G. KYALYAN R. PETROSYAN

HUMAN ANATOMY

Adapted course for foreign students

Volume III The control and communication

Yerevan 2002 LITERATURE

1. Human Anatomy M.R. Sapin Russian edition, 1993

2. Human Anatomy M. Prives N. Lysenkov V. Bushkovich English edition, 1985

3. Human Anatomy Robert Carola John P. Harley Charles P. Woback English edition, 1992

4. Gray’s Anatomy Edited by Peter L. Williams English edition, 1993

2 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. Consequently, the nervous system is “an indescribably complex and fine instrument of relations involving the connection of numerous parts of the organism between one another and with the organism as a whole in a complex system with an infinite number of external influences” (I.P. Pavlov).

3 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 (I.M. Sechenov). The simple reflex arc consists of at least two neurons, one of which connects with a sensory surface (the skin, for

4 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 ocated. 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 a third in ternuncial 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. In man and other higher animals neurons also form temporary reflex connections of the highest order on the basis of simple and complex reflexes. These temporary reflex connections are known as conditioned reflexes (Pavlov). 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; the analysis begins from this phenomenon (Pavlov). 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

5 transformation of the impulse received by the centre into an external reaction. This synthesis “evidently represents the phenomenon of the nerve connector” (Pavlov). This is why Pavlov calls this neuron the connector. 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. Besides the reflex arc, a reflex circle has been found recently which participates as a basic component of nervous system activity.

6 Modern cybernetics has established the common feedback principle of connections in the control and coordination of processes in both modern automatons and living organisms. From this viewpoint a feedback connection can be distinguished in the nervous system between the working organ and the nerve centres. This phenomenon called feedback afferentation (Anokhin) involves the transmission of impulses about the activity of the organ at any given moment from the working organ to the central nervous system. When the centres of the nervous system send efferent impulses to the executive organ, certain actions (movement, secretion) are triggered in this organ. These actions, in turn, stimulate nervous (sensory) impulses which return along afferent routes to the spinal cord and brain signalling that a certain action has just been performed by the working organ. Thus, the essence of feedback afferentation, is, figuratively speaking. a report to the centre that its command has been fulfilled by the periphery. When the hand reaches for an object, for example, the eyes constantly measure the distance between the hand and the object and dispatch the information as afferent signals to the brain. A contact is made in the brain with efferent neurons which convey motor impulses to the muscles of the hand reaching for the object. At the same time the muscles act upon the receptors within them to transmit continuous sensitive signals to the brain and thus report on the position of the hand at every moment. This two-way signalization along the reflex circuits continues until the distance between the hand and the

7 object is, reduced to zero, i.e. until the hand grasps the object. The action of the working organ is thus constantly selfcontrolled by the mechanism of feedback afferentation, which functions as a closed circuit in the following succession: from the centre (the instrument setting the programme of action) to the effector (motor) to the tool (working organ) to the receptor (receiver) and back to the centre. 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

8 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.

GENERAL DEVELOPMENT OF THE NERVOUS SYSTEM

Phylogenesis of the nervous system briefly amounts to the following. The single-celled protozoa (the amoeba) have no nervous system and their connection with the environment is accomplished by means of fluids present both in and outside of the organism. This is the humoral, preneural form of control. Later, when the nervous system originates, another form of control, i.e. neural control, appears. With the gradual development of the nervous system the humoral control becomes more' and more subordinate to neural control so that a single neurohumoral control forms in which the nervous system plays the leading role. In the process of phylogenesis the nervous system passes through a series of principal stages. Stage I, a network nervous system. In this stage (the coelenterata) the nervous system, e.g. that of the hydra, is

9 formed of nerve cells whose numerous processes are interconnected in different directions to form a network diffusely piercing the whole body of the animal. In stimulation of any point of the body, the stimulus spreads along the entire nervous network and the animal responds by movement of the whole body. The network structure of the intramural nervous system in man is a reflection of this stage (E.K. Sepp). Stage II, a ganglionic nervous system. In this stage (higher worms) the nerve cells come together to form aggregations or groups; the aggregations of cell bodies form the nerve ganglia, the centres, whereas aggregations of the processes form trunks, the nerves. The number of processes in each cell reduces and they acquire a definite direction. According to the segmental structure of the animal's body, e.g. in the segmented worm, each segment has segmental nerve ganglia and nerve trunks. The trunks join the ganglia in two directions: the transverse trunks connect the ganglia of the given segment while the longitudinal trunks join the ganglia of different segments. As a result, nerve impulses occurring in any point of the body spread not over the whole body but only along the transverse trunks within the given segment. The longitudinal trunks unite the nerve segments into a single whole. Sensory organs develop in the animal's head end, which comes in contact with different objects in the environment when the animal moves forward due to which the head ganglia are more developed than the other ganglia and are a prototype of the future brain. The persistence in man of primitive features

10 (scattered ganglia and microganglia on the periphery) in the structure of the vegetative nervous system is a reflection of this stage. Type III, a tubular nervous system. The motor apparatus played an especially significant role in the initial developmental stage of animals. The principal condition for the anima1's existence, nutrition, depends on the perfection of this apparatus (movement in search of food, its grasping and absorption). Such a central nervous system occurred in the chordates (the lancelet) in the form of a metameric neural tube giving off segmental nerves to all body segments, including the motor apparatus: this is the truncal cord. In vertebrates and man it becomes the spinal cord. The appearance of the truncal cord is therefore linked with the perfection of the animal's motor equipment in the first place. In addition, receptors (olfactory and light) are already present in the lancelet. Further development of the nervous system and the origin of the brain are predominantly determined by the perfection of the receptor equipment. Since most sensory organs arise in the end of the animal's body facing in the direction of the movement, i.e. forward, to perceive the external impulses arriving through them the anterior end of the truncal cord develops and the brain forms, which coincides with the differentation of the anterior part of the body into the head (cephalization, Gk kephale head). The discussed patterns of phylogenesis determine the embryogenesis of the human nervous system. The nervous

11 system originates from the outer germinal layer of the embryo, or the ectoderm. The ectoderm forms a longitudinal thickening called the medullary or neural plate which is bounded on the sides by the remaining part of the ectoderm, the skin or horny layer. The medullary (neural) plate soon transforms into a medullary (neural) groove whose margins (medullary or neural folds) are gradually raised, approach each other, and fuse so converting the groove into a tube (neural tube). After fusion of the margins the neural tube separates completely from the skin ectoderm and the mesoderm grows between them. At first the neural plate consists only of a single layer of epithelial cells. During its closure to form the neural tube the number of cells in the wall of the tube increases as a result of which three layers are formed: an inner layer (facing the cavity of the tube) from which the epithelial lining of the cerebral cavities is derived (the ependyma of the central canal of the spinal cord and the ventricles of the brain); a middle layer which gives rise to the grey matter of the brain (the nerve cells, neuroblasts); and, finally, an outer layer almost devoid of cell nuclei which develops into the white matter (processes of the nerve cells, neurites, or axons). Bundles of the neuroblast axons spread either in the thickness of the neural tube to form the white matter or leave it to pass into the mesoderm and then become joined with the young muscle cells (myoblasts). In this manner the motor nerves arise. The sensory nerves arise from the rudiments of the spinal ganglia already noticeable on the margins of the neural

12 groove where it is continuous with the horny layer. When the groove has been converted into the neural tube the rudiments are displaced to the midline of its dorsal surface. The cells of these rudiments then move ventrally and are again arranged on the sides of the neural tube to form the neural crests. Both crests are shaped like a string of beads according to the segments of the dorsal surface of the embryo as a result of which a series of spinal ganglia (ganglia spinalia s. intervertebralia) form on each side. In the cephalic part of the neural tube they reach only to the level of the posterior brain vesicle where they form the rudiments of the ganglia of the sensory cranial nerves. Neuroblasts develop in the ganglionic rudiments in the form of bipolar nerve cells one of whose processes pierces the neural tube while the other passes to the periphery and becomes the sensory nerve. Owing to fusion of both processes for some distance from their origin, the bipolar cells are converted to false unipolar cells possessing a single process which separates to form the letter “T”; these cells are characteristic of the intervertebral ganglia of the adult.

