous labyrinth is suspended on the compact wall of the bony labyrinth by a complex system of threads and membranes. This prevents its displacement during forceful movements. Neither the perilymphatic nor the endolymphatic spaces are sealed off completely from the environment. The perilymphatic space communicates with the middle ear via the fenestra vestibuli and the fenestra cochleae which are both elastic and yielding. The endolymphatic space is connected by means of the endolymphatic duct with the endolymphatic sac lying in the cranial cavity; it is a more elastic reservoir which communicates with the inner space of the semicircular canals and the rest of the labyrinth. This creates physical prerequisites for the response of the semicircular canals to progressive movement.

STRUCTURE OF THE AUDITORY ANALYZER

The anterior part of the membranous labyrinth, the duct of the cochlea, (ductus cochlearis), enclosed in the bony cochlea, is the most vital component of the organ of hearing. The duct of the cochlea begins with a blind end in the cochlear recess of the vestibule somewhat posteriorly of the ductus reuniens that connects the duct with the saccule. Then it passes along the entire spiral canal of the bony cochlea and ends blindly at its apex. On cross section the duct of the cochlea is triangular in shape. One of its three walls is fused with the external wall of the bony canal of the cochlea; another wall, termed the spiral membrane (membrana spiralis), is a continuation of the osseous spiral lamina which stretches between the free edge of the latter and the outer wall. The third, a very thin wall of the duct of the cochlea (paries vestibularis ductus cochlearis) stretches obliquely from the spiral lamina to the outer wall. The basilar membrane (membrana spiralis) encloses the basilar lamina which carries the apparatus appreciating sounds: the organ of Corti, or the spiral organ. The duct of the cochlea separates the scala vestibuli from the scala tympani except for a place in the dome of the cochlea where they communicate through an opening called helicotrema. Scala vestibuli communicates with the perilymphatic space of the vestibule and scala tympani ends blindly at the fenestra cochlea.

THE SPIRAL ORGAN

The organ of Corti, or the spiral organ (organum spirale) is located along the length of the duct of the cochlea on the basilar lamina occupying the part nearest to the osseous spiral lamina. The basilar lamina (lamina basilaris) consists of a large number (24 000) of fibrous fibres of different length tightened like strings (acoustic strings). According to the well-known theory of Helmholtz (1875), they are resonators, the vibrations of which make it possible to appreciate tones of different pitch. According to the findings of electron microscopy, however, these fibres form an elastic network which as a whole resonates with strictly graded vibrations. The organ of Corti itself is composed of several rows of epithelial cells among which the neurosensory acoustic hair cells can be distinguished. Certain authors claim that this organ performs the role of a “reverse microphone” converting mechanical (acoustic) vibrations into electric oscillations.

THE PATHWAYS OF SOUND CONDUCTION

From the functional viewpoint, the organ of hearing (the peripheral part of the auditory analyzer) is divided into two: (1) the sound-conducting apparatus, i.e. the external and middle ear, as well as certain components of the internal ear (peri- and endolymph); and (2) the sound-appreciating apparatus, i.e. the internal ear. The air waves collected by the ear pass into the external auditory meatus, hit the tympanic membrane and cause it to vibrate. The vibrations of the tympanic membrane, the degree of tension of which is regulated by the contraction of the tensor tympani muscle (innervation from the trigeminal nerve) move the handle of the malleus fused with the membrane. The malleus moves the incus; and the incus moves the stapes fitted in the fenestra vestibuli leading into the internal ear. The displacement of the stapes in the fenestra vestibuli is regulated by the contraction of the stapedius muscle (innervation by the nerve supplied to it from the facial nerve). Thus, the chain of ossicles which are linked in mobility, conducts the vibrating movements of the tympanic membrane in a definite direction, namely, to the fenestra vestibuli. The movement of the stapes in the fenestra vestibuli stirs the labyrinth fluid which protrudes the membrane of the vestibuli cochlea to the exterior. These fluid movements are necessary for the functioning of the highly sensitive elements of the organ of Corti. The first to move is the perilymph in the vestibule. Its vibrations in the perilymph of scala vestibuli reach the apex of the cochlea and are conducted via the helicotrema to the perilymph in the scala tympani; then they descend along it to the secondary tympanic membrane and close the fenestra cochleae (which is a vulnerable place in the osseous wall of the internal ear) and return, as it were, to the tympanic cavity. From the perilymph the sound vibrations are conducted to the endolymph and through it to the organ of Corti. Thus, thanks to the system of the auditory ossicles of the tympanic cavity, the vibrations of air in the external and internal ear ale converted into vibrations of the fluid in the membranous labyrinth which stimulate the special acoustic hair cells of the organ of Corti comprising the receptor of the auditory analyzer. In the receptor, which plays the role of a “reverse microphone” the mechanic vibrations causing fluctuations in the fluid (endolymph) are converted into electric oscillations characterizing the nerve process spreading along the conductor to the cerebral cortex. The conductor of the auditory analyzer is made up of auditory conductors consisting of a number of links. The cellular body of the first neuron lies in the spiral ganglion. The peripheral process of the bipolar cells enters the organ of Corti and ends at the receptor cells, while the central process passes as the cochlear division of the auditory nerve to its nuclei, nucleus dorsalis and nucleus ventralis, located in the rhomboid fossa. According to the electrophysiological data, different parts of the auditory nerve conduct sounds of various frequency of vibration. The nuclei mentioned above contain the bodies of the secondary neurons, the axons of which form the central acoustic fasciculus, which, in the region of the posterior nucleus of the trapezoid body, crosses with the fasciculae of the same name on the opposite side, forming the lateral lemniscus. The fibres of the central acoustic fasciculus coming from the ventral nucleus form a trapezoid body and, on passing the pons, become part of the lateral lemniscus of the opposite side. The fibres of the central fasciculus coming out of the dorsal nucleus, run along the floor of the fourth ventricle in the form of auditory striae (striae medullares ventriculi quarti), penetrate the reticular formation of the pons and, together with the fibres of the trapezoid body, become part of the lateral lemniscus on the opposite side. The lateral lemniscus ends partly in the inferior quadrigeminal bodies of the tectal lamina and partly in medial geniculate body, where the third neurons are located. The superior quadrigeminal bodies serve as the reflex centre for auditory impulses. They are connected with the spinal cord by the tectospinal tract through which motor responses to auditory stimuli entering the mesencephalon are made. Reflex responses to auditory impulses may also be received from other intermediate auditory nuclei, namely nuclei of the trapezoid body and lateral lemniscus, connected by short pathways with the motor nuclei of the mesencephalon, the pons and medulla oblongata. Ending in structures related to hearing (the inferior quadrigeminal bodies and the medial geniculate body) the auditory fibres and their collaterals also join the medial longitudinal fasciculus by means of which they establish connections with the nuclei of the oculomotor muscles and the motor nuclei of other cranial and spinal nerves. These connections provide an explanation for the reflex responses to auditory stimuli. The inferior quadrigeminal bodies have no centripetal connections with the cortex. The medial geniculate body contains the cellular bodies of the last neurons whose axons as part of the internal capsule reach the cortex of the temporal lobe of the brain. The cortical end of the auditory analyzer is located in the superior temporal gyrus (Heschl’s gyrus, area 41). Here the vibrations of air in the external ear causing movement of the auditory ossicles in the middle ear and fluctuation of fluid in the internal ear are converted into nerve impulses further in the receptor, transmitted along the conductor to the brain cortex, and perceived in the form of auditory sensations. Consequently, thanks to the auditory analyzer, the air vibrations, i.e. the objective phenomenon existing independently of our conscious awareness of the surrounding reality, is reflected in our consciousness in the form of subjectively perceived images, i.e. auditory perceptions. Thanks to the auditory analyzer, various sound stimuli received in our brain as sound perceptions and complexes of perceptions, sensations, become signals (primary signals) of vitally important environmental phenomena. According to Pavlov, this constitutes the first signalling system of reality, i.e. concretely visible thinking also inherent in animals.

THE ORGAN OF VISION

Light became the stimulus which gave rise, in the animal world, to a special organ of vision. The main component of this organ in all animals are specific sensory cells which originate from the ectoderm and are capable of receiving stimuli from rays of light. For the most part these cells are surrounded by pigment which serves to channel the rays of light in a definite direction and absorb superfluous rays.

THE EYEBALL

The eye (oculus) consists of the eyeball (bulbus oculi) and the auxiliary apparatus surrounding it. The eyeball is spherical in shape and is situated in the eye socket. The anterior pole, corresponding to the most convex point on the cornea, and the posterior pole, located lateral to the exit of the optic nerve are distinguished in the eyeball. The straight line connecting both poles is called the optic axis, or the external axis of the eye (axis opticus). The part lying between the posterior surface of the cornea and the retina is called the internal axis of the eye. This axis intersects at a sharp angle with the socalled visual line (linea visus) which passes from the object of vision, through the nodal point, to the place of the best vision in the central pit of the retina. The lines connecting both poles along the circumference of the eyeball form meridians, whereas the plane perpendicular to the optic axis is the equator of the eyeball dividing it into the anterior and posterior halves. The horizontal diameter of the equator is slightly shorter than the external optic axis (the latter is 24 mm and the former 23.6 mm); its vertical diameter is even shorter (23.3 mm). The internal optic axis of a normal eye is 21.3 mm; in myopic eyes it is longer, and in long- sighted people (hypermetropic eyes) it is shorter. Consequently, the focus of converging rays in myopic people is in front of the retina, and in hypermetropic people it is behind the retina. To achieve clear vision, the hypermetropic people must always resort to accommodation. To relieve such anomalies of sight, adequate correction by means of eyeglasses is essential. The eyeball has three coats surrounding its inner nucleus; a fibrous outer coat, a vascular middle coat, and an inner reticular coat (the retina).

THE COATS OF THE EYEBALL

I. The fibrous coat (tunica fibrosa bulbi) forms an external sheath around the eyeball and plays a protective role. In the posterior, largest of its parts, it forms an opaque tunic called the sclera, and in the anterior segment, a transparent cornea. Both areas of the fibrous coat are separated one from the other by a shallow circular sulcus (sulcus sclerae). 1. The sclera consists of dense connective tissue, white in colour. Its anterior part is visible between the eyelids and is commonly referred to as the “white of the eye”. At the junction with the cornea in the thickness of the sclera there is a circular venous canal, the sinus venosus sclerae (Schlemmi) called Schlemm’s canal. Since light must penetrate to the light-sensitive elements of the retina lying in the eyeball, the anterior segment or the fibrous tunic becomes transparent and develops into the cornea. 2. The cornea is a continuation of the sclera and is a transparent, rounded plate, convex toward the front and concave in the back, which, like the glass of a watch, is fitted by its edge (limbus corneae) into the anterior segment of the sclera. II. The vascular coat of the eyeball (tunica vasculosa bulbi) is rich in vessels, soft, dark-coloured by the pigment contained in it. It lies immediately under the sclera and consists of three parts: the choroid, the ciliary body, and the iris. 1. The choroid (chorioidea) is the posterior largest segment of the vascular coat. Due to the constant movement of the choroid in accommodation a slit-like lymphatic perichoroidal space (spatium perichorioideale) is formed here between the layers. 2. The ciliary body (corpus ciliare), the anterior thickened part of the vascular tunic, is arranged in the shape of a circular swelling in the region where the sclera is continuous with the cornea. Its posterior edge, which forms the ciliary ring (orbiculus ciliaris), is continuous with the choroid. This place corresponds to the retinal ora serrata (see below). In front the ciliary body is connected with the external edge of the iris. Anteriorly of the ciliary ring the ciliary body carries about 70 fine radially arranged whitish ciliary processes (processus ciliares). Due to the abundance and the particular structure of the vessels in the ciliary processes, they secrete a fluid, the aqueous humour of the chambers. This part of the ciliary body is comparable with the choroid plexus of the brain and is known as the secernent part (L secerne to separate). The other part, the accommodating part, is formed by the smooth ciliary muscle (musculus ciliaris) which lies in the thickness of the ciliary body externally of the ciliary processes. Formerly this muscle was divided into three portions: external meridional (Bruecke); middle radial (Ivanov) and internal circulatory (Muller). Now only two types of fibres are distinguished, namely, meridional (fibrae meridionales) arranged longitudinally, and circular (fibrae circulares) arranged in rings. The meridional fibres forming the principal part of the ciliary muscle, begin from the sclera and terminate posteriorly in the choroid. On contracting, they tighten the choroid and relax the sac of the lens in adjusting the eyes for short distances (accommodation). Circulatory fibres help accommodation, advancing the frontal part of the ciliary processes and this is why they are particularly well developed in hypermetropics who must tense their accommodation apparatus very greatly. Thanks to the elastic tendons, the muscle after contraction resumes its initial position and there is no need for an antagonist. Fibres of both kinds intertwine and form a single muscular-elastic system which in childhood consists mostly of meridional fibres, and in old age of circulatory fibres. During the lifespan the muscle fibres become gradually atrophied and replaced by connective tissue, which explains the weakening of accommodation in old age. In females degeneration of the ciliary muscle begins 5 to 10 years earlier than in males, that is, with the onset of the menopause. 3. The iris is the most anterior portion of the vascular coat of the eye and is a circular vertically standing plate with around aperture called the pupil (pupilla). The pupil is not exactly in the middle, but is slightly displaced toward the nose. The iris plays the role of a diaphragm regulating the amount of light entering the eye. Due to this, when there is strong light the pupil contracts and when the light is weak, it dilates. With its outer edge (margo ciliaris) the iris is connected with the ciliary body and the sclera. Its inner edge surrounding the pupil (margo pupillaris) is free. The iris has an anterior surface (facies anterior) facing the cornea and a posterior surface (facies posterior) adhering to the lens. The anterior surface which is visible through the transparent cornea is of different colour in different people and determines the colour of their eyes. This depends on the amount of pigment contained in the superficial layers of the iris. If there is much pigment, the eyes are brown to the point of being very dark, and, on the contrary, if the pigmentary layer is weakly developed or practically absent, then the colour tones are mixed greenish- grey and light blue. The last two colours are mainly due to the transparency of the black retinal pigment on the posterior surface of the iris. In performing the function of a diaphragm, the iris displays remarkable mobility; this is ensured by fine adaptation and the correlation of its components. Thus, the basis of the iris (stroma iridis) consists of connective tissue with the architecture of a lattice in which vessels have been fitted radially from the periphery to the pupil. These vessels are the sole carriers of elastic elements (since the connective tissue of the stroma contains no elastic fibres) and together with the connective tissue form an elastic skeleton of the iris, permitting it to change easily in size. The actual movements of the iris itself are accomplished by the muscle system lodged within the stroma. This system consists of smooth muscle fibres which are partly arranged in a ring around the pupil to form the sphincter of the pupil (m. sphincter pupillae) and partly fan out radially from the pupillary aperture to form the dilator of the pupil (m. dilatator pupillae). Both muscles are interrelated and affect each other: the sphincter stretches the dilator, while the dilator straightens out the sphincter. Because of this, each muscle returns to its initial position, which explains the rapidity of the movements of the iris. This integral muscle system has a punctum fixum on the ciliary body. The sphincter of the pupil is innervated by parasympathetic fibres coming from the nucleus of Yakubovich’s as part of n. oculomotorius, while the dilator of the pupil is innervated by sympathetic fibres from the sympathetic trunk. The impermeability of the diaphragm to light is due to the presence on its posterior surface of a double layer of pigmentary epithelium. On the anterior surface washed by fluid it is covered by the endothelium of the anterior chamber. Due to the position of the vascular membrane between the fibrous and retinal coats its pigmentary layer prevents superfluous rays from falling on the retina and the vessels are distributed to all layers of the eyeball. III. The retina is the innermost of the three coats of the eyeball adhering to the vascular coat along its entire length until it reaches the pupil. Unlike the other coats it develops from the ectoderm and according to its origin consists of two layers; the external pigmented layer (stratum pigmenti retinae) and the internal layer which is the retina proper. The retina proper is divided in structure and function into two parts: the posterior, which contains light-sensitive elements, constitutes the optic part of the retina (pars optica retinae) and the anterior part which does not have these elements. The junction between the two is an idented line (ora serrata) passing at the level where the choroid is continuous with the ciliary ring of the ciliary body. The optic part is almost fully transparent and opacifies only in a cadaver. On examination with an ophthalmoscope the fundus of the eye in a living person is dark red because the blood in the vascular coat is seen through the transparent retina. On this crimson background a white round spot is visible on the fundus which is the site of the exit of the optic nerve from the retina. As it emerges the optic nerve forms an optic disk (discus n. optici) with a crater-like depression in the centre, the excavation of the optic disc (excavatio disci). In examination with a mirror the vessels of the retina arising from this excavation are also well visible. The fibres of the optic nerve deprived of their myelin sheath spread from the disk in all directions over the optic part of the retina. The optic disk which is about 1.7 mm in diameter lies slightly medially (toward the nose) of the posterior pole of the eye. Laterally from it and, at the same time, slightly in the temporal direction from the posterior pole, is an oval 1 mm area, the macula; it is reddish brown in a living person with a pinpoint depression (fovea centralis) in the centre. This is the site of sharpest acuity of vision. There are light-sensitive visual cells in the retina, whose peripheral ends are shaped as rods and cones. Since they are situated in the external layer of the retina and adhere to the pigmentary layer, to reach them, the rays of light must penetrate the entire thickness of the retina. The rods contain visual purple (rhodopsin) which is responsible for the pink hue of afresh retinal membrane in the dark; in the light it is rendered colourless. The formation of visual purple is attributed to the cells of the pigmentary layer. The cones do not contain visual purple. It should be noted that there are only cones in the macula and no rods. In the region of the optic disk there are no light-sensitive elements at all, as a result of which this place produces no visual sensation and is therefore called the blind spot.

THE REFRACTING MEDIA OF THE EYE

The transparent light-refracting media of the eye are the cornea (already described), the vitreous body and the lens which serve to form the image on the retina, and the aqueous humour which fills the chambers of the eye and provides nutrition for the avascular structures of the eye. A. The vitreous body (corpus vitreum) fills the space between the lens and the retina and is an absolutely transparent mass resembling jelly. The lens presses into the anterior surface of the vitreous body as a result of which a depression, the hyaloid fossa (fossa hyaloidea) forms; its edges are joined with the capsule of the lens by a special ligament. B. The lens is a very important light-refracting medium of the eyeball. It is completely transparent and shaped like a lentil or a biconvex glass. The central points of the posterior and anterior convexities are called the poles of the lens (polus anterior and posterior), while the peripheral circumference of lens, where both surfaces join, is called the equator. The axis of the lens joining both poles is 3.7 mm long when looking at a distance and 4.4 mm in accommodation, when the lens becomes more convex. The equatorial diameter is 9 mm. The equator plane is at a right angle to the optical axis. The anterior surface of the lens is in contact with the iris, the posterior surface is in contact with the vitreous body. The lens is enclosed in a thin, also absolutely transparent structureless capsule (capsula lentis) and is held in position by a special ligament called the ciliary zonule, or zonule of Zinn (zonula ciliaris [Zinni]) which is made up of a multitude of fine fibres running from the capsule of the lens to the ciliary body where they are mainly distributed among the ciliary processes. Between the fibres are spaces filled with aqueous, the zonular spaces (spatia zolunaria [Petiti]), or (Petit’s canal) communicating with the chambers of the eye. Due to the elasticity of the capsule, the lens easily changes its curvature depending whether the object we are looking at is far or near. This phenomenon is called accommodation. In the first case Zinn’s ligament pulls at the lens, flattening it; in the second case, when the eye must be set at a short distance, the ciliary muscle contracts, relaxing Zinn's ligament together with the capsule of the lens, bulging the lens more. As a result the rays coming from a nearby object are refracted more strongly by the lens and can merge on the retina. The lens, like the vitreous body, has no vessels. C. The chambers of the eye. The space between the anterior surface of the iris and the posterior surface of the cornea is called the anterior chamber of the eye (camera anterior bulbi). The anterior and posterior surfaces of the chamber meet along its circumference in the angle formed by the junction of the cornea with the sclera on the one hand, and with the ciliary margin of the iris, on the other. This is the iridocorneal angle (angulus iridocornealis); it is rounded off by a network of trabeculae comprising, in their entirety, the pectinate ligament (lig. pectinatum anguli iridocornealis). Between the trabeculae of the ligament are slit-like Fontana’s spaces. The iridocorneal angle is of vital physiological significance from the viewpoint of the circulation of aqueous in the chamber, which drains through Fontana's spaces into Schlemm's canal located nearby within the sclera. Behind the iris is a narrower postertor chamber of the eye (camera posterior bulbi), which includes the spaces between the fibres of Zinn's ligament. It is bounded posteriorly by the lens and on the side by the ciliary body. The posterior chamber communicates with the anterior chamber through the pupil. Both chambers of the eye are filled with a transparent fluid, aqueous humour (humor aquosus) which drains into Schlemm’s canal.

THE RESPIRATORY SYSTEM (SYSTEMA RESPIRATORIUM)

The respiratory organs are concerned with the supply of oxygen to the blood, by which it is brought to the tissues of the body, and with the removal of carbon dioxide into the atmosphere. The respiratory organs in mammals develop from the ventral wall of the foregut and retain their connection with it throughout life. This explains the existence of the intersection of the respiratory and digestive tracts in the pharynx also in man, a fact mentioned above. To accomplish the act of respiration, an adjustment providing for the flow of fresh air current along the respiratory surface, i.e. the circulation of air, is necessary. In view of this, air passages exist, in addition to the lungs, namely the nasal cavity and pharynx (upper air passage) and then the larynx, trachea, and bronchi (the lower air passage).

THE CAVITY OF THE NOSE

Before coming in contact with the fine tissue of the lungs the inspired air must be cleansed of dust, warmed, and humidified. This is accomplished in the nasal cavity (cavitas nasi). There is also the external nose (nasus externus) which communicates anteriorly with the atmosphere by means of the nostrils and posteriorly with the pharynx by means of the choanae. The walls of the cavity as well as the septum and conchae are lined with mucous membrane, which is continuous with the skin in the region of the nostrils and with the pharyngeal mucosa posteriorly. THE LARYNX

The larynx is situated on the level of the fourth, fifth, and sixth cervical vertebrae immediately below the hyoid bone, on the anterior surface of the neck. To the back of it is the pharynx with which it communicates directly through an opening called the inlet of the larynx (aditus laryngis). On both sides of the larynx pass large vessels of the neck while in front it is covered with muscles of the infrahyoid group (sternohyoid, sternothyroid, and omohyoid muscles), the cervical fascia, and the superior parts of the lateral lobes of the thyroid. Below the larynx is continuous with the trachea.

CARTILAGES OF THE LARYNX

The cricoid cartilage (cartilago cricoidea) is hyaline and shaped like a signet ring with a wide plate (lamina) at the back and an arch (arcus) in front and on the sides. The border of the lamina and its lateral surface bear articular fossae for uniting with the arytenoid and thyroid cartilages. The thyroid cartilage (cartilago thyroidea) is the largest of the laryngeal cartilages; it is hyaline in structure and consists of two laminae, which fuse in front at an angle. The site of union of the laminae in children and females is rounded and there is, therefore, no conspicuous prominence in them as in adult males (Adam's apple). In the superior border on the midline is the thyroid notch (incisura thyroidea superius). The posterior, thickened border of each plate is continuous with a larger superior horn (cornu superius) and a shorter inferior horn (cornu inferius). The inferior horn bears on the inner surface of its apex an area for articulating with the cricoid cartilage. The arytenoid cartilages (cartilagines arytenoideae) are directly related to the vocal cords and muscles. They are pyramidal in shape with the base (basis) sitting on the superior border of the cricoid cartilage while the apex faces upward. Two processes arise from the base: (1) an anterior process (of elastic cartilage) is the site of attachment of the vocal cord and is therefore called vocal (processus vocalis) and (2) a lateral, muscular, process (of hyaline cartilage) for the attachment of muscles (processus muscularis).The corniculate cartilages (cartilagines corniculatae) are seated on the apices of the arytenoid cartilages in the thickness of the aryepiglottic folds. The cuneiform cartilages (cartilagines cuneitormes) are directly in front of the corniculate cartilages, also in the aryepiglottic folds. Sometimes they are absent. The epiglottis cartilage (cartilago epiglottica) is a leaf-shaped lamina of elastic cartilaginous tissue situated in front of aditus laryngis and directly behind the root of the tongue.