THE CENTRAL NERVOUS SYSTEM

THE SPINAL CORD

Development of the spinal cord. As it is pointed out above, phylogenetically the spinal cord (the truncal cord of the

13 lancelet) appears in stage III of the development of the nervous system (tubular nervous system). There is still no brain and the truncal cord has therefore centres for the control of all processes in the organism, both vegetative and animal (visceral and somatic centres). In accordance with the segmental structure of the body, the truncal cord also has a segmental structure and consists of interconnected neuromeres within the limits of which the simplest reflex arc closes. The metameric structure of the spinal cord is preserved in man too as a consequence of which short reflex arcs exist. With the appearance of the brain (the stage of cephalization) the higher centres for the control of the whole organism appear in it, whereas the spinal cord becomes subordinate to it. It does not remain simply a segmental apparatus but becomes the conductor of impulses from the periphery to the brain and in the opposite direction and two- way connections with the brain develop in it. Thus, two apparatus develop during evolution of the spinal cord: one is an older, segmental apparatus concerned with the own connections of the spinal cord, and the other is a newer, suprasegmental apparatus of two-way conduction pathways of the brain. This structural principle is also encountered in man.

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

14 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. Knowledge of this fact is of practical importance (to avoid inflicting damage to the spinal cord in lumbar puncture for collecting the cerebrospinal fluid or for producing spinal anaesthesia the needle must be inserted between the spinous processes of the third and fourth lumbar vertebrae). The conus medullaris is continuous caudally with a thread-like structure called filum terminale which is the atrophied lower segment of the spinal cord; it is a continuation of the pia mater and is attached to the second coccygeal vertebra. 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), which form as the result of thickening of the lateral walls of the neural tube and stretch on the midline, divide the spinal cord into two symmetrical halves, right and left. Each half, in turn, has a slightly pronounced

15 longitudinal sulcus running on the line of entry of the posterior roots; it is called sulcus dorsolateral is s. posterolateralis. 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). Both pass above to the posterior surface of the medulla oblongata where they are known by the same names. 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. In inflammation of the funiculus (funiculitis) segmental disorders of the motor and sensory spheres occur simultaneously; in a lesion of the root (radiculitis) segmental disorders of one sphere, either sensory or motor, develop; in inflammation of nerve branches (neuritis)

16 the disorders correspond to the zone of the distribution of this nerve. 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.

17 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. A. The grey matter (substantia grisea) developing from the neural tube, from the middle layer of epithelial cells, is embedded inside the spinal cord and 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. The central canal is a remnant of the primary neural tube cavity. That is why 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. With age the central canal becomes narrower and at places completely obliterates so that after the age of 40 (in 93 per cent of people) it is no longer a continuous canal. The grey matter around the central canal is termed the substantia intermedia centralis. In each column of grey matter two columns are distinguished, columna anterior and columna posterior. On transverse section or the spinal cord they are seen as a dilated anterior horn (cornu anterius) and a pointed posterior horn (cornu posterius). As a result the

18 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, internuncial, 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,

19 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 musculature and called the somatico-motor nuclei. They have the appearance of short columns located in two groups, medial and lateral. The medial group innervates muscles developing from the dorsal part of the myotomes (the autochthonous musculature of the back); the lateral group supplies muscles derived from the ventral part of the myotomes (the ventrolateral muscles of the trunk and the muscles of the limbs). The more distal the muscles, the more lateral is the position of the cells innervating them. The anterior and posterior horns in each half of the spinal cord are connected to each other by the intermediate

20 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. B. 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. This apparatus includes the grey matter of the spinal cord with the posterior and anterior roots and the main fasciculi proprii of the white matter which form a narrow band around

21 the grey matter. In development. the spinal-cord apparatus is a phylogenetically older structure and preserves consequently primitive structural features, namely a segmental structure; it is therefore also known as the segmental apparatus of the spinal cord as distinct from the non-segmental apparatus of two-way connections with the brain. Thus, a nerve segment is a transverse segment of the spinal cord and the right and left spinal nerves connected with it and which had developed from 11 single neurotome (neuromere). 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 short reflex arc closes within the boundaries of a nerve segment. In view of the fact that the segmental spinal-cord apparatus developed when there was still no brain, its function consists in the accomplishment of those reactions to external and internal stimuli that had developed earlier in the process of evolution, i.e. inborn reactions, or, according to Pavlov, unconditioned reflexes. The apparatus of two-way connections with the brain is younger phylogenetically because it originated only with the appearance of the brain.

22 With the gradual development of the brain, the conducting pathways. connecting the spinal cord with the brain grew outwardly. This, explains the fact that the white matter of the spinal cord as though surrounds. completely the grey matter. Through the conducting apparatus the spinal-cord apparatus is connected with the brain apparatus which integrates the whole nervous system activity. The nerve fibres are grouped to form fasciculi demonstrable on preparations only by special methods (see course on microscopic anatomy), whereas the fasciculi compose the posterior, lateral, and anterior funiculi visible to the naked eye. The posterior funiculus adjacent to the posterior (sensory) horn contains fasciculi of ascending nerve fibres; in the anterior funiculus adjoining the anterior (motor) horn lie fasciculi of descending fibres, and, finally in the lateral funiculus both types of nerve fibres are present. 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

23 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.

24 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;

25 (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) derived from the mesoderm around the neural tube. 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

26 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

GENERAL SURVEY

The brain (encephalon) occupies the cavity of the skull and its shape corresponds in outline to the inner contours of the cranial cavity. Its superolateral, or dorsal, surface is convex in conformity with the calvaria, whereas the ventral surface, or the

27 base of the skull, is more or less flat and uneven. Three large parts are distinguished in the brain, namely, the cerebral hemispheres (hemispheriae cerebri), the cerebellum, and the brain stem (truncus cerebri s. encephalicus).

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. The anterior vesicle is closed in front by lamina terminalis. This stage of three vesicles transforms during subsequent differentiation into five vesicles which give rise to the five main parts of the braing. 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. A narrow constriction, the isthmus rhombencephali, separates the metencephalon from the midbrain. The common cavity of the rhombencephalon,

28 rhomboid-shaped on frontal section, forms the fourth ventricle communicating with the central canal of the spinal cord. Its ventral and lateral walls are considerably thickened due to the development of the nuclei of the cranial nerves in them, while the dorsal wall is thin. A greater part of this dorsal wall in the region of the medulla oblongata consists only of a single epithelial layer which fuses with the pia mater (tela chorioidea inferior). The walls of the mesencephalon thicken more uniformly with the development of the medullary matter in them. Ventrally they give 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. Besides, lateral evaginations of these walls form two optic vesicles from which ultimately develop the retina and the optic nerves. The dorsal wall of the diencephalon remains thin in the form of an epithelial plate which fuses with the pia mater (tela chorioidea superior). A protrusion of this wall forms posteriorly, from which the pineal body (corpus pineale) is derived. 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,

29 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 human brain grows intensively in the embryonic period and during the first years of life when the organism grows vigorously, becomes adapted to the new environment, and acquires an erect posture, and when the second, verbal, signalling system is formed; the growth of the brain is completed by the age of 20. The brain of a newborn weighs (on the average) 340 g in boys and 330 g in girls; the brain of an adult male weighs 1375 g and that of an adult female 1245 g.

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:

30 (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

31 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,

32 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.