LIGAMENTS AND JOINTS OF THE LARYNX

The larynx appears to be suspended from the hyoid bone on the thyrohyoid membrane (membrana thyreohyoidea) stretching between this bone and the thyroid cartilage. The membrane consists of an unpaired median thyrohyoid ligament (lig. thyreohyoideum medianum) and the paired lateral thyrohyoid ligaments (ligamenta thyreohyoideum) stretched between the ends of greater horns of the hyoid bone and the superior horns of the thyroid cartilage in the thickness of which a small cartilago triticea, resembling a grain of wheat, is embedded. The hyoid bone is also connected with the epiglottis by the hyoepiglottic ligament (lig. hyoepiglotticum); the epiglottis is in turn attached to the thyroid cartilage by the thyroepiglottic ligament (lig. thyreoepiglotticum). Between the arch of the cricoid cartilage and the margin of the thyroid cartilage stretches on the midline a strong cricothyroid ligament (lig. cricothyroideum) composed of elastic fibres. The lateral fibres arising from the superior margin of the cricoid cartilage pass medially and uni te posteriorly with the arytenoid cartilage; these bundles together with the cricothyroid ligament form the cricovocal membrane (conus elasticus) which narrows toward the top; its superior free margin is the vocal ligament (lig. vocale). The vocal ligament (lig. vocale) is attached anteriorly to the angle of the thyroid cartilage in close vicinity to its contralateral fellow, and posteriorly to the vocal process of the arytenoid cartilage.The medial margin of the vocal ligament is pointed and free; laterally and downward the ligament is continuous with the cricovocal membrane mentioned above. Above and parallel to the vocal ligament is the paired vestibular ligament (lig. vestibulare). It is so called because it bounds inferiorly the vestibule of the larynx. Besides ligaments, there are joints between the cartilages at the junction of the thyroid and arytenoid cartilages with the cricoid cartilage. 1. The paired combined cricothyroid joint (art. cricothyroidea) forms between the inferior horns of the thyroid cartilage and the cricoid cartilage. 2. The paired cricoarytenoid joints (articulationes cricoarytenoideae) form between the base of each arytenoid cartilage and the cricoid cartilage. 3. The corniculate cartilages articulate with the apices of the arytenoid cartilages by means of small joints or synchondroses (synchondrosis arycorniculata).

THE CAVITY OF THE LARYNX

The cavity of the larynx (cavum laryngis) opens by means of an inlet of the larynx (aditus laryngis) bounded anteriorly by the free margin of the epiglottis, posteriorly by the apices of the arytenoid cartilages and the fold of mucosa, interarytenoid fold (plica interarytenoidea) stretching between them, and laterally by mucosal folds stretched between the epiglottis and the arytenoid cartilages, the aryepiglottic fold (plica aryepiglottica). The cavity itself resembles an hourglass in shape; it is narrowed in the middle but expands upward and downward. The upper, expanded part of the cavity is called the vestibule of the larynx (vestibulum laryngis). It extends from the laryngeal inlet to a paired mucosal fold on the lateral wall of the cavity; this is the vestibular fold (plica vestibularis), or the false vocal cord, in the thickness of which is the vestibular ligament. The walls of the vestibule are formed: anteriorly, by the dorsal surface of the epiglottis, posteriorly, by the upper parts of the arytenoid cartilages and the interarytenoid fold, and laterally, by a paired elastic membrane of the larynx (membrana fibroelastica laryngis) extending from the vestibular fold to the aryepiglottic fold. The middle, constricted part of the laryngeal cavity is most complex in structure. It is bounded above and below by mucosal folds situated on the lateral walls of the larynx. The upper fold is the paired vestibular fold mentioned above. The free margins of these folds form the boundaries of an unpaired rather wide rima vestibuli. The lower, vocal fold (plica vocalis), or true vocal cord, protrudes into the cavity more than the upper fold does and contains the vocal ligament and the vocal muscle. The recess between the vestibular and vocal folds is the sinus of the larynx (ventriculus laryngis). A sagittally lying fissure of the glottis, rima glottidis, is formed between the two vocal folds; it is the narrowest part of the laryngeal cavity. In it are distinguished an anterior, larger part between the folds themselves, the intermembranous part (pars intermembranacea), and a posterior, smaller one, between the vocal processes of the arytenoid cartilage, the intercartilaginous part (pars intercartilaginea). The lower expanded part of the larynx, the infraglottic cavity (cavum infraglotticum) gradually narrows inferiorly and is continuous with the trachea.

THE TRACHEA

The trachea is a continuation of the larynx. It begins at the level of the lower border of the sixth cervical vertebra and terminates at the level of the upper border of the fifth thoracic vertebra by separating into two bronchi, right and left. The place of the separation is called the bifurcation of the trachea (bifurcatio tracheae). The length of the trachea ranges from 9 to 11 cm, the transverse diameter from 15 to 18 mm on the average. Topography of the trachea. The cervical segment is embraced above by the thyroid gland. Posteriorly the trachea is related to the oesophagus, while on both sides of it stretch the common carotid arteries. Anteriorly the trachea is covered by the isthmus of the thyroid gland. The thoracic segment of the trachea is covered in front by the manubrium sterni, remnants of the thymus, and by vessels. The position of the trachea in front of the oesophagus is associated with its development from the ventral wall of the foregut. Structure of the trachea. The trachea is formed of 16 to 20 incomplete rings of cartilage (cartilagines tracheales) united by means of fibrous annular ligaments (ligamenta anularia); each ring extends only over two thirds of the circumference. The posterior, membranous wall of the trachea (paries membranaceus), is flat and contains transverse and longitudinal fascicles of smooth muscle tissue, which cause active movement of the trachea in respiration, coughing, etc.

THE BRONCHI

The principal bronchi (Gk bronchos windpipe), right and left (bronchi principales dexter and sinister) arise from the tracheal bifurcation almost at a right angle and pass to the hilum of the corresponding lung. The right bronchus is a little larger than the left because of the larger volume of the right lung. At the same time, the left bronchus is almost twice the length of the right and contains 9 to 12 cartilaginous rings while the right bronchus is formed of 6 to 8 rings. The right bronchus takes a more vertical direction than the left and is as if a continuation of the trachea.

THE LUNGS

The lungs (pulmones) (Gk pneumon lung, hence pneumonia, inflammation of the lungs) are situated in the thoracic cavity (cavum thoracis) lateral of the heart and large vessels. They are invested in pleural sacs separated one from the other by the mediastinum, which extends from the vertebral column at the back to the anterior thoracic wall in front. The right lung is larger in volume than the left (approximately by 10 per cent) but is a little shorter and wider, firstly because the right diaphragmatic dome is situated higher than the left (the effect of the large right hepatic lobe) and, secondly, because the heart is located more to the left than to the right. As a result the width of the left lung is reduced. Each lung (pulmo) has an irregular conical shape with a base (basis pulmonis) below and a rounded apex (apex pulmonis) protruding 3-4 cm above the first rib or 2-3 cm above the clavicle in front; posteriorly the apex rises to the level of the seventh cervical vertebra. A small groove, sulcus subclavius, is formed on the apex from pressure of the subclavian artery passing here. Three surfaces are distinguished in the lungs. The inferior diaphragmatic surface (facies diaphragmatica) is concave and fits the convexity of the superior surface of the diaphragm to which it is adjacent. The large costal surface (facies costalis) is convex in accordance with the concavity of the ribs, which, together with the intercostal muscles, are components of the wall of the thoracic cavity. The medial surface (facies medialis) is concave in adaptation of its greater part to the outlines of the heart sac and is divided into an anterior part adjacent to the mediastinum (pars medtastinalis) and a posterior part (pars vertebralis) adjacent to the spine. The surfaces are separated by borders: the sharp border of the base is called the inferior border (margo inferior), the other, also a sharp one separating the medial and costal surfaces, is called the anterior border (margo anterior). On the medial surface, above and behind a depression formed by the pericardium, is the hilum of the lung (hilus pulmonis) through which the bronchi and pulmonary artery (as well as nerves) enter the lungs and the two pulmonary veins (and lymphatic vessels) leave; them; the whole complex forms the root of the lung (radix pulmonis). The bronchus is situated dorsally in the root while the position of the pulmonary artery on the right side differs from that on the left. In the root of the right lung, the pulmonary artery is below the bronchus, whereas in the root of the left 1ung it crosses the bronchus and lies above it. The pulmonary veins on both sides lie in the root of the lung below the pulmonary artery and bronchus. There is no sharp border at the junction of the costal and medial surfaces; the rounded part of each lung is lodged in a groove in the thoracic cavity on both sides of the spine; this is the pulmonary sulcus of the thorax (sulci pulmonales). Each lung is divided by interlobar sutures into lobes (lobi). The oblique fissure (fissura obliqua) is present in both lungs. In each lung it separates the upper lobe from the lower one. The right lung has, in addition to this fissure, a horizontal fissure (fissura horizontalis. It separates a wedge-shaped middle lobe from the upper lobe of the right lung. There is, therefore, three lobes in the right lung: upper lobe (lobus superior), middle lobe (lobus medius), and lower lobe (lobus inferior). The left lung has only two lobes: an upper lobe (lobus superior), lower lobe (lobus inferior). In the lower part of the anterior border of the left lung is the cardiac notch (incisura cardiaca pulmonis sinistra).

STRUCTURE OF THE LUNGS

Branching of the bronchi. Correspondingly to the separation of the lungs into lobes, each of the principal bronchi on reaching the pulmonary hilum begins separating into lobar bronchi (bronchi lobares). The right upper lobe bronchus running to the centre of the upper lobe passes above the pulmonary artery and is, therefore, called an eparterial bronchus; all the other lobe bronchi of the right lung and all the lobe bronchi of the left lung pass below the level of the pulmonary artery and are called hyparterial. On entering the substance of the lung, the lobe bronchi give off several smaller, tertiary bronchi called segmental bronchi (bronchi segmentales) because they ventilate definite areas of the lung, the pulmonary segments. The segmental bronchi divide in turn dichotomously (each into two parts) into smaller bronchi of the fourth and next orders to end as terminal and respiratory bronchioles. The skeleton of extrapulmonary bronchi consists of cartilaginous semirings; as the bronchi approach the hilum of the lung, cartilaginous bonds form between the cartilaginous semirings as a result of which the annular structure is replaced by a latticed structure. The cartilages of the segmental bronchi and those of their branchings no longer have the shape of semirings but break down into separate laminae whose size reduces with the diminution of the calibre of the bronchi; in the terminal bronchioles the cartilages disappear completely. The mucous glands also disappear in the terminal bronchioles, but the ciliary epithelium remains. The muscular layer is composed of smooth muscle fibres arranged circularly on the inner surface of the cartilages. The macromicroscopic structure of the lung. The pulmonary segments consist of pulmonary lobules (lobuli pulmonales), which are small (0.5-1.0 cm in diameter) pyramidal areas of pulmonary parenchyma separated from one another by connective-tissue (interlobular) septa. A single small (1 mm in diameter) bronchus (of the eighth order on the average) still containing cartilage in its walls (lobular bronchus) enters the apex of each lobule. The number of lobular bronchi in both lungs comes up to a thousand. Each lobular bronchus branches out in the lobule into 12-18 terminal bronchioles (bronchioli terminales), which contain neither cartilage nor glands. All the bronchi, from the principal bronchi to the terminal bronchioli, constitute a single bronchial tree and serve as passages for conducting the stream of air in inspiration and expiration; respiratory exchange of gases between the air and the blood does not take place in them. The terminal bronchioles branch dichotomously to give origin to the respiratory bronchioles (bronchioli respiratorii) which are characterized by the appearance of alveoli of the lung (alveoli pulmonum) on their walls. Alveolar ducts (ductuli alveolares) arise radially from each respiratory bronchiole and terminate as blind air saccules (sacculi alveolares). The wall of each saccule is surrounded by a thick network of blood capillaries. The exchange of gases takes place through the alveolar walls. The respiratory bronchioles, alveolar ducts, and air saccules with the alveoli compose a single alveolar tree, or the respiratory parenchyma of the lung. They form the functional and anatomical unit of the parenchyma called the acinus (L grape).

THE PLEURAL SACS AND THE MEDIASTINUM

Three absolutely isolated serous sacs are present in the thoracic cavity, one for each lung and one, middle, sac, for the heart. The serous covering of the lung is called the pleura. It has two layers: the viscerdl pleura (pleura visceralis) and the parietal pleura (pleura parietalis). The visceral, or pulmonary pleura, covers the lung itself and fuses with the pulmonary substance so closely that it cannot be removed without injury to the tissue. It enters the pulmonary fissures and separates in this manner the pulmonary lobes one from another, The parietal pleura is the outer part of the pulmonary serous sac. The outer surface of the parietal pleura fuses with the thoracic walls, while the inner surface faces directly the visceral pleura. The potential space between the adjoining parietal and visceral pleurae is called the pleural cavity (cavum pleurae). The parietal pleura is a single closed sac, but for descriptive purposes it is subdivided into the costal, diaphragmatic, and mediastinal pleurae. Besides, the superior part of each pleural sac is distinguished as the cervical pleura, or cupula of pleura (cupula pleurae). The costal pleura, the most extensive part of the parietal pleura, lines the inner surfaces of the ribs and intercostal spaces. The diaphragmatic pleura covers the superior surface of the diaphragm except for the middle part where the pericardium is in direct contact with the diaphragm. The mediastinal pleura stretches anteroposteriorly from the posterior surface of the sternum and the lateral surface of the spine to the root of the lung; it bounds laterally the mediastinal organs. Reserve spaces are formed by two parietal layers of the pleura where the pulmonary borders do not coincide with the pleural boundaries; these are the pleural recesses, or sinuses (recessus s. sinus pleurales). The lung enters them only during very deep inspiration. The largest one, the costodiaphragmatic recess or phrenicocostal sinus (recessus costodiaphragmaticus s. sinus phrenicocostalis) (BNA), is situated on the right and left sides along the inferior boundary of the pleura, between the diaphragm and the thoracic cage; the inferior pulmonary borders do not reach the pleural boundaries here. Another, smaller, reserve space is at the anterior border of the left lung. It extends for the distance of the cardiac notch between the costal and mediastinal pleurae and is called the costomediastinal recess, or sinus (recessus s. sinus costomediastinalis). The pleural cavity is the space between the visceral and parietal pleurae; pleural sinuses are reserve spaces of the pleural cavity between two layers of the parietal pleura.

THE HEART The heart (cor) is a hollow muscular organ, which receives blood from the venous trunks draining into it and pumps the blood into the arterial system. The cavity of the heart is subdivided into for chambers: two atria and two ventricels. The left atrium and the left ventricle comprise the left heart, also called the arterial heart, beacause of the type of blood it contains; the right atrium and right ventricle comprise the right or venous heart. Contractiono of the right and left atria occurs simultaneously. The ventricles also contract simultaneously, but in regular sequence with the atria. Contraction of the walls of the heart chambers is called systole. Their relaxation is called diastole. The heart is shaped like a slightly flattened cone. In it we distinguish an apex, a base, the anterosuperior and iferior surfaces, and the right and left borders, separating these surfaces. The rounded apex of the heart (apex cordis) facec downward, forward, and to the left, reaching the fifth intercostal space 8-9 cm to the left of the midline; the apex is formed by the left ventricle.The base of the heart (basis cardis) faces upward, backward, and to the right. It is formed by the atria and,in front, by the aorta and pulmonary trunk.The opening of the superior vena cava into the heart is the upper right angle of the rectangle formed by the atria, that of the inferior vena cava in the lower right angle. The two right pulmonary veins enter immediately to the left. The two left pulmonary veins enter on the left border. The anterosuperior, or sternocostal surface of the heart (facies sternocostalis) faces forward, upward, and to the left and is located behind the body of the sternum and the cartilages of the third to sixth ribs. The atrioventricular groove (sulcus coronarius) passinig transversely to the longitudinal heart axis and separating the atria from the ventricles, divides the heart into an upper area formed by the atria and a larger, lower area formed by the ventricles. An anterior interventricular groove (sulcus interventricularis anterior) on the sternocostal surface passes on the borderline between the ventricles;the greater part of the anterior surface is formed by the right ventricle, the smaller part by the left ventricle. The inferior or diaphragmatic surface (facies diaphragmatica) adjoins the central tendon at the diaphragm. An inferior interventricular groove (sulcus interventricularis posterior) runs on the surface and separates the surface of the left ventricle (larger) from the surface of the right ventricle (smaller). The lower ends of the anterior and inferior interventricualr grooves of the heart fuse to form the incisure of the apex. The right and left borders of the heart differ in configuration: the right is sharp, while the left border, less sharp because the wall of the left ventricle is thick, is rounded. It is considered that the size of an individual’s heart is equal to the size of his fist. On the average, the heart is 12-13 cm in lenght and no wider than 9-10.5cm. The average antero-posterior dimension is 6- 7cm. The male heart weighs 300g, on the average (1/215 of total body weight), and a female heart, 220g (1/250 of total body weight).

CHAMBERS OF THE HEART The atria are blood-receiving chambers. The ventricles, in contrast, pump blood from the heart into the arteries. The right and left atria are separated by a septum just as the right ventricle communicate by the right atrioventricular orific (ostium atrioventriculare dextrum) and the left atrium with the left ventricle by thr ostium atriventriculare sinistrum. Through these openings blood is directed from the atrial cavities during their contraction into the cavities of the ventricles.

THE RIGHT ATRIUM The right atrium (atrium dextrum) is shaped like a cube. In the back it receives the superior vena cava above and the inferior vena cava below. In front, the atrium is continuous with a hollow process, the auricle of right atrium (auricula dextra). The right and left auricles embrace the base of the aorta and the pulmonary trunk. The atrial septum (septum interatriale) it set obliquely. It passes to the back and to the right from the anterior wall, so that the right atrium is located to the right and anteriorly, and the left atrium to the left and posteriorly. The inner surface of the right atrium is smooth except for a small frontal area and for the inner surface of the auricle where the pectinate muscles (musculi pectinati) form a series of small vertical columns.Superiorly the pectinate muscles are continuous with a crest (crista terminalis), to which the sulcus terminalis corresponds on the external surface of the atrium. This sulcus points to the site where the primary sinus venosus is connected with the atrium of the embryo. On the septum separating the right atrium from the left is an oval depression (fossa ovalis), which is bounded superiorly and anteriorly by a raised edge, annulus ovalis (limbus fossae ovalis). This depression is a remnant of the foramen ovale through which the atria communicate during the intrauterine period. The foramen ovale may persist throughout life (in one- third of all cases), as a consequence of which arterial and venous blood may occasionally mix if contraction of the atrial septum does not close it. On the posterior wall between the orifices of the superior and inferior venae cavae is a small ridge, intervenous tubercle (tuberculum intervenosus) to the back of the superior part of the fossae ovalis.This ridge is thought to direct the flow of blood in the embryo from the superior vena cava into the right atrioventricular orifice. A variably sized crescentic fold (valvula venae cava inferioris) stretches from the inferior margin of the orifice of the inferior vena cava to the limbus fossae ovalis. This fold is of vital importance for the embryo because it directs blood from the inferior vena cava through the foramen ovale into the left atrium. Below this valve , between the openings of the inferior vena cava and the right atrioventricular orifice, the sinus coronarius cordis, which collects blood from the veins of the heart, drains into the right atrium; moreover, small veins of the heart drain independently into the right atrium. Their small openings (foramina venarum minimarum) are scattered over the surface of the atrial walls. A small endocardial fold (valvula sinus coronarii) is found close to the opening of the venous sinus. In the inferoanterior section of the atrium, the wide right atrioventricular orifice (ostium atrioventriculare dextrum) leads into the cavity of the right ventricle.

THE LEFT ATRIUM The left atrium (atrium sinistrium) adjoins posteriorly the descending aorta and the oesoohagus. Two pulmonary veins drain into it from each side. The auricle of the left atrium(auricula sinistra) protrudes anteriorly, passing around the left side of the aorta and pulmonary trunk. The auricle contains pectinate muscles. In the inferoanterior secton, an oval-shaped, left atrioventricular orifice(ostium atrioventriculare sinistrum) leads into the cavity of the left ventricle.

THE RIGHT VENTRICLE The right ventricle (ventriculus dexter) is shaped like a traingular pyramid. The base of the pyramid directed upward makes up the right atrium, with the exception of the upper left angle where the pulmonary trunk, truncus pulmonaris, leaves the right ventricle. The cavity of the ventricle is subdivided into two parts; the section nearest the right atrioventricular orifice, the corpus, and an anterossuperior section close to the orifice of the pulmonary trunk, the conus arteriosus, which is continuous with the pulmonary trunk. The right atrioventricular orifice leading from the cavity of the right atrium into the cavity of the right ventricle is supplied with a tricuspid valve (valva atrioventricularis dextra s. valva tricuspidalis), which prevents the return of blood into the atrium during systole; the blood flows into the pulmonary trunk. Three cusps of the valve are designated, according to their location, as the cuspis anterior, cuspis posterior, and cuspis septalis. The free margins of the cusps face into the ventricle. Fine tendinous threads (chordae tendineae) are attached to them. At their other ends these chordae are attached to the apices of the popillary muscles (musculi papillares). The papillary muscules are conical muscular projections, with the apex projecting into the cavity of the ventricle and the base continuous with the ventricular wall. There are usually three papillary muscules in the right ventricle. The wall of the right ventricle is smooth in the region of the conus arteriosus, but elsewhere there are inwardly projecting muscular trabeculae (trabeculae carneae). Between the longitudinal trabeculae lies a series of transverse ridges, as a result of which a network of trabeculae is produced. Blood from the right ventricle enters the pulmonary trunk through an orifice, the ostium trunci pulmonalis, supplied with a valve, the valva trunci pulmonalis, which prevents the return of blood from the pulmonary trunk into the right ventricle during diastole. The valve is composed of three semilunar cusps called the semilunar valvulae. One of them is attached to the anterior third of the circumference of the pulmonary trunk (valvula semilunaris anterior) and the other two to the posterior section of the circumference (valvulae semilunares dextra and sinistra). A small nodule, the nodulus valvulae semilunaris, is found in the middle of the free inner border of each valve; on each side of this nodule there are thin marginal segments of the valve called the lunulae valvulae semilunares. The nodules make the valves close more lightly.

THE LEFT VENTRICLE The left ventricle (ventriculus sinister) has a conical shape. The thickness of its walls is two or three times that of the wall of the right ventricle (10-15mm, and 5-8mm, respectively). This difference is explained by the muscular layer and by the greater workload of the left ventricle (systematic circulation) compared with that of the right ventricle (pulmonary circulation). The walls of the stria are, nevertheless, thinner than those of the ventricles (2-3mm), in accordance with their function. The trabeculae carneae are thinner and more numerous in the left than in the right ventricle, and there are more of them on the diaphragmatic wall and in the region of the apex. The upper part of the sternocostal surface and the septum, in comparison, are relatively smooth. The orifice leading from the cavity of the left atrium into the left ventricle, the ostium atrioventricularae sinistrum, is oval, and it is supplied with a bicuspid valve (valva atrioventricularis sinistra (mitralis) s. biscuspidalis). The smaller cusp is to the left and back (cuspis posterior), the larger to the right and front (cuspis anterior). The free margins of the valve face into the ventricular cavity; the chordae tendineae are attached to them. There are two apillary muscles, anterior and posterior, in the left ventricle. They are much larger than the papillary muscles in the right ventricle. Each muscle sends tendinous threads to both cusps of the mitral valve. The aortic orifice is called ostium aortae, and the part of the ventricle closest to it is called the infundibulum (conus arteriosus). The aortic valve (valva aortae) is similar in structure to the valve of the pulmonary trunk. One of the valvulaes, the valvula semilunaris posterior, occupies the posterior one-third of the aortic circumference. The other two, the valvulae semilunares dextra and sinistra, occupy the right and the left sides of the orifice. The nodules on their free margins, the noduli semilunarium nortae, are more conspicuous than those on the valves of the pulmonary trunk; lunulae semilunarium aortae are also present. The ventricular septum (septum interventriculare) consists mainly of muscle tissue (pars muscularis), except for the uppermost area where it is formed of fibrous tissue covered on both sides with the endocardium (pars membranacea).