33 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

34 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; these form the bulbothalamit; tract (tractus bulbothalamicus). 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, between the olives (the interolivary looped layer). 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.

THE METENCEPHALON

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

35 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

36 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. The pons is naturally most pronounced in man, which is a specific feature in the structure of his brain. 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).

37 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

38 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. The flocculus is joined to the nodulus by means of a fine strand, the peduncle of the flocculus (pedunculus flocculi), which is continuous medially with a thin crenate lamina, the inferior medullary velum (velum medullare inferius).

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. and has a mouth open medially (the hilus of the nucleus dentatus). 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 nu. cleus.

39 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

40 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

41 to the extrapyramidal system along the second tract and thus it itself has an influence on the spinal cord through this system.

ISTHMUS RHOMBENCEPHALI

Isthmus rhombencephali is the junction of the rhombencephalon with the mesencephalon. Its components are as follows: (1) the superior cerebellar peduncles; (2) the superior medullary velum stretched between these peduncles and the cerebellum; (3) trigonum lemnisci formed by the audiory fibres passing to the lateral lemniscus. This grey- coloured triangle is bounded by the inferior quadrigeminal brachium anteriorly, by the superior cerebellar peduncle posteriorly, and by the cerebral peduncle laterally.

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 apex of the roof juts

42 out into the inferior surface of the cerebellum. 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 lateral angles terminate as two blind lateral recesses of the fourth ventricle (recessus laterales ventriculi quarti) curving ventrally around the inferior cerebellar peduncles. 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

43 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

44 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.

Topography of the Rhomboid Fossa Grey Matter

The grey matter of the spinal cord is continuous with the grey matter of the brain stem and partly spreads in the rhomboid fossa and on the walls of the aqueduct (see the mesencephalon) and partly breaks up into the nuclei of the cranial nerves or into nuclei of the bundles of the conduction tracts. To understand the arrangement of these nuclei one must bear in mind that, as it is said above, the closed neural tube of the spinal cord opened on its dorsal side at the junction with the medulla oblongata and spread out to form the rhomboid fossa. As a result the posterior horns of grey matter of the spinal cord diverged, as it were. The somatic sensory nuclei embedded in the posterior horns acquired a lateral position in the rhomboid

45 fossa while the somatic motor nuclei corresponding to the anterior horns remained in a medial position. As to the vegetative nuclei located in the lateral horns of the spinal cord, these, in accordance with the position of the lateral horns between the posterior and anterior horns, proved to be between the somatic sensory and somatic motor nuclei in the rhomboid fossa when the neural tube spread out. As a result, in contrast to their anterolateral position in the spinal cord, in the wall of the rhomboid fossa the grey matter nuclei are arranged in rows, medially and laterally. For instance, the somatic motor nuclei of the twelfth and sixth pairs lie in the medial row, the vegetative nuclei of the tenth, ninth, and seventh pairs in the middle row, and the somatic sensory nuclei of the eighth pair are located laterally. Projection of the cranial nerve nuclei onto the rhomboid fossa. The twelfth pair, the hypoglossal nerve (n. hypoglossus), has a single, motor nucleus, located in the inferior part of the rhomboid fossa, deep in the hypoglossal triangle (trigonum n. hypoglossi). The eleventh pair, the accessory nerve (n. accessorius), has two nuclei (both motor): one is located in the spinal cord and is called the spinal nucleus of the accessory nerve (nucleus spinalis n. accessorii), the other is a caudal continuation of the nuclei of the tenth and ninth pairs of nerves and is called the nucleus ambiguus; it is located in the medulla oblongata dorsolateral to the olivary nucleus.

46 The tenth pair, the vagus nerve (n. vagus), has the following three nuclei. 1. The sensory nucleus of the tractus solitarius (nucleus tractus solitarii), lies next to the nucleus of the hypoglossal nerve, deep in the vagal triangle. 2. The vegetative dorsal nucleus of the vagus nerve (nucleus dorsalis n. vagi), lies in the same region. 3. The motor nucleus ambiguus (dual) common with the nucleus of the ninth pair; it is embedded in the reticular formation, deeper than the dorsal nucleus. The ninth pair, the glossopharyngeal nerve (n. glossopharyngeus), also contains three nuclei. 1. The sensory nucleus of the tractus solitarius, lies lateral to the nucleus of the hypoglossal nerve. 2. The vegetative (secretory) inferior salivary nucleus (nucleus salivatorius inferior); its cells are scattered in the reticular formation of the medulla oblongata between the nucleus ambiguus and the olivary nucleus. 3. The motor ambiguus nucleus in common with the vagus and accessory nerves. The eighth pair, the auditory nerve (n. vestibulocochlearis s. n. octavus), has numerous nuclei which are projected onto the lateral angles of the rhomboid fossa in the vestibular area. The nuclei are separated into two groups according to the two divisions of the nerve. One division of the nerve, pars cochlearis, the cochlear nerve or the nerve of hearing proper, has two nuclei: a dorsal one, nucleus cochlearis

47 dorsalis, and a ventral nucleus, nucleus cochlearis ventralis, lying lateral to and in front of the dorsal nucleus. The other division of the nerve, pars vestibularis, the vestibular nerve, has four nuclei (nuclei vestibulares). 1. The medial vestibular nucleus (main) (Schwalbe's nucleus). 2. The lateral vestibular nucleus (Deiters' nucleus). 3. The superior vestibular nucleus (Bechterew's nucleus). 4. The inferior vestibular nucleus. The presence of four nuclei in man reflects the early stages of phylogenesis in which fish had a number of separate perceiving static apparatuses. The seventh pair, the facial nerve (n. facialis), has one motor nucleus lying in the reticular formation of the dorsal part of the pons. The nerve fibres arising from it form a loop deep in the tissue of the pons; this loop protrudes in the rhomboid fossa as the facial colliculus (colliculus facialis). The nervus intermedius, whose course is intimately connected with that of the facial nerve, has two nuclei. 1. The vegetative (secretory) superior salivary nucleus (nucleus salivatorius superior) embedded in the reticular formation of the pons dorsal to the nucleus of the facial nerve. 2. The sensory nucleus of the tractus solitarius (nucleus tractus solitarii). The sixth pair, the abducent nerve (n. abducens), has one motor nucleus embedded in the loop of the facial nerve and

48 the facial colliculus on the surface of the rhomboid fossa corresponds, therefore, to this nucleus. The fifth pair, the trigeminal nerve (n. trigeminus), has four nuclei. 1. The superior sensory nucleus of the trigeminal nerve (nucleus sensorius principalis n. trigemini) is projected onto the dorsolateral part of the superior part of the pons. 2. The nucleus of the spinal tract of the trigeminal nerve (nucleus tractus spinalis n. trigemini) is a continuation of the superior sensory nucleus of the trigeminal nerve and extends for the whole length of the medulla oblongata to the cervical part of the spinal cord where it comes in contact with the gelatinous matter of the posterior horns. 3. The motor nucleus of the trigeminal nerve (nucleus motorius n. trigemini) lies medial to the sensory nucleus; it is concerned with mastication. 4. The mesencephalic nucleus of the trigeminal nerve (nucleus tractus mesencephalici n. trigemini) lies lateral to the aqueduct. It is a nucleus of proprioceptive sensation for the muscles of mastication and the muscles of the eyeball. It possibly reflects the independent development of the first division of the trigeminal nerve, the ophthalmic nerve (called n. ophthalmicus profundus in animals), related to the organ of vision, which explains the location of the nucleus in the mesencephalon.

49 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

50 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 colltculi 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.

51 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. Syluii). 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 aqueduct is bounded dorsally by the tectal lamina and ventrally by the tegmentum of the cerebral peduncles. 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 accordance with the development of the mesencephalon under the influence of the visual receptor, it contains different nuclei related to innervation of the eye. 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

52 the tegmentum (Meynert's dorsal fountain 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. The cerebral aqueduct is surrounded by central grey matter functionally related to the vegetative system. In this matter, 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. Above (oral to) the nucleus of the oculomotor nerve, in the

53 tegmentum, lies the nucleus of the medial longitudinal fasciculus (Darkshevich's nucleus). Lateral to the aqueduct is the mesencephalic nucleus of the trigeminal nerve. 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

54 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 and the olfactory, ascend in these lemnisci to the cerebrum. The reticular formation (formatio reticularis) and the medial longitudinal bundle (fasciculus longitudinalis medialis) are also continuous with the tegmentum of the midbrain. The medial longitudinal bundle takes its origin in different parts. One part arises from the vestibular nuclei, passes on either side of the midline directly under the grey matter of the floor of the aqueduct and the fourth ventricle, and consists of ascending and descending fibres running to the nuclei of the third, fourth, sixth, and ninth cranial nerves. The medial longitudinal bundle

55 is an important association pathway that connects different nuclei of the nerves of the eye muscles, which causes the conjoint movements of the eyes in deviation to the side. Its function is also boncerned with movements of the eyes and head in stimulation of the equilcirium apparatus.

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

56 (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 dorsal surface is covered with a fine layer of white matter, the stratum zonale. Its lateral surface faces the cavity of the lateral ventricle and is separated from the adjacent caudate nucleus by the terminal sulcus (sulcus terminal is). This sulcus forms the borderline between the telencephalon, to which the caudate nucleus belongs, and the diencephalon to which the thalamus is related. A band of brain matter, the stria terminalis, passes in the sulcus. The medial surface of the thalamus is covered by a layer of central grey matter; it is set vertically and faces the cavity of the third ventricle whose lateral wall it forms. Above, the

57 medial surface 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. The lateral wall of the thalamus borders upon the internal capsule. The inferior surface of the thalamus is above the cerebral peduncle and fused with the tegmentum. It can be seen on sections that layers of white matter, laminae medullar is thalami, divide the grey matter of the thalamus into nuclei which are named, according to their topography, the anterior, central, medial, lateral nuclei and some ventral nuclei. The thalamus is of great functional significance. As it is pointed out above, it is a relay station for the afferent tracts: part of the fibres of the optic tract (the subcortical visual centre) terminate in the pulvinar, the bundle of Vicq d'Azyr passing from the mamillary bodies and connecting the thalamus with the olfactory sphere terminates in the anterior nucleus, and, finally, all the other afferent sensory tracts running from the lower parts of the central nervous system terminate in its other 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).

58 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

59 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, including the subthalamic region in the narrow sense. According to the embryonic development, two parts are distinguished in the hypothalamus: the anterior, optic part (pars optic a hypothalami), 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.

60 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. Two grey nuclei are contained inside each of the bodies under the surface layer of white matter. The mamillary bodies are functionally related to the subcortical olfactory centres. D. The subthalamic region (regio subthalamica) (in the narrow sense) is a small area of brain matter under the thalamus from which it is separated by the hypothalamic sulcus (sulcus hypothalamicus). This sulcus can be seen on a median section of the brain. In the subthalamic region, lateral to the substantia nigra, lies an oval body, the subthalamic, or Luys' nucleus (corpus subthalamicus Luysi), which belongs to the diencephalon. It is one of the links of the extrapyramidal system and is also believed to be concerned with vegetative functions.

THE THIRD VENTRICLE

The third ventricle (ventriculus tertius) lies on the midline; on a frontal section of the brain it has the appearance of a slit-like cavity. The lateral walls of the third ventricle are

61 formed by the medial surfaces of the thalami which are bridged almost in the middle by the interthalamic adhesion. The anterior wall of the third ventricle is formed below by a fine terminal lamina and further upward by the columns of the fornix (columnae fornicis) with the white anterior commissure (commissura cerebri anterior) lying across them. On both sides, at the anterior ventricular wall, 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 superior wall (roof) of the third ventricle lies under the fornix and the corpus callosum. It is actually formed of tela chorioidea of the third ventricle and its components are the underdeveloped wall of the cerebral vesicle remaining as an epithelial lamina and the pia mater fused with it. The choroid plexus of the third ventricle (plexus chorioideus ventriculi tertii) lies on either side of the midline in the tela chorioidea. In the posterior wall of the ventricle are the habenular and posterior cerebral commissures between which is a caudally directed blind projection of the ventricle, the pineal recess. The aqueduct communicates with the third ventricle by means of funnel-like opening ventral to the posterior commissure. The narrow inferior wall (floor) of the third ventricle is separated on the inside from the lateral walls by the hypothalamic sulci and corresponds to the posterior perforated substance, the mamillary bodies, and the tuber cinereum with the optic

62 chiasma on the base of the brain. The floor of the ventricle forms two recesses: the infundibular recess (recessus infundibuli) projecting into the tuber cinereum and the infundibulum, and the optic recess (recessus opticus) situated in front of the chiasma. The inner surface of the third ventricle is lined with the ependyma.

THE TELENCEPHALON

As it is pointed out above, 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. The forebrain, from which the endbrain arises, originates initially, in association with the olfactory receptor (the rhinencephalon) and later becomes the organ controlling the animal's behaviour. This organ consists of centres of instinctive behaviour based on species reactions (unconditioned reflexes), i.e. the subcortical ganglia, and centres of individua behaviour based on individual experience (conditioned reflexes), the cerebral cortex. According to this, the following groups of centres are distinguished in the telencephalon in the order of historical development.

63 1. The part of the brain concerned with the function of olfaction, the rhinencephalon, the oldests and at the same time the smallest part, which is located ventrally. 2. The basal, or central, ganglia of the hemispheres, the “subcortex”, the old part of the endbrain, the palaeencephalon, hidden at the depth. 3. The cortex composed of grey matter, the youngest part, the neencephalon, and at the same time the largest part covering all the other parts like a cloak or mantle., hence its other name, the pallium (L palliare to cloak). In addition to the two forms of behaviour recorded in animals, a third form develops in man; this is collective behaviour based on the experience of a human collective formed in the process of the labour activity of man and intercourse of humans by means of speech. This form of behaviour is linked with the development of the youngest surface layers of the cerebral cortex composing the material substrate of the second signalling (verbal) system of reality (Pavlov). 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

64 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, the columns, continue downward to the base of the brain, pass through the grey matter of the hypothalamus, and end in the mamillary bodies. The columns of the fornix form the boundaries of the interventricular foramina lying behind, by means of which the third ventricle communicates with the lateral ventricles. In front

65 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 lucidum (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), seems to be laid in folds as the result of which the surface of the pallium has a very complicated pattern formed of sulci alternating in different directions with elevations, called gyri. The total area of the cortex of an adult human is about 220000 mm2, of which two thirds are lodged deeply between the gyri and only one third lies on the surface. The size and shape of the sulci are marked by considerable individual variability as a consequence of which not only the brain of

66 different persons but even the hemispheres of a given person are not quite similar in the pattern of the sulci. 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 lateral is s. Sylvii) arises on the basal surface of the hemisphere from the Sylvian fossa and then passes on the superolateral surface to the back andslightly upward. It terminates approximately at the junction of the middle and posterior third of the superolateral surface of the hemisphere. The anterior part of the lateral sulcus gives off two small branches, one horizontal ascending ramus (ramus ascendens), and the other horizontal anterior ramus (ramus anterior) which pass into the frontal lobe. 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 lower end of the central sulcus does not reach the lateral sulcus. The area of the hemisphere in front of the

67 central sulcus is related to the frontal lobe; part of the brain surface to the back of the central sulcus composes the parietal lobe which is separated from the temporal lobe lying below by the posterior part of the lateral sulcus. 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; this boundary, however, is incomplete because the parieto-occipital sulcus does not stretch far on the superolateral surface as a result of which the parietal lobe is directly continuous with the occipital lobe. The latter also has no clearly defined boundary separating it from the temporal lobe situated in front. In view of this, the boundary between these lobes is drawn artificially by means of a line passing from the parieto-occipital sulcus to the inferior border 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.