STRUCTURE OF THE HEART WALLS

The walls of the heart are made up of threee layers; an inner layer, the endocardium, a middle layer, the myocardium, and an outer layer, the epicardium, which is the visceral membranae of the pericardium. The thickness of the cardiac walls consists mainly of the middle layer, the myocardium, made up of muscle tissue. The outer layer, the epicardium, is the visceral lining of the serous pericardium. The inner layer, the endocardium, lines the heart cavities. Two layers are distinguished in the heart musculature: the muscular layers of the atrium and the muscular layers of the ventricles. The fibres of the arise from two fibrous rings (anull fibrosi) one of which surrounds the right atrioventricular orifice and the other, the left atrioventricular orifice. Since the fibres of the atrium are not, as a rule, continuous with those of the ventricle, the atria can contract separately from the ventricles. A superficial and a deep muscular layer are distinguished in the atria: the superficial layer consists of circular or transverse fibres, whereas the deep layer is made up of longitudinal fibres arising from the fibrous rings and encircling the atrium like a loop. The large venous trunks draining into the atria are surrounded by circular fibres like sphincters. The fibres of the superficial layer encircle both atria; the deep fibres are separate in each atrium. The musculature of the ventricles is even more complex. Three layers can be distinguished in it. A thin superficial layer is composed of longitudinal fibres which arise from the right fibrous ring and descend obliquely, passing also onto the left ventricle; on the heart apex, they from a whorled mass (vortex cardis), making loops in the depths of the muscle and forming the longitudinal inner layer; the upper ends of the fibres of this layer are attached to the fibrous rings. The fibres of the middle layer, between the longitudinal outer and inner layers, are more or less circular. In contrast to the fibres of the superficial layer, however, they do not pass from one ventricle to the other, but are independent components of each ventricle. An important role in the rhythmic work of the heart and in the coordination of the activity of the musculature of the separatae heart chambers is played by the conducting system of the heart. Although the atrial musculature is separated from the ventricualr musculature by fibrous rings, the two musculatures are, nevertheless, connected by the conducting system, which is composed of a complex of muscle fibres of a special structure (fibres of Purkinje). The cells of these muscle fibres are poor in myofibrils but rich in sarcoplasm, and the fibres are, therefore, lighter in colour than the fibrous rings. They are sometimes visible to the naked eye as light-coloured threads and are the least differentiated part of the primary syncytium, although they are larger than the common muscle fibres of the heart. The following nodes are bundles are distinguished in the conducting system. 1. The atrioventricular bundle (fasciculus atrioventriciularis) arises as a thickening of the atrioventricualr node, nodus atrioventricularis (the node of Aschoff and Tawara), lying in the wall of the right atrium near the cuspis septalis of the tricuspid valve. The fibres of the node are directly connected with the atrial musculature and continue into the septum between the ventricles as the bundle of His (discovered slightly earlier by Kent). In the septum between the ventricles, the bundle of His divides into right and left crura (crus dextrum and crus sinistrum), which pass into the walls of the walls of the respective ventricles and give off branches under the endocardium in the musculature. The atrioventricualr bundle is significant in the work of the heart since it conducts the wave of contraction from the atria to the ventricles that regulates the rhythm of the systole of the atria and ventricles. 2. The sinoatrial node (nodus sinuatrialis) or Keith-Flack’s sinoatrial bundle is located in an area of the right atrial wall corresponding to the sinus venosus in cold-blooded animals (in sulcus terminalis, between the superior vena cava and the right suricle). It is connected with the musculature of the atria and is important for their rhythmic contraction. The atria are, therefore, connected to each other by the sinoatrial bundle and to the ventricles by the atrioventricular bundle. Stimulation from the right atrium is usually conducted from the sinoatrial node to the atrioventricular node and, from there, along the bundle of His to both ventricles. The epicardium covers the outer surface of the myocardium and is an ordinary serous membranes lined by the mesothelium. The endocardium forms the lining of the inner surface of the heart cavities. It is made up, in turn, of a layer of connective tissue rich in elastic fibres and smooth muscle cells, a layer of connective tissue over the first alyer with an admixture of elastic fibres, and an inner endothelial layer, the presence of which distinguishes the endocardium from the epicardium. In origin, the endocardium corresponds to the vascular wall, while its three layers correspond to the three vascualr coats. All the heart valves are folds (duplicatures) of the endocardium.

THE PERICARDIAL SAC The pericardial sac (pericardium) is a closed serous sac, in which two layers are distinguished: an outer fibrous layer, the pericardium fibrosum, and an inner serous layer, the pericardium serosum. The fibrous pericardium merges with the adventitia of the large vessels and attaches anteriorly to the inner surface of the sternum by short cords of connective tissue called ligamenta sternopericardia. The serous pericardium is, in turn, divided into two layers: a visceral layer or the epicardium mentioned above, and a parietal layer, which fuses with the inner surface of the fibrous pericardium and lines it. A slit-like serous cavity (cavum pericardii) containing a small amount of serous fluid (liquor pericardii) exists between the visceral and parietal layers. On the trunks of large vessels close to the heart, the visceral and parietal layers are continuous. The aorta and pulmonary trunk are surrounded on all sides by a common pericardium so taht when the pericardial sac is open a finger can be passed around them. The passage behind the aorta and pulmonary trunk is called the transverse sinus of the pericardium (sinus transversus pericardii). The venae cavae and the pulmonar veins are only partly covered with the serous layer and, therefore, a finger cannot be passed around them. The space bounded below and to the right by the inferior vena cava and above and to the left by the left pulmonary veins is the oblique sinus of the pericardium (sinus obliquus paericardii).

THE TOPIGRAPHY OF THE HEART The heart is located asymmetrically in the anterior mediastenum. Its greater part is to the left of the midline,and only the right atrium and both venae cavae are to the right. The long axis of the heart extends obliquely downward from right to left and from back to front, forming an angle of about 40 degrees with the body axis. the heart is thus rotated so that the right venous part lies more to the front and the left arterial part more to the back. The greater part of the anterior surface of the heart and pericardium, sternocostal surface (facies sternocostalis) is covered by the lungs, whose anterior borders, together with the corresponding part of both pleuras, extend in front of the heart and separate it from the anterior thoracic wall, except for one area of the anterior surface of the heart, which by the pericardium attaches to the sternum and cartilages of the fifth and sixth left ribs. The heart borders are projected onto the chest wall as follows: the apex beat may be felt in the fifth intercostal space 1 cm from the left mamillary line toward the midline. The supeior border of the cardiac projection passes on a level with the superior margin of the third costal cartilages. The right border of the heart passes 2-3 cm to the right of the right sternal border between the third and fifth ribs; the inferior border stretches transversaly from the cartilage of the fifth right rib to the heart apex; the left border passes from the cartilage of the third rib to the heart apex.

THE VESSELS OF SYSTEMIC (GREATER) CIRCULATION

THE ARTERIES OF SYSTEMIC CIRCULATION.

THE AORTA The aorta, the main arterial trunk of systemic circulation, carries blood from the left ventricle of the heart. The following sections can be distinguished in the aorta: (1) the bulbus aortae, (2) aorta ascendens, the ascending aorta, (3) arcus aortae, the arch of the aorta, and (4) aorta descendens, the descending aorta. The aorta ascendens begins as a marked bulbous expantion (bulbus aorte). Both the ascending aorta and the pulmonary trunk behind which it is located are still covered at this point with the pericardium. Behind the manubrium of the sternum, the ascending aorta is continuous with the arcus aortae, which bends posteriorly and to the left, curves over the left bronchus at its very origin, and is then continuous with the descending aorta on a level with the fourth thoracic vertebra. The aorta descendens lies in the posterior mediastinum to the left of the spine. It then deviates slightly to the right so that, in passing through the hiatus aorticus of the diaphragm on the level of the twelfth thoracic vertebra, the aortic trunk is located in front of the spine on the midline. To the level of the hiatus aorticus, the descending aorta is called the thoracic aorta (aorta thoracica); below the hiatus aorticus, when the aorta is already in the abdominal cavity, it is called the abdominal aorta (aorta abdominalis). here, on a level with the fourth lumbar vertebra, it gives off two large lateral branches, the common iliac arteries (bifurcatio aortae), and then continues into the pelvis as a small thin trunk (a. sacralis mediana). To arrest bleeding in the lower arteries, the abdominal aorta is pressed to the vertebral column in the region of the umbilicus located above the bifurcation (the umbilicus marks the level of the aorta).

BRANCHES OF THE ASCENDING AORTA The first vessels that arise from the aorta, therefore, are its branches to the heart, the right and left coronary arteries (aa. coronariae dextra and sinistra).

THE COMMON CAROTID ARTERY The common carotid artery (a. carotis communis) (GK karos heavy sleep) arises from the brachiocephalic trunk on the right and independently from the arch of the aorta on the left side. Both common carotid arteries run upward on the side of the trachea and oesophagus. The right common carotid artery consists only of a cervical section and is, thus, shorter than the left, which consist of a thoracic section (from the arch of the aorta to the left sternoclavicular joint) and a cervial section. The common carotid artery passes in the trigonum caroticum and, at the level of the superior border of the thyroid cartilage or greater horn of the of the hyoid bone, or at the level or the 4 cervical vertebrae divides into its terminal branches, the external and internal carotid arteries (a. carotis externa and a. carotis interna) (bifurcation).

The External Carotid Artery The external carotid artery (a. carotis externa) supplies blood to the external part of the head and neck and is, therefore, distinct from the internal carotid artery penetrating into the cavity of the skull. From its point of origin, the external carotid artery runs upward, passing medial to the posterior belly of the digastric and stylohyoid muscles, pierces the parotid gland, and divides into its terminal branches a.temporalis superficialis and a. maxillaries at the neck of the manibular condyloid process. The anterior group is conditioned by the development and position of organs supplied by the arteries of this group and derived from the visceral arches, namely, the thyroid gland and larynx (the suprior thyroid artery), the tongue (lingual artery) and the face (facial artery). 1.The superior thyroid artery (a. thyreoidea superior) arises from the external caroid artery immediately above its origin and runs downward and anteriorly to the thyroid gland where it anastomoses with the other thyroid arteries. It gives off superior laryngeal artery which with the n vagus ( n. laryngeus superior) penetrates the thyreohyoid membrane and supplies mucouse membrane, ligaments and the muscles of the laryngs, and infrahyoid muscles, part of the sternocleidomaastoid muscle. 1. The lingual artery (a. lingualis) arises at the level of the horns of the hyoid bone, runs upward through Pirogov’s triangle, where it is covered by the hyoglossus muscle, and then passes to the tongue. Before entering the tongue it gives off branches to the hyoid bone, palatine tonsils, and the sublingual gland. The lingual artery extends to the tip of the tongue, where it is called the deep lingual artery (a. profunda linguae). It gives off multiple branches to the back of the tongue (rr. dorsale lingaue) along the way. 2. The facial artery (a. facilais) arises slightly above the lingaul artery at the level of the mandibular angle, passes medially of the posterior belly of the dignosrtic muscle, and reaches the anterior border of the masseter muscle where it bends at a right angle over the border of the mandible onto the face. It passes further to the medial angle of the eye where its terminal branch (a. angularis) anastomoses with the dorsal artery of the nose (a. dorsalis nasi) (a branch of the ophtalmic artery from the system of the internal carotid artery). Before bending over the mandible it gives off branches to the adjacent structures: the pharynx and soft palate, palatine tonsiles ( a. palatine ascendens), submandibualr gland and oral diaphragm (a. submentalis), to suprahyoid muscles, auditory tube. After the bend it gives rise to branches to the upper lip (a. labialis superior) and lower lip (a. labialis inferior). The posterior group 4. The occipital artery (a. occipitalis) arisis below the posterior belly of the digastric muscle, lies in the sulcus on the mastoid process, emerges under the skin in the occipital regions, and gives off branches which stretch to the vertex. Gives off a. meningea posterior for the posterior part of the dura mater by the foramen jugulare or canalis hypoglossus. 5. The posterior auriculur artery (a. auricularis posterior) arises above the posterior belly of the digastric muscle, runs upward and to the back to the skin behind the auricle, cavum tympany and the occipital muscles and reaches the region of the parietal tubers. 6. The sternocleidomastoid artery (a. sternocleidomastoidea) runs to the muscle of the same name, aa.thyroidea superior and inferior supply this muscle. The middle group: 7. The ascending pharyngeal artery (a. pharyngea ascendens) arises from the external carotid artery close to its origin and ascends on the wall of the pharynx and supplies superior and middle constuctors of the pharyngs, the soft palate, the palatine tonsil, the eustachian tube, the tympanic cavity, and the posterior part of the dura mater by the foramen jugulare. 8. The superficial temporal artery (a. temporalis superficialis), one of two terminal branches of the external carotid artery, stretches as the continuation of this artery in front of the external acoustic meatus onto the temple under the skin on the fascia of the temporal muscle. The artery can be pressed here to the temporal bone. On the level of the superior orbital border, it divides into two large anterior and posterior branches (ramus frontalis and ramus parietalis), which branch out in the region of the vertex and temple. 9. The maxillary aretry (a. maxillaris) is another terminal branch of the external carotid artery. For the sake of convenience it is classified into three sections: the first curves around the neck of the maxilla; the second passes in the infratemporal fossa on the surface of the lateral pterygoid muscle; and the third penetrates the pterygopalatine fossa. The branches of the first section ascend to the external acoustic meatus and entre into the tympanic cavity through the fissura petrygotympanica. Its largest branch, the middle meningeal artery (a. meningea media), passes to the dura mater of the middle cranial fossa, which it enters through the foramen spinosum. The first section also gives off the inferior alveolar artery, (a. alveolaris inferior), which stretches downward to the teeth and passes into the mandible through the canalis mandibulae. Before entering the canal, the inferior alveolar artery gives off the ramus mylohyoideus to the milohyoid muscle, while in the canal it supplies the lower teeth with branches. It leaves the canal through the foramen mentale where it is called a. mentalis, which branches out in the skin and muscles of the chin. The branches of the second section pass to the mucous membrane of the maxillary sinus and upper molars (aa. alveolares superiores posteriors) and to all the muscles of mastication and the cheek muscles, receiving names that correspond to the muscles. The branches of the third section: (1) a. infratorbitalis enters the eye socket through the fissura orbitalis inferior, then passes through the canalis infraorbitalis to the anteriorn surface of the maxilla, and sends branches to the lower eyelid, to the lacriaml sac, and down to the upper lip and cheek, lateral part of the skin of the nose. Here it anastamoses with the branches of the facial artery so that when the flow of blood in the trunk of a. maxillaris is disturbed, circulation can occur through a. facialis. While still in the eye socked, a. infraorbitalis gives off branches to the muscles of the eyeball (m. rectus inferior, m.obliquus inferior). As it passes through the inferior orbital canal, it supplies the canine tooth and incisors (aa. alveolares superiores anteriores and media) and the mucosa of the maxillary sinus with small branches; (2) some of the branches reaching the pharynx and the eustachin tube descend to the canalis palatinus major, pass through the foramina palatina major and minor, and spread out in the soft and hard palates, gives off a. canalis pterygoidei which by the pterygoid canal reaches to the pharyngs and auditory tube; (3) a. sphenopalatina runs through the orifice to the same name into the nasal cavity, giving off branches to its lateral wall and to the septum a. nasals posteriors and laterals and a. septalis posterior. One of the branches is longer which by the canalis incisivus (a. incisiva, a. nasopalatina) reaches to the anterior 2/3 of the hard palate. The anterior part of the nasal cavity receives blood through aa. etmoidales anterior and posterior (from a. opthalmica).