68 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. Branches of the lateral sulcus which project into the inferior frontal gyrus divide the last named into three parts: a posterior part (pars opercularis) between the lower end of the precentral sulcus and the horizontal ascending ramus of the lateral sulcus; a triangular part (pars triangularis) between both rami of the lateral sulcus; and, finally, an orbital part (pars orbitalis) in front of the horizontal anterior ramus of the lateral sulcus. 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

69 parietal lobule (lobulus parietalis inferior) which passes to the back, curves round the ends of the lateral sulcus and the superior temporal sulcus, and is lost in the occipital lobe. 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 transverse occipital sulcus (sulcus occipitalis transversus) is distinguished among them; it is usually joined to the end of the intraparietal sulcus. The insula. This lobule becomes visible when the margins of the lateral sulcus which overhang it are drawn apart

70 or removed. These margins are related to the frontal, parietal, and temporal lobes and are termed the operculum. The insula is triangular and its apex is directed forward and downward. Anteriorly, superiorly, and posteriorly the insula is separated from the adjoining parts by a deep circular sulcus (sulcus circularis insulae) .The surface of the insula is covered by several short gyri. The inferior surface of the hemisphere. Its anterior part is related to the frontal lobe. Here, parallel to the medial border of the hemisphere passes the olfactory sulcus (sulcus olfactorius) lodging the olfactory bulb and tract. Between this sulcus and the medial border of the hemisphere stretches the gyrus rectus, a continuation of the superior frontal gyrus. Several inconstant orbital sulci (sulci orbitales) are seen on the inferior surface lateral to the olfactory sulcus; they separate the orbital gyri (gyri orbitales) which can be regarded as a continuation of the middle and inferior frontal gyri. The posterior part of the basal hemispheric surface is formed by the inferior surface of the temporal and occipital lobes whose boundaries are unclearly defined here. Two sulci are seen on this area: the occipitotemporal sulcus (sulcus occipitotemporalis) passing from the occipital to the temporal pole, and the collateral sulcus (sulcus collateralis) extending parallel to it (this sulcus is continuous anteriorly with the rhinal sulcus). Between them runs the lateral occipitotemporal gyrus (gyrus occipitotcmporalis). Two gyri are located medial to the collateral sulcus: the medial occipitotemporal, or lingual

71 gyrus (gyrus occipitotemporalis medialis s. gyrus lingualis) lying between the posterior end of the collateral sulcus and the calcarine sulcus, and the hippocampal gyrus (gyrus parahippocampalis) between the anterior end of the collateral sulcus and the rhinal sulcus on one side and the hippocampal sulcus, curving around the brain stem, on the other. The hippocampal gyrus, being adjacent to the brain stem, is on the medial surface of the hemisphere. 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. The space between this hemispheric border and the sulcus cinguli is related to the frontal lobe, to the superior frontal gyrus. 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) because it corresponds to the medial surface of the superior ends of both central gyri where they are continuous one with the other. 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

72 subparietalis), and posteriorly by the deep parieto-occipital sulcus. The precuneus belongs to the parietal lobe. 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 hippocampal gyrus is bounded on one side by the hippocampal sulcus curving around the brain stem, and on the other side by the collateral sulcus and its anterior continuation called the rhinal sulcus. The isthmus, a constricted part at the junction of the gyrus cinguli and the hippocampal gyrus, is behind the splenium of the corpus callosum at the end of the sulcus formed at the fusion of the parieto-occipital and the calcarine sulci. 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. On drawing apart the margins of the hippocampal sulcus one can see a narrow, denticulate grey band which is the rudimentary dentate gyrus (gyrus dentatus). Structure of the cerebral cortex. The cortex of tho cerebral hemispheres consists principally of six layers differing mainly in the shape of the nerve cells composing them: (1) a

73 molecular layer lying directly under the pia mater and containing the end branchings of the nerve cell processes which form a network; (2) all external granular layer called so because it contains numerous small cells resembling granules; (3) a layer of pyramids composed of small and medium-size pyramidal nerve cells; (4) an internal granular layer composed, like the external granular layer, of small cells, granules; (5) a ganglionic layer containing large pyramidal cells (Betz's cells); (6) a layer of multiform cells which adjoins the white substance. The lower (fifth and sixth) layers are mainly the site of the origin of the efferent tracts; the fifth layer, in particular, is composed of Betz's pyramidal cells whose axons form the pyramidal system. The middle (third and fourth) layers are predominantly connected with the afferent tracts, whereas the upper (first and second) layers are related to the association tracts of the cortex. The six-layer type of the cortex alters in the different areas both in thickness and arrangement of the layers and in the cell composition (this is discussed in detail in the course of histology).

THE RHINENCEPHALON

The rhinencephalon is phylogenetically the oldest part of the forebrain; it arises in association with the olfactory receptor when the forebrain has still not become the organ that controls the animal's behaviour. All its components are therefore different parts of the olfactory analyser.

74 As a result, the human rhinencephalon contains a number of structures of different origin which can be separated topographically into two parts. A. Peripheral part, the olfactory lobe (lobus olfactorius) composed of structures lying on the base of the brain: (1) the olfactory bulb; (2) the olfactory tract; (3) the olfactory pyramid (trigonum olfactorium), and (4) the anterior perforated substance. B. Central part, the gyri of the brain: (1) the hippocampus (2) the dentate gyrus; and (3) the gyrus fornicatus with its anterior part, the uncus, lying close to the temporal pole.

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 whole thickness of the brain matter separates them from the superolateral surface of the hemispheres. 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

75 projection to the back, into the occipital lobe – this is the posterior horn (cornu posterius). The medial wall of the anterior horn is formed by the septum pellucidum which separates it from the similar horn in the other hemisphere. The lateral wall and part of the floor of the anterior horn are occupied by a grey projection, the head of the caudate nucleus (caput nuclei caudati), while the superior wall is formed by the fibres of the corpus callosum. The roof of the central, the narrowest part of the lateral ventricle is also composed of fibres of the corpus callosum, whereas the floor is formed by the body of the caudate nucleus (corpus nuclei caudati) and part of the superior surface of the thalamus. The posterior horn is surrounded by a layer of white nerve fibres arising from the corpus callosum; this layer is called the tapetum (a carpet); the medial wall of the posterior horn carries an elevation, the calcar avis, formed by projection of the calcarine sulcus which stretches on the medial surface of the hemisphere. The superolateral wall of the inferior horn is formed by the tapetum which is a continuation of a similar structure surrounding the posterior horn. The thinned part of the caudate nucleus, its tail (cauda nuclei caudati), curving downward and forward, passes on the superior wall of the inferior horn from the medial side. A white projection, the hippocampus, or Ammon's horn (cornu Ammonis) extends for the whole length of the medial wall of the inferior horn; it is produced because the hippocampal sinus presses in deeply from the outside. Small

76 sulci divide the anterior end of the hippocampus into several small projections. The medial border of the hippocampus carries a fringe, fimbria hippocampi, which is a continuation of the posterior column of the fornix (crus fornicis). On the floor of the inferior horn is an elevation, the collateral eminence (eminentia collateralis), produced by the collateral sinus pressing in from the outside. On the medial side of the lateral ventricle in its central part and inferior horn is a projection of the pia mater forming here a vascular plexus, the choroid plexus of the lateral ventricle (plexus chorioideus ventriculi lateralis). The plexus is covered by epithelium which is a rudiment of the undeveloped medial wall of the ventricle. The choroid plexus of the lateral ventricle is the lateral margin of the tela chorioidea of the third ventricle

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.

77 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) forms the lateral wall of the anterior horn of the lateral ventricle, while its attenuated parts, the body and tail (corpus and cauda nuclei caudati), stretch to the back on the floor of the central part of the lateral ventricle; the tail curves onto the superior wall of the inferior horn. Medially the caudate nucleus adjoins the thalamus from which it is separated by a band of white matter called the stria semicircularis (stria terminalis). Anteriorly and inferiorly the head of the nucleus approaches the anterior perforated substance where it is united with the lentiform nucleus (with the part called the putamen). In addition to this wide union of 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

78 the caudate nucleus, and the posterior side is parallel to the thalamus. The lateral surface of the nucleus is slightly convex and is directed at the lateral side of the hemisphere in the region of the insula. As it is pointed out above, anteriorly and ventrally the lentiform nucleus is fused with the head of the caudate nucleus. On a frontal section the lentiform nucleus is wedge-shaped, the apex of the wedge is directed medially, and the base laterally. Two parallel white layers called the medullary laminae (laminae medullares) 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

79 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. It is separated from the putamen by a thin layer of white matter, the external capsule (capsula externa) and from the cortex of the insula by a similar layer called the capsula extrema. 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 (it is noted above in the description of the thalamus under the name terminal stria).

80 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 (itinerant) fibres. A. The association fibres connect different cortical areas of o 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 area.s 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.

81 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 related to the neencephalon. 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. As it is said above, 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 frontal section through the brain the internal capsule is seen as an oblique white band which is continuous with the cerebral peduncle. 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

82 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.

83 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, namely Gratiolet's visual bundle, the optic radiation (radiatio optica) stretching from the lateral

84 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. Three points of view on the subject exist. According to one of them, a cortical centre is a strictly demarcated group of nerve cells with a clearly defined lineal boundary; the given function is located only in the given centre; this is the theory of narrow localism. Another point of view rejects the existence of

85 cortical centres in the form of localized cortical areas differing in quality, but claims the whole cortex to be equipotential; this is the theory of equipotentionalism. However, as early as 1874 the Kiev anatomist V.A. Betz asserted that each cortical area differed in structure from the other parts of the brain. This initiated the theory of the different quality of the cerebral cortex, i.e. cytoarchitectonics (Gk kyto cell, architekton master builder). Research conducted by Brodman, Economo and scientists of the Moscow Institute of the Brain (headed by Sarkisov) revealed over 50 different cortical areas, or cortical cytoarchitectonic fields, each differing in structure and localization; division of the cortex into more than 200 areas is also accepted (U. Fogt and 0. Fogt). From these areas, designated by numbers, a special “map” of the human cerebral cortex has been drafted. Pavlov fought against both theories, the theory of narrow localism and that of equipotentionalism, and created a new concept of the brain centre. According to Pavlov, the centre is the cerebral end of an analyser. The analyser is a nervous mechanism whose function is “to decompose the intricacy of the outer and inner worlds into their separate elements and components”, i.e. analysis. At the same time, due to the wide connections with the other analysers synthesis occurs here, the connection of analysers one with the other and with manifold activities of the organism. Pavlov taught that “the analyser is a complicated nervous mechanism beginning with the receiving external apparatus and ending in the brain”. From his point of view, the brain centre,

86 or the cortical end of the analyser, has no strictly demarcated boundaries but consists of a nuclear and a scattered parts; this comprises the theory of the nucleus and “scattered elements”. The “nucleus” is a detailed and exact projection in the cortex of all elements of the peripheral receptor and is necessary for accomplishing the highest analysis and synthesis. The “scattered elements” are on the periphery of the nucleus and can be found at a far distance from it; they are concerned with a simpler and elementary analysis and synthesis. In a lesion of the nuclear part, the scattered elements can compensate, to a certain degree, for the lost function of the nucleus, which is very important in clinical practice for the restoration of the given function. Before Pavlov, a motor zone, or motor centres (the anterior central gyrus) and a sensory zone, or sensory centres located behind the central (Rolando's) sulcus were distinguished in the cortex. Pavlov showed that the so-called motor zone corresponding to the anterior central gyrus was, like the other zones of the cerebral cortex, a receiving area (the cortical end of the motor analyser). He claimed that the motor area was a receptive area and that this determined the unity of the entire cortex of the hemisphere. The whole cerebral cortex is now regarded as a continuous receiving surface. According to Pavlov, the cortex is just a receptor apparatus analysing and synthesizing in many ways the arriving stimuli which then reach the true efferent apparatuses only by means of descending connecting fibres.

87 The cortex is a sum of the cortical ends of the analysers. It is from this standpoint that we shall discuss the topography of the cortical parts of the analysers, i.e. the most important receiving areas of the cortex of the cerebral hemispheres. We shall deal first of all with the cortical ends of the internal analysers. 1. The nucleus of the motor analyser, i.e. the analyser of proprioceptive (kinaesthetic) stimuli arising in the bones, joints, and the skeletal muscles and their tendons, is located in the anterior central gyrus (areas 4 and 6) and in the paracentral lobule. Previously regarded as merely a motor zone, it was shown by Pavlov to be primarily a receptive zone, like the other areas: the visual, auditory, and so on. The motor conditioned reflexes close here. Motor paralyses developing in lesions of the motor zone are attributed by Pavlov not to injury to the motor efferent neurons but to a disorder of the nucleus of the motor analyser as a consequence of which the cortex cannot receive kinaesthetic stimuli and movements become impossible. The cells of the motor analyser nucleus lie in the middle layers of the cortex of the motor zone. In its deep layers (the fifth and partly the sixth) lie the giant pyramidal cells of Betz which are efferent neurons considered by Pavlov to be internuncial neurons connecting the cerebral cortex with the subcortical ganglia, and nuclei of the cranial nerves and the anterior horns of the spinal cord, i.e. with the motor neurons. The human body is projected in the anterior central gyrus, like in the posterior central gyrus, head first. The right motor area is

88 connected with the left half of the body (and vice versa) because the pyramidal tracts arising in it partly decussate in the medulla oblongata and partly in the spinal cord. The muscles of the trunk, larynx, and pharynx are controlled by both hemispheres. Besides the cortex of the anterior central gyrus, that of the posterior central gyrus also receives proprioceptive impulses (muscular-articular sense). 2. The nucleus of the motor analyser concerned with concord turning of the head and eyes in the opposite direction is situated in the middle frontal gyrus, in the premotor area (area 8). Such turning is also accomplished in stimulation of area 17 located in the occipital lobe, adjacent to the nucleus of the visual anlilyser. In view of the fact that on contraction of the muscles of the eye not only impulses from the receptors of these muscles but also impulses from the retina (visual analyser, area 17) always arrive in the cerebral cortex (motor analyser, area 8), different visual stimulations are inevitably combined with setting of the eyes in various positions by contraction of their muscles. 3. The nucleus of the motor analyser by means of which habitual purposeful combined movements are synthesized is located in the left (in a right-handed individual) inferior parietal lobule, in the supramarginal gyrus (the deep layers of area 40). These coordinated movements formed on the principle of temporary connections and elaborated by the practice of individual life are made through the connection of the supramarginal gyrus with the anterior central gyrus. In affection

89 of field 40, the ability to move in general is preserved but purposeful movements and actions cannot be performed; this is apraxia (Gk prassen to do). 4. The nucleus of the analyser of the position and movement of the head, the static analyser (the vestibular apparatus) has not been localized exactly in the cerebral cortex. It can be assumed that the vestibular apparatus is projected in the same cortical area as the cochlea, i.e. in the temporal lobe. For instance, lesions in areas 21 and 20 situated in the middle and inferior temporal gyri are attended with ataxia, i.e. disorder of balance, rocking of the body in an upright position. This analyser, which plays the decisive role in man's erect posture, is particularly important for jet-aircraft pilots because the sensitivity of the vestibular apparatus diminishes considerably in a plane. 5. The nucleus of the analyser of impulses passing from the viscera and vessels (vegetative functions) is in the inferior parts of the anterior and posterior central gyri. Centripetal impulses from the viscera, vessels, smooth muscles, and skin glands arrive in this part of the cortex from which centrifugal pathways to the subcortical vegetative centres arise. The vegetative and animal functions are united in the premotor area (areas 6 and 8). Not only this cortical area affects the activity of the viscera, however. The condition of the whole cerebral cortex has an effect on the internal organs. Nerve impulses from the external environment enter the cortical ends of the analysers of the external world.