The Internal Carotid Artery Having branched off from the common carotid artery (a. carotis interna) rises to the base of the skull and enters the canalis caroticus in the temporal bone. It does not give off branches in the region of the neck; at its origin it lies lateral to a. carotis externa, correponding to the development of the dorsal aorta from the laterally located trunk, but it soon begins to make its way onto the medial surface of the dorsal aorta. In accordance with the convolutions of the canalis caroticus, the internal carotid artery at first runs vertically and then curves anteromedially. At the top of the temporal bone, the artery enters the caity of the skull through the foramen caroticum internum. Bending upward, it rises along the sulcus caroticus of the sphenoid bone, turning forward again at the level of the bottom of the Turkish saddle. Passing through the thickness of the cavernous sinus, the artery makes its last turn at the canalis opticus upward and somewhat posteriorly, giving off its first branch, a. ophthalmica, after which it perforates the dura mater and pia mater and, finally, divides into its terminal branches. The branches of a. carotis internae include: 1. The caroticotympanic branches (rr. caroticotympanic) are small, thin branches penetrating the posterior wall of the canalis caroticus into the tympanic cavity. 2. The opthalmic artery (a. ophtalmica) together with n. opticus, penetrates the canalis opticus into the orbital cavity where it divides into its terminal branches and supplies coats of the eyeball, muscles, the skin opf the forehead. Along the way to the orbit, it gives off several branches. 1. a. meningea anterior by the fissure orbitalis superior entres into the anterior cranial cavity and supplies anterior part of the dura mater; (2) the lacrimal gland (a. lacrimalis); (3) the eyeball, aa. ciliares, ending in the vascualr coat of the eye; one of these branches, a. centralis retinae, penetrates the optic nerve and, together with it, branches out in the retina; (4) the muscles of the eyeball except inferior rectus and inferior oblique muscles; (5) the eyelids, (6) the mucosa of the nasal cavity, aa. ethmoidales anterior by the same name foramen entres into the cranial cavity then by the lamina cribrosa enres into the nasal cavity and supplies anterior part of the nasal cavity and aa. ethmoidales posterior supplies sinus sphenoidalis, ethmoid labytints and the anterior part of the nasal cavity; (7) a.supraorbitalis, exiting from the orbit through the incisura supraorbitalis; and (8) a. dorsalis nasi, descending along the ridge of the nose. 3. The anterior cerebral artery (a. cerebral anterior), which is smaller in size, stretches forward medially to the start of the logitudinal sulcus of the brain, curves around the knee of the corpus callosum, and stretches along the inner surface of the cerebral hemisphere back to the beginning of the occipital lobe, giving off branches along the way to the cerebral cortex. At the origin of the longitudinal sulcus of the brain, it joins with the artery of the same name from the other side by means of a transverse trunk, a. communicans anterior. 4. The medial cerebral artery (a. cerebral media) passes laterally into the depth of the lateral cerebral sulcus. On the surface of insula Reilil, it starts dividing into branches that emerge onto the surface of the hemispheres and supply the external surface of the frontal, temporal, and parieatal lobes with blood, with the exception of the posterior parts of the brain, which receive blood from the system of a. vertebralis. 5. The posterior comunicating artery (a. communicans posterior) branches off from a. carotis interna. It then passes back and drains into a. cerebri posterior (from a. vertebralis). In the subaranchoid space, a. communicans anterior, the beginning segments of aa. cerebri anteriores, aa. communicantes posteriores, and aa. cerebri posteriores (from a. vertebralis) join to form a closed arterial circle - circulus arteriosus cerebri (Willis; circle).

THE SUBCLAVIAN ARTERY Only the left subclavian artery (a. subclavia) belongs to the group of arteries branching directly off the aortic arch. The artery on the right is a branch of the truncus branchiocephalicus. As a result, the right subclavian artery is slightly shorter than the left. The artery forms a convex arch, curving around the cupola of the pleura. It leaves the thoracic cavity through the apertura superior, approaches the clavicle, and fits into the sulcus of a. subclaviae of the first rib and bends over it. Here the subclavian artery may be compressed to stop the flow of blood to the first rib behind the tuberculum m. scaleni. The artery proceeds further into the axillary fossa where, beginning with the external edge of the first rib, it has been given the name a. axillaris. Along the way, the subclavian artery, together with the brachial plexus nervosus, passes through the spatium interscalnum. Thus, three segments can be distinguished in the artery: the first from the beginning of the artery to the entrance into the spatium interscalenum, the second in the spatium interscalenum, and the third from its exit to the transition into a. axillaris. The branches of the first segment of the subclavian artery (up to its entry into the spatium interscalenum): 1. The vertebral artery (a. vertebralis), the first branch defects upward into the interval between m. scalenus anterior and m. longus colli, passes into the foramen transversarium of the sixth cervical vertebra, and rises through the openings in the transversae processes of the cervical vertebrae to the membrana atlantooccipitalis posterior, which is perforates to enter the cranial cavity through the foramen magnum of the occipital bone. Within the skull, the vertebral arteries from both sides pass torward the median line and from a single unpaired artery (a. basilaris) near the posterior edge of the pons. Along the way. this artery gives off small branches to the muscles, the spinal cord and dura mater of the posterior cranial fossa, as well as the large branches. (a) A. spinalis anterior branches off in the cranial cavity near the junction of two vertebral arteries and passes down to the median line to meet with the branch of the same name from the contralateral side; they merge into a single trunk. (b) A. spinalis posterior branches off the vertebral artery as soon as it enters the cranial cavity and also runs down both sides of the spinal cord. as a result, three arterial trunks descend along the spinal cord; the unpaired trunk of the anterior surface (a. spinalis anterior) and two paired trunks on the posterolateral surface, one on each side (aa. spinales posteriores). (c) A. cerebelli inferior posterior is the largest branch of a. vertebralis; it arises near the pons, turns back and, bypassing the medulla oblongata, branches out on the lower surface of the cerebellum. The basilar artery (a. basilaris) is formed by the union of both vertebral arteries; it is unpaired and lies in the median sulcus of the pons. At the anterior margin of the pons, it divides into two aa. cerebri posteriores (one on each side), which pass back and upward, curving around the lateral surface of the cerebral peduncles, and branch out on the lower, inner, and external surfaces of the octipital lobe. Receiving the aa. communicantes posteriores described above from a carotis interna, the posterior cerebral arteries, participate in the formation of circulus arteriosus cerebri (Willis’ circle). Small branches lead from the trunk of a. basilaris to the pons, into the internal ear, passing through the meatus acusticus internus. Two branches lead to the cerebelum: a. cerebelli inferior anterior and a. cerebelli superior. A. vertebralis, which passes parallel to the trunk of the common carotid artery and participates, together with it, in supplying the brain with blood, is a collateral vessel for the head and neck. A bazilaris, two vertebrai arteries joined in one trunk and two aa. spinales anteriores also joined in one trunk, form Zakharchenko’s arterial circle, which, together with circulus arteriosus cerebri, is important in the collateral circualtion of the medulla oblongata. 2. The thyrocervical trunk (truncus thyreocervicalis) divides into the following branches: (a) a. thyroidea inferior passes to the posterior surface of the thyroid gland giving off a. laryngea inferior, which branches out in the muscles and the mucosa of the larynx and anastomoses with a laryngea superior with branches to the trachea, oesophagus infrahyoid muscles, and thyroid gland; these last branches anastamose with branches of a. thyreoidea superior in the system a. carotis externa; (b) a. cervicalis ascendens supplies the deep muscles of the neck; (c) a. suprascapularis passes down and laterally from the trunk to the incisura scapuale and, bending over lig. transversum scapulae, branches out in the dorsal muscles of the shouleder blade, anastamosing with a. circumflexa scapulae, d), a.cervicalis superficialis for trapezoid muscle. 3. The internal thoracic artery (a. thoracica interna) originates from a. subclavia opposite the origin of a. vertebralis and descends medially, close to the pleura; beginning from the cartilage of the first rib, it extends vertically at a distance of about 12 mm from the edge of the sternum. When it reaches the lower edge of the seventh rib cartilage, a. thoracia interna divides into two terminal branches: a. musculophrenica for the diaphragm, and a. epigastrica superior, which continues the descent of a. thoracia interna, penetrating sheath of the rectus abdominis muscle and, on reaching the level of the navel, anastomoses with a. epigastrica inferior (from a. iliaca externa). Along its way, a. thoracia interna gives off branches to the nearest anatomical formations: to the connective tissue of the anterior medianstinum, thymus, lower end of the trachea, and the bronchi, to six upper intercostal spaces, and to the mammary glnds. Its long branch, a. pericardiacophrenica, passes, together with n. phrenicus, to the diaphragm, giving of small branches along the way to the pleura and pericardium. Its rami intercostales anteriores run through the six superior intercostal spaces and anastamoses with aa. intercostales posteriores (from the aorta). The branches of the second segment of the subclavian artery: 4. The costocervial trunk (truncus costocervicalis) branches to the spatium interscalenum and passes back and up to the neck of the first rib where it divides into two branches: (a) deep cervical artery (a. cervicalis profunda), which penetrates the posterior muscle of the neck and gives off small branches in the canalis vertebralis to the spinal cord; and (b) superior intercostal artery (a. intercostalis suprema), which branches into the first and second intercostal spaces. The branches of the third segment of the subclavian artery: 5. The transverse cervical artery (a. transversa colli) perfoartes the plexus brachialis, supplies the neighbouring, and descends along the vertebral edge of the shoulder blade to its inferior corner.

THE VEINS OF SYSTEMIC CIRCULATION THE SYSTEM OF VENA CAVA SUPERIOR The superior vena cava (vena cava superior) is located to the right and somewhat posteriorly of the ascending aorta. It is formed by the merger of the right and left innominate veins (vv. brachiocephalicae dextra and sinistra) behind the junction of the first right rib with the sternum. From there it passes downward at the level of the third rib, concealed behind the right auricle of the heart; it drains into the right atrium. V. azygos drains into the vena cava superior.