90 1. The nucleus of the auditory analyser lies in the middle part of the superior temporal gyrus on the surface facing the insula (areas 41, 42 and 52) where the cochlea is projected. Its injury results in cortical deafness. 2. The nucleus of the visual analyser is in the occipital lobe (areas 17, 18 and 19). The optic tract terminates on the medial surface of the occipital lobe on the margins of the calcarine sulcus, in area 17. The retina is projected here and the visual analyser of each hemisphere is connected with the fields of vision and the corresponding halves of the retina of both eyes (e.g. the left hemisphere is connected with the lateral half of the left eye and the medial half of the right eye). A lesion of the nucleus of the visual analyser leads to blindness. Above field 17 lies area 18 whose damage is attended with loss of visual memory but vision itself is preserved. Still higher is area 19; in injury to it, orientation in unusual surroundings is lost. 3. The nucleus of the olfactory analyser lies in the phylogenetically oldest part of the cerebal cortex within the limits of the base of the rhinencephalon (the uncus) and partly the hippocampus (area 11). 4. The nucleus of the taste analyser is situated, according to certain data, in the lower part of the posterior central gyrus close to the centres of the mouth and tongue muscles, and according to others, in the uncus in close vicinity to the cortical end of the olfactory analyser, which explains the close relation of the olfactory and taste senses. It has been

91 established that the sense of taste is impaired in damage to area 43 (Bechterew). The olfactory, taste, and auditory analysers of each hemisphere are connected with the receptors of the respective organs on both sides of the body. 5. The nucleus of the skin analyser (the sense of touch, pain, and temperature) is in the posterior central gyrus (areas 1,2, and 3) and in the cortex of the superior parietal area (areas 5 and 7). The body is projected in the posterior central gyrus head foremost, as a result of which the lower limb receptors are projected in the upper part of the gyrus and the head receptors in its lower part. Since the receptors of general sensitivity in animals are particularly developed in the cephalic end of the body, in the region of the mouth, which is very important in food grasping, man has also markedly developed mouth receptors. In view of this, the region of these receptors occupies an excessively large area of the cortex of the posterior central gyrus. Moreover, with the development of the human hand as an organ of labour the tactile receptors in the skin of the hand sharply increased; as a result the hand also became an organ of the sense of touch. In accordance with this, the cortical areas related to the upper limb receptors are much larger than those related to the lower limb receptors. Therefore, if we make a drawing of the human figure in the posterior central gyrus with the head directed at the base of the skull and the feet directed at the superior border of the hemisphere, we will have to draw a very large face with an unreasonably enormous mouth, a large

92 upper limb with a particularly large hand and thumb sharply exceeding the other fingers in size, a small trunk, and a tiny lower limb. Each posterior central gyrus is connected with the contralateral part of the body due to decussation of the sensory conduction pathways in the spinal cord and partly in the medulla oblongata. A particular type of cutaneous sensitivity, stereognosis (Gk stereos solid, gnosis knowledge), or recognition of objects by touch is linked with a cortical area of the superior parietal lobule (area 7) on the opposite side: the left hemisphere corresponds to the right hand, and the right hemisphere to the left hand. Affection of the superficial layers of area 7 results in loss of the ability to recognize objects by touch alone. The cortical ends of the analysers described are located in definite areas of the cerebral cortex which, as Pavlov put it, is an enormous mosaic, and enormous switchboard. Signals from the organism's inner and outer world reach this “board” through the analysers. Aceording to Pavlov, these signals compose the first signaling system of reality expressed as concrete, visually aided thought (sensations and complexes of sensations, i.e. perceptions). Animals also have the first signaling system. Pavlov claimed, however, that man's evolution involved an essential addition to the mechanisms of nervous activity found in the developing animal world. An animal perceives reality almost entirely through stimuli and their traces left in the cerebral hemispheres. The stimuli flow directly into special cells of the visual, auditory, and other

93 receptors of the body. We perceive these stimuli as external impressions, sensations, and ideas from both the general natural environment and the social environment. This first signaling system which excludes the spoken or written word is common to both man and animals. But speech and later the written word make up a second signaling system of reality which serves as a set of symbols or signs for the first system. This use of the word which is specifically human is exactly what distinguishes man from the animals. Thus, Pavlov identified two cortical systems: the first and the second signaling systems of reality. The second signaling system appeared later than the first and developed only in man. The second signaling system encompasses human thought and is, thus, a verbal system since speech is the material substrate and direct reality of thought. Through long repetition, temporary connections formed between certain signals (sounds heard and signs seen) and movements of the lips, tongue, and muscles of the larynx on the one hand, and real stimuli or the concept of them, on the other. In this way the second signaling system developed on the basis of the first one. In reflection of this phylogenetic process, the first signaling system is laid down in ontogenesis in humans before the second system. The second system begins functioning if the child has contact with other persons and acquires the habits of spoken and written speech, a process which takes several years. If an infant is born deaf or loses the ability to hear before starting to speak, his inherent speech abilities will not be used and the child will remain dumb

94 although he is capable of uttering sounds. Similarly, a person will remain illiterate all his life if he is not taught reading and writing. This is evidence of the decisive influence of the environment on the development of the second signaling systein, which is linked with the activity of the entire cerebral cortex. However, certain cortical areas, the nuclei of the speech analysers, playa special role in the production of speech. To understand the anatomical substrate of the second signaling system it is therefore necessary not only to know the structure of the cerebral cortex as a whole but also to take into account the cortical ends of the speech analysers. 1. Since speech was the means of communication among people during their joint labour activity, the motor analysers of speech developed near the nucleus of the common motor analyser. The motor analyser of speech articulation (the speech motor analyser) lies in the posterior part of the inferior frontal gyrus (Broca's speech area, area 44) close to the lower part of the motor zone where stimuli from the muscles responsible for speech production are analysed. The function of speech is linked to the motor analyser of the lip, tongue, and laryngeal muscles in the lower part of the anterior central gyrus, which explains the close relation of the speech motor analyser to the motor analyser of these muscles.When area 44 is damaged the speech muscles preserve the ability to make simple movements, shout and sing, but the ability to utter words is lost; this condition is motor aphasia (Gk phasis speech). Area 45 located

95 in front of area 44 is related to speech and singing. Damage to it results in vocal motor amusia (inability to carry a tune or to compose melodies), and agrammatism (Sepp) or agrammaphasia (loss of the power of grammatical and syntactical expression). 2. The development of spoken speech is associated with the organ of hearing. Therefore the auditory analyser of spoken speech developed near the sound analyser. Its nucleus is located in the posterior part of the superior temporal gyrus, deep in the lateral sulcus (area 42, or Wernicke's centre). The auditory analyser allows man to perceive different combinations of sounds as words designating various objects and phenomena, whose signals they become (the second signals). This analyser helps man control his speech and understand the speech of others. If it is damaged the ability to hear sounds is preserved, b.ut the meaning of both written and spoken words is not understood; this condition is word deafness or sensory aphasia. Affection of area 22 (the middle one third of the superior temporal gyrus) results in music deafness (inability to appreciate the pitch of a musical sound and music is perceived as a confused noise). 3. At a still higher developmental stage man learned not only to speak but also to write. Written speech requires specific hand movements for forming letters or other signs, which are associated with the motor analyser (the common motor analyser). Therefore, the motor analyser of written speech is located in the posterior part of the middle frontal gyrus, close to

96 the zone of the anterior central gyrus (the motor zone). The activity of this analyser is associated with the analyser of hand movements necessary in writing (area 40 in the inferior parietal lobule). Nearly all types of movement are preserved in damage to area 40, but the ability to perform the fine movements necessary for forming letters, words, and other signs is lost (agraphia) as is the power to express ideas in written form. 4. Since the development of written speech is linked also with the organ of vision, the visual analyser of written speech developed near the visual analyser, connected, naturally, with the calcarine sulcus in which the common visual analyser is situated. The visual analyser of written speech located in the inferior parietal lobule, in the angular gyrus (area 39). Vision is preserved in damage to area 39, but the ability to read, i.e. to analyse written letters and form words and sentences from them, is lost (alexia). All speech analysers are laid down in both hemispheres but develop only on one side (on the left in a right-handed person and on the right in a left-handed person). All analysers are functionally asymmetric. This similarity between the motor analyser of the hand and the speech analysers is explained by the close relationship between labour and speech, which had a decisive influence on the development of the brain. As mentioned above labour and later articulate speech led to the development of the brain. The relationship between speech and the brain can be used for therapeutic purposes. In damage to the speech motor analyser, for example, the

97 elementary motor ability of the speech muscles is preserved but the power to utter words is lost (motor aphasia). In such cases speech can sometimes be restored by persistent exercise of the left hand (in a right-handed person) which favourably affects the development of the rudimentary nucleus of the motor speech analyser on the right side. The analysers of spoken and written speech receive verbal signals (the signals of signals or second signals, in Pavlov's terminology). These verbal signals constitute the second signaling system of reality which is manifested as abstract thought (general ideas, concepts, conclusions, generalizations) and is inherent only in man. The analysers mentioned above, however, are not the only components of the morphological substrate of the second signaling system. Since the function of speech is the youngest phylogenetically, it is the least localized and inherent in the whole cortex. The cortex grows along the periphery and its most superficial layers are related to the second signaling system. These layers consist of a great number of nerve cells (one billion) with short processes which make possible an unlimited closing function and facilitate extensive associations. These properties are essential in the activity of the second signaling system. Pavlov argued that the basic laws governing the work of the first signaling system must control that of the second signaling system as well, since the same nervous system is involved in both instances.

98 The above general discussion of the structure of the central nervous system gives us a basis to consider the specifically human features of the structure of the brain, i.e. those features that distinguish man from the animals. 1. The predominance of the brain over the spinal cord. The brain of carnivores (e.g. cats) is 4 times heavier than the spinal cord, while the brain of primates (e.g. the macaca) is 8 times heavier than the spinal cord. The human brain (1500 g) is 45 times heavier than the spinal cord (30 g). According to Ranke, the weight of the spinal cord is 22-48 per cent the weight of the brain in mammals, 5-6 per cent in gorillas, and only 2 per cent in man. 2. The weight of the brain. In absolute weight the human brain is smaller than that of large animals. The average human brain weighs 1500 g compared with that of dolphins (1800 g), elephants (5200 g), and whales (7000 g). The “square index of the brain” (Roginsky) or the ratio of the weight of the brain to that of the body, can be determined by multiplying the absolute weight of the brain by its relative weight. This index which distinguishes man from the whole animal world is 0.19 in rodents, 1.14 in carnivores, 6.27 in cetaceans (dolphins), 7.35 in anthropoid apes, 9.82 in elephants, and, fInally, 32.0 in man. 3. The predominance of the pallium over the brain stem, i.e. the new brain (neencephalon) over the old brain (palaeoencephalon).

99 4. The particular development of the frontal lobe of the brain. According to Brodman, the frontal lobes account for roughly 8-12 per cent of the whole surface of the hemispheres in lower animals, 16 per cent in anthropoid apes, and 30 per cent in man. 5. The predominance of the new cortex of the cerebral hemispheres over the old cortex. 6. The predominance of the cortex over the “subcortex”, which reaches maximum in man: according to Dalgert, the cortex accounts for 53.7 per cent and the basal nuclei for only 3.7 per cent of the bulk of the brain. 7. The sulci and gyri. These increase the area of the cortex of grey matter; therefore the greater the development of the cerebral cortex, the greater is the number of convolutions in the brain. The increase in the convoluted area of the brain is caused by greater development of the small (third category) sulci, and the depth and asymmetric position of the gyri and sulci. No other animal has such a large number of both deep and asymmetric sulci and gyri as man. 8. The existence of the second signaling system whose anatomical substrate the most superficial layers of the cerebral cortex.

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

100 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

101 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. It is attached to the margins of the sinus sulcus transversi of the occipital bone and along the anterior side of the pyramid of the temporal bone on both sides, up to the posterior clinoid process of the sphenoid bone. The tentorium cere belli 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; this small process projects into the posterior cerebellar notch. 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; they differ from veins in the structure of their walls which are composed of tightly stretched layers of the dura mater and consequently do not collapse when cut and gape on being injured. The inflexibility of the walls of the venous sinuses

102 provides free drainage of venous blood in changes of intracranial pressure; this is important for uninterrupted activity of the brain, which explains why such venous sinuses are present only in the skull. The sinuses are as follows. The transverse sinus (sinus transversus), the largest and widest sinus which runs along the posterior margin of the tentorium cerebelli in the sulcus sinus transversi of the occipital bone. 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, are located the so-called blood lacunae which are small cavities communicating with the sinus and the diploic veins on the one side and with the veins of the dura mater and brain on the other). 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 and then (after

103 bifurcating) along both margins of the foramen magnum of the occipital bone. 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), or torcula herophili. 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. It is connected with a similar sinus on the other side by means of two transverse communications, intercavernous sinuses (sinus intercavernosus), passipg in front of and behind the hypophyseal fossa as a consequence of which a venous circle forms in the region of the sella turcica. According to certain data, the cavernous sinus is an intricate anatomical complex whose components, in addition to the sinus itself, are the internal carotid artery, the nerves and the connective tissue surrounding them. All these structures compose, as it were, a special instrument which plays an important role in regulation of the intracranial flow of venous

104 blood. The cavernous sinus receives anteriorly the superior ophthalmic vein passing through the superior orbital fissure, as well as the inferior end of the sphenoparietal sinus (sinus sphenoparietalis) running on the margin of the ala parva. The cavernous sinus is drained of blood by two sinuses located behind it, namely the inferior and superior petrosal sinuses (sirtus petrosus superior and inferior) located in the superior and inferior petrosal sulci. Both inferior petrosal sinuses communicate by means of several venous canals which lie within the dura mater on the basal part of the occipital bone and are united under the term plexus basilaris. This plexus is connected with the venous plexuses of the vertebral canal, into which blood from the cranial cavity flows. Blood drains from the sinuses mainly into the internal jugular veins, but the sinuses are also connected with the veins of the outer surface of the skull through emissary veins (venae emissariae) transmitted through openings in the skull bones (foramen parietale, foramen mastoideum, canalis condylaris. Small veins leaving the skull together with nerves through foramen ovale, foramen rotundum and canalis hypoglossi play a similar role. The diploic veins and the veins of the spongy substance of the cranial bones also drain into the sinuses of the dura mater, while their other end may be connected with the veins on the external surface of the head. The diploic veins (venae diploicae) are canals anastomosing