THE INNOMINATE VEINS The right and left innominate veins (vv. brachiocephalicae dextra and sinistra), which form the vena cava superior, are, in their turn, each formed by the merger of the subclavian and internal jugular veins (v. subclavia and v. jugular is interna) behind the sternoclavicular joint. The angle which is formed by the subclavian and the internal jugular vein is called venos angle. The right innominate vein, just 2-3 cm in length, is shorter than the left. Vv. thyreoideae inferiores drain into the innominate veins.

THE INTERNAL JUGULAR VEIN The internal jugular vein (v. jugularis interna) carries blood from the cavity of the skull and the organs of the neck. Beginning at the foramen jugulare, where it forms a distention (the bulbus superior venae jugularis internae), the vein passes downward, laterally of a. carotis interna and, further down, laterally of a. carotis communis. As second distention (the bulbus inferior v. jugularis internae) is formed at the lower end of v. jugularis internae before it joins v. subclavia; there are one or two valves in the vein in the region of the neck above this distention. In the back region the internal jugular vein is covered by mm sternocleidomastoideus and omohyoideus. The following extracranial veins flow into v. jugularis interna: 1. v. facialis communis which is formed by the merger of the v. facialis and the v. retromandibularis. The facial vein (v. facialis) passes under m. stylohyoideus and m . sternocleidomastoideus. (It sometimes drains into v. jugularis ext.) The veins draining into the facial vein correspond to the branches arising from the facial artery. At the angle of the mandible connectes whith the retromandibular vein. It carries blood from the upper and lower lips, palatal and the submandibular region. 2. v. retromandibularis collects blood from the temporal region. Further down, a trunk carrying blood out of the plexus pterygoideus (a dense plexus between mm. pterygoidei) drains into v. retromandibularis after which the vein, passing through the thickness of the parotid gland with the external carotid artery, merges with v. facialis below the angle of the lower maxilla. 3. T'he pharyngeal veins (vv. pharyngeae) form a plexus on the pharynx and flow either directly into v. jugularis interna or empty into v. facialis. 4. The lingual vein (v. lingualis) accompanies the artery of the same name. 5. The superior thyroid veins (vv. thyreoideae superiores) collect blood from the upper parts of the thyroid gland and the larynx. 6. The middle thyroid vein (v. thyreoidea media) THE EXTERNAL JUGULAR VEIN The external jugular vein (v. jugularis externa) originates behind the floor of the auricle and emerges at the level of the maxilla angle, from the region behind the mandibular fossa. From there it descends into the subcutaneous tissue covered by m. platysma and travels along the external surface of the sternocleidomastoid muscle. On reaching the posterior edge of the sternocleidomastoid muscle, the vein penetrates the superficial layer of the fascia colli, enters the supraclavicular region where it immediately descends and drains into the internal jugular vein or into the venouse angle. Behind the floor of the auricle, v. auricularis posterior and v. occipitalis drain into v. jugularis externa. The external jugular vein Has connection with the retromandibular vein.

THE ANTERIOR JUGULAR VEIN The anterior jugular vein (v. jugularis anterior). It is made up of small branches located over the hyoid bone, from which point it descends. Both the right and left anterior jugular veins perforate the deep fascia colli propriae, enter the spatium interaponeuroticum suprasternale and drain into the subclavian vein. In the suprasternal space the anterior jugular veins anastomose, with either one or two trunks. Thus a venous arch, arcus venosus juguli, is formed over the top edge of the sternum and the clavicles. In some cases the right and left veins are replaced by a single unpaired vein, v. jugularis anterior, which descends along the median line and drains below into the venous arch formed, in such instances, by an anastomosis between vv. jugulares externae.

THE MENINGES OF THE BRAIN

The membranes of the brain, the meninges, are the direct continuation of the meninges of the spinal cord, the dura mater, the arachnoid mater, and the pia mater. The arachnoid mater and the pia mater together, like those in the spinal cord, are called the leptomeninges.

THE DURA MATER

The cerebral dura mater (dura mater encephali), or pachymeninx, a thick whitish connective-tissue membrane is outermost in position. Its external surface is in direct contact with the cranial bones for which it serves as the periosteum; this is the main feature distinguishing it from the spinal dura mater. The inner surface facing the brain is lined with endothelium and is therefore smooth and shiny. Between it and the cerebral arachnoid mater is a narrow slit-like subdural space (cavum subdurale) filled with a small amount of fluid. At places the dura mater is separated into two layers, namely, in the region of the venous sinuses (see below) and in the region of the fossa at the apex of the pyramid of the temporal bone (to form the cavum trigeminale) where the trigeminal nerve ganglion is located. The dura mater gives off several processes from its inner surface, which penetrate between the parts of the brain and separate one part from another. The falx cerebri, a large sickle-shaped process, lies sagittally between both cerebral hemispheres. On the midline of the calvaria it is attached to the margins of the sulcus sinus sagittalis superioris, its anterior narrow end grows into the crista galli, while the wide posterior end blends with the superior surface of the tentorium cerebelli. The tentorium cerebelli is a horizontally stretched plate slightly convex upward like a roof with two sloping surfaces. The tentorium cerebelli separates the cerebral occipital lobes from the cerebellum lying below them. The falx cerebelli, a small sickle-shaped process, lies, like the falx cerebri, on the midline along the crista occipitalis interna and stretches to the foramen magnum whose sides it embraces with two limbs and separates the cerebellar lobes. The diaphragma sellae is a plate forming the roof over the fossa in which the hypophysis cerebri is lodged on the floor of the sella turcica. The dura mater contains several reservoirs collecting blood from the brain; these are the sinuses at the dura mater (sinus durae matris). The sinuses are venous canals (triangular on transverse section) devoid of valves and located in the thickness of the dura mater at the attachment of its processes to the skull. The sinuses are as follows. The transverse sinus (sinus transversus), the largest and widest sinus and is formed by the sulcus sinus transverses of the occipital bone and the dura mater. From here it descends into the sulcus sinus sigmoidei under the name of the sigmoid sinus (sinus sigmoideus), and at the jugular foramen is continuous with the orifice of the internal jugular vein. As a result, the transverse and sigmoid sinuses form the main receptacle for all the venous blood of the cranial cavity. All the other sinuses drain into it either directly or indirectly. The following sinuses drain directly into it. The superior sagittal sinus (sinus sagittalis superior) runs on the upper margin of falx cerebri for the whole length of sulcus sinus sagittalis superioris from crista galli to the internal occipital protuberance (on either side of the superior sagittal sinus, within the dura mater. The occipital sinus (sinus occipitalis) is a continuation, as it were, of the superior sagittal sinus along the attachment of falx cerebelli to the internal occipital crest. The straight sinus (sinus rectus) runs on the line of attachment of falx cerebri to tentorium cerebelli. It receives anteriorly the inferior sagittal sinus (sinus sagittalis inferior) stretching on the free lower margin of falx cerebri and vena cerebri magna (Galeni) carrying blood from the deep parts of the brain. At the confluence of these sinuses (transverse, superior sagittal, straight, and occipital) a common expansion forms; it is called the confluence of the sinuses (confluens sinuum). The cavernous sinus (sinus cavernosus) is located on the base of the skull lateral to the sella turcica. It has the apperance of either a venous plexus or a wide lacuna surrounding the internal carotid artery. The cavernous sinus is drained of blood by two sinuses located behind it, namely the inferior and superior petrosal sinuses (sinus petrosus superior// drains into the sigmoid sinus// and inferior// drains into the v. jugularis interna//) located in the superior and inferior petrosal sulci.

THE ARACHNOID MATER

The cranial arachnoid mater (arachnoidea encephali) like the spinal arachnoid membrane is separated from the dura mater by a capillary subdural slit-like space. In contrast to the pia mater it does not penetrate into the sulci and depressions of the brain but bridges them as a result of which a subarachnoid space (cavum subarachnoideale) filled with a clear fluid forms between these two membranes. In places, mainly on the base of the brain, the subarachnoid spaces are particularly well developed and form wide and deep reservoirs for the cerebrospinal fluid-cisternae. These cisternae are as follows. 1. The cerebellomedullary cisterna (cisterna cerebellomedullaris) is the largest and is located between the posterior border of the cerebellum and the medulla oblongata. 2. The interpeduncular cisterna (cisterna interpeduncularis) lies between the cerebral peduncles. 3. The chiasmatic cisterna (cisterna chiasmatis) lies in front of the optic chiasma. 4. The cisterna of the ponse lies on the ventral surface of the ponse. THE PIA MATER

The pia mater of the brain (pia mater encephali) is in intimate contact with the brain and entres into all sulci and fissures on its surface; it contains blood vessels and vascular plexuses, in some spaces form plexus chorioideus.

THE CEREBROSPINAL FLUID

The cerebrospinal fluid (liquor cerebrospinalis) from the 4- th ventricle by Magendie's and Luska’s foramina drains into the subarachnoid space( cavum subarachnoidale) into the cerebellomedullary cisterna and the cisterna of the ponse then reaches to the sinus sagitalis superior and by the pachion bodies which form arachnoidea drains into the sinus sagitalis superior.

THE CERVICAL PLEXUS

The cervical plexus (plexus cervicalis) is formed by the anterior branches of four superior cervical nerves

(CI-CIV) which are located laterally of the transverse processes between the prevertebral muscles and vertebral muscles, anastomosing with n. accessorius, n. hypoglossus and tr. sympathicus. In front the plexus is covered by m. sternocleidomastoideus. The branches arising from the plexus are divided into cutaneous, muscular and mixed. The cutaneous branches: 1. The lesser occipital nerve (n. occipitalis minor), runs to the skin of the lateral part of the occipital region. 2. The great auricular nerve (n. auricularis magnus), the thickest nerve of the cutaneous branches of the cervical plexus, passes to the concha auriculae, supplying it and the external acoustic meatus. 3. The anterior cutaneous nerve of the neck (n. transversus colli) supplying the skin of the neck, passes under the platysma muscle. 4. The supraclavicular nerves (nn. supraclaviculares) descend under the platysma nearly perpendicularly along the supraclavicular fossa into the skin above the pectoralis major and deltoideus. The muscular branches: 1. Branches to the deep muscles of the neck. 2. Radix inferior, passes in front of the v. jugularis interna under the sternocleidomastoid muscle and at the intermediary tendon of the m. omohyoideus joins with radix superior( 12-th nerve) forming a cervical loop (ansa ceroicalis) By means of the branches arising from the ansa, the fibres of the cervical plexus innervate the m. sternohyoideus, m. sternothyreoideus and m. omohyoideus. 3. Branches to the m. sternocleidomastoideus and m. trapezius which participate in the innervation of these muscles together with the accessory nerve. The mixed branches: The phrenic nerve (n. phrenicus) descends along m. scalenus anterior into the thoracic cavity where it passes between the subclavian artery and vein. Further the right phrenic nerve descends nearly vertically in front of the root of the right lung and passes along the lateral surface of the pericardium to the diaphragm. The left phrenic nerve descends with the left v. brachiocepfalica, crosses the anterior surface of the aortic arch and passes in front of the root of the left lung along the left lateral surface of the pericardium to the diaphragm. Both nerves pass in the anterior mediastinum between the pericardium and pleura. The phrenic nerve is a mixed nerve: its motor branches innervate the diaphragm thus functioning as a nerve that is responsible for respiration; it sends sensory nerves to the pleura and pericardium. Some of the terminal branches of the nerve pass through the diaphragm into the abdominal cavity (nn. phrenicoabdominales) sending small branches to the peritoneum. The hepatic ligaments and to the liver itself; as a result, when liver is affected, a special phrenicus-symptom may arise.

THE LYMPHATIC SYSTEM The lymphatic system (systema lymphaticum) constitutes part of the vascular system and may be considered a supplementary bed of the venous system. It develops in close relationship with the venous system and shares with it similar structural features (the existence of valves, the direction of the flow of lymph from the tissues to the heart). The main functions of the lymphatic system consist in conducting lymph from the tissues into the venous bed, in rendering harmless the foreign particles, bacteria and so on, that find their way into the organism, and in forming lymphoid elements (lymphopoiesis), which participate in immunological reactions. In addition, the cells of malignant tumours spread along the lymphatic system; a thorough knowledge of the anatomy of the lymphatic system is essential in determining these paths. The lymphatic system is classified according to function into the following groups: (1) paths that conduct lymph: lymph capillaries, vessels, and ducts; (11) sites at which lymphoid elements develop: (1) the bone marrow and the thymus (central organs of lymphopoiesis); (2) lymphoid organs in mucous membranes: a) solitary lymph nodes, folliculi lymphatici solitarii; b) lymph follicles bunched in groups, folliculi , lymphatici aggregati; c) formations of almond-shaped lymphoid tissue, tonsillae; d) accumulations of tissue in the vermiform process; e) white pulpa of the spleen (3) lymph nodes, nodi lymphatici (s. lymphonodi). The organs included under (2) and (3) are the peripheral organs of lymphopoiesis. All the formations mentioned above perform simultaneously a barrier role in neutralizing foreign particles in the organism. The presence of lymph nodes distinguishes the lymphatic from the venous system. The two systems are further distinguished since the venous capillaries communicate with the arterial capillaries, whereas the lymphatic system is a system of tubes closed at one end (the peripheral end) and opening at the other end (central) into the venous bed. The lymphatic system consists of the following parts: (1) the closed end of the lymphatic bed begins with a meshwork of lymph capillaries piercing the tissue of the organs; (2) the lymph capillaries develop into intraorganic plexuses of small lymphatic vessels possessing valves; (3) these vessels emerge from the organs in the form of still larger, extraorganic, abducent lymphatic vessels interrupted by lymph nodes; (4) the large lymphatic vessels drain into collectors and, further, into the main lymph trunks of the body- right and left (thoracic) lymphatic ducts, which, in their turn, drain into the big veins of the neck. The lymph capillaries absorb and resorb colloid solutions of protein substances that are not absorbed by the blood capillaries from the tissues. The lymph capillaries also help the veins to drain the tissues by absorbing aqueous fluid and the crystalloids dissolved in it. They also remove foreign particles, bacteria, and so on from the tissues under pathological conditions. The lymph capillaries perform these functions through a system of endothelial tubes that penetrate all organs of the body except the brain, splenic parenchyma, epithelial coat of the skin and mucous membranes, cartilages, cornea, lens of the eye, and placenta. The structure of the initial lymphatic networks varies. It is determined by the structure and function of the organ and the properties of the connective tissue in which the lymph capillaries have been laid. The direction of the loops of these capillaries corresponds to the direction and position of the fasciculi of connective tissue, muscle fibres, glands, and other structural elements of the given organ. Lymphatic vessels. The transition of lymph capillaries into lymphatic vessels is determined by changes in the structure of the wall and the appearance of a valve in the vessel, which serves as a border between it and the capillary. The intraorganic lymphatic vessels form widely looped plexuses and run parallel to the blood vessels in the connective-tissue layers of the organ. Abducent lymphatic vessels (extraorganic) leading to various lymph nodes arise from every organ or part of the body. G.I. Iosifov divides them into main (or collector) vessels and secondary vessels. The collectors are the main lymphatic vessels, which form when the secondary and attendant arteries or veins join. After passing through the last group of lymph nodes (see below), the lymph collectors form lymph trunks, corresponding in number and position to the large parts of the body. Thus, the main lymph trunk for the lower limb and pelvis is the truncus lumbalis, which forms from the efferent vessels of the lymph nodes located by the aorta and vena cava inferior. For the upper extremity the main lymph trunk is the truncus subclavius which passes along v. subclavia. The head and neck are supplied by the truncus jugularis passing along v. jugularis interna. In addition, the thoracic cavity is served by the paired truncus bronchomediastinalis. The unpaired truncus intestinali is sometimes encountered in the abdominal cavity. All these trunks eventually join in two terminal ducts: the ductus lymphaticus dexter and ductus thoracicus, which drain into the large veins, mainly the internal jugular vein . Lymph nodes. Before entering the thoracic duct or the right lymphatic duct, the lymph passes through several lymph nodes, nodi lymphatici s.lymphonodi, which are situated singly or, more frequently, in groups on the paths of the lymphatic, vessels. The nodes are round or oval formations, ranging in size from a grain of wheat to a bean. The lymph node is covered by a connective-tissue capsule from which the septa trabeculae protrude into the node. Lymphoid tissue, in the form of cortical, paracortical (paracortical zone), or cerebral matter, lies between the trabeculae. There are spaces (lymphatic sinuses) between the trabeculae and the lymphoid tissue. The lymph comes to the lymph node through afferent lymphatic vessels, vasa afferentia, which enter its convex side and open into sinuses. There are special cells (macrophages) in the walls of the lymphatic sinuses which accomplish phagocytosis of foreign bodies brought by the flowing lymph. In the sinuses the lymph flow slows down and sweeps along with it the lymphocytes forming in the tissue of the node; the lymph then departs through efferent lymph vessels, vasa efferentia, which exit through the porta on the concave side of the node. Thus, lymph nodes are distinguished from lymphoid organs and tonsils, which have only efferent lymphatic vessels and lack afferent lymphatic vessels. The lymphatic vessels of any organ pass through certain regional groups of nodes located near the given organ. The regional nodes for the internal organs are usually located at their hilum. In the "soma" large accumulations of lymph nodes are situated in protected and mobile places near the joints, the movements of which help the lymrh to pass through the nodes. Thus, large groups of nodes are concentrated on the lower extremity in the popliteal fossa and the groin; on the upper extre.mity by the elbow joint and the axillary fossa; on the trunk in the lumbar region and the neck, i.e. , near the most mobile parts of the spinal column. The lymph nodes have arteries and veins which are branches (arteries) and affluents (veins) of neighbouring vessels. The nodes are also characterized by afferent and efferent innervation. Since lymph nodes can arrest foreign bodies (bacteria, tumorous cells, etc.) that enter them through the lymphatic vessels, substances of pathogenic origin may accumulate in the nodes. Knowledge of their topography can be of great diagnostic and therapeutic significance.

THE THORACIC DUCT The thoracic duct (ductus thoracicus) is from 30 to 41 cm long and originates at the junction of the right and left lumbar trunks (truncus lumbalis dexter and sinister). Usually described in textbooks as the third root of the thoracic duct, the intestinal trunk (truncus intestinalis) is not often encountered. It is sometimes paired, and it drains, most frequently, into the left and, occasionally, into the right lumbar trunk. The level of the origin of the thoracic duct varies between the eleventh thoracic and second lumbar vertebrae. At its origin there is a dilatation in the thoracic duct, the cisterna chyli, which may often (in 42 per cent of all cases) be absent. Arising in the abdominal cavity, the thoracic duct passes into the thoracic cavity through the aortal orifice. There it grows together with the right peduncle of the diaphragm, the contractions of which help the lymph to move along the duct ("passive lymphatic heart"). On entering the thoracic cavity, the duct rises in front of the spinal column to the right of the thoracic aorta, behind the oesophagus and, then, behind the aortic arch. Reaching the aortic arch at the level of the third-fifth thoracic vertebra, the duct begins inclining to the left. (Lymph nodes are nearly always situated along the thoracic duct.) At the level of the seventh cervical vertebra, the thoracic duct forms an arch with its convexity facing upward and drains into the left internal jugular vein or into the angle of its confluence with the left subclavian vein (angulus venosus sinister). The thoracic duct is supplied inside with two well-developed folds which prevent blood from flowing into it when it drains. Three trunks drain into the upper part of the thoracic duct: the left mediastinal trunk (truncus bronchomediastinalis sinister), which collects lymph from the left half of the walls and organs of the thoracic cavity, the left subclavian trunk (truncus subclavius sinister), which collects lymph from the left upper extremity, and the left jugular trunk (truncus jugularis sinister), which collects lymph from the left half of the neck and head. Thus, the thoracic duct collects three-fourths of all lymph in the body with the exception of lymph from the right half of the head and neck, the right arm, and the right half of the chest and thoracic cavity. The lymph from these areas flows into the right lymphatic duct.

THE RIGHT L YMPHATIC DUCT The right lymphatic duct (ductus lymphaticus dexter) is no more than 10-12 mm long and is formed by the confluence of three trunks: the right jugular trunk (truncus jugularis dexter), which receives lymph from the right side of the head and neck, the right subclavian trunk (truncus subclavius dexter), which carries lymph from the right upper extremity, and the right mediastinal trunk (truncus bronchomediastinalis dexter), which collects lymph from the right half of the thoracic internal organs and the right half of the thoracic wall. The right lymphatic duct drains into the right subclavian vein. It is quite often absent, in which case the three trunks mentioned above drain independently into the subclavian vein.

THE HEAD AND NECK Lymph from the head and neck collects in the right and left jugular lymph trunks, the truncus jugularis dexter and sinister, which run on each side of the neck parallel to the internal jugular vein. The right trunk drains into the ductus lymphaticus dexter or directly into the right venous angle, the left into ductus thoracicus or directly into the left venous angle. Before flowing into the duct the lymph passes through the regional lymph nodes. On the head lymph nodes group mainly along the border between the head and the neck. Among these groups of nodes we can note the following. 1 Occipital lymph glands (nodi lymphatici occipitales) drain the lymphatic vessels coming from the posteroexternal part of the temporal, parietal, and occipital regions of the head. 2. Mastoid lymph glands (nodi lymphatici retro-auriculares) collect lymph from the same areas as the occipital nodes as well as from the posterior surface of the floor of the auricle, the external acoustic meatus, and the tympanic membrane. 3. Parotid lymph glands (nodi lymphatici parotidei), superficiales and profundi, collect lymph flowing from the forehead, temple, lateral part of the eyelids, external surface of the floor of the auricle, temporomandibular articulation, parotid and lacrimal glands, walls of the external acoustic meatus, tympanic membrane, and both auditory tubes. 4. Submandibular lymph glands (nodi lymphatici submandibulares) collect lymph from the lateral side of the chin, the upper and lower lips, cheeks, nose, gums and teeth, medial part of the eyelids, hard and soft palates, body of the tongue, and submandibular and sublingual salivary glands. 5. Mandibular lymph glands (nodi lymphatici mandibulares) collect lymph from the eyeball, muscles of facial expression, mucous surface of the cheek, lips, and gums, muciparous glands of the oral 'cavity, periosteum of the region of the mouth and nose, and submandibular and sublingual glands. 6. Submental lymph glands (nodi lymphatici submentales) collect lymph from the same regions of the head as the mandibular nodes, as well as from the tip of the tongue. 7. Buccal lymph glands (nodi lymphatici buccales) collect lymph from the mucous membrane of the cheek and the cheek muscle. 8. Retropharyngeal lymph glands (nodi lymphatici retropharyngei) drain lymph from the mucous membrane of the nasal cavity and its accessory air-passage cavities, from the hard and soft palates, the root of the tongue, nasal and oral parts of the throat, as well as from the middle ear . Lymph from the above nodes flows to the cervical nodes. On the neck there are two groups or lymph nodes: superficial (nodi) limphatici cervicales superficiales) and deep (nodi lymphatici cervicales profundi). The superficial lymph nodes are divided into anterior nodes, located below the hyoid bone between the two main neurovascular nodes of the neck, and the lateral nodes, situated along v. jugularis externa. The deep lymph nodes form a chain along the length of v. jugularis interna as well as along the nerves of the spinal cord and a. transversa colli. They play major role in the outflow of lymph from the organs and tissues of the head and neck. It is possible to distinguish nodes in front of the larynx, the thyroid gland, and the trachea, nodes running parallel to the sides of the trachea along the recurrent nerves, and supraclavicular nodes. All deep cervical lymph nodes can be divided generally into two large groups: superior and inferior . Among the superior deep cervical lymph nodes, the nodus lymphaticus jugulodigastricus deserves special attention. Located on the internal jugular vein at the level of the greater horn of the hyoid bone, this node receives lymph from the vessels of the posterior third of the tongue and becomes greatly enlarged when this part of the tongue is affected by cancer. The nodus lymphaticus jugulo-omohyoideus is one of the most important of the inferior deep cervical lymph nodes. This node lies on the internal jugular vein directly over m. omohyoideus. It receives the lymphatic vessels of the tongue either directly or through the mental or submaxillary lymph nodes, which may contain cancerous cells. The lymphatic vessels of the skin and muscles of the neck pass to nodi lymph cervicales superficiales. Those of the larynx (the lymph plexus of the mucosa above the vocal chords) flow through the membrana thyreohyoidea to nodi lymph. cervicales profundi (superiores). The lymphatic vessels of the mucosa below the rima glottidis pass along two routes-forward through the membrana thyreohyoidea to nodi lymph. cervicales profundi (prelaryngeal) and backward to the nodes located along n. laryngeus recurrens (tracheal). The lymphatic vessels of the thyroid gland flow mainly to nodi lymph cervicales profundi (prethyroid) and from the isthmus to the anterior superficial cervical nodes. From the pharynx and palatine tonsils, lymph flows to nodi lymph. retropharyngei and cervicales profundi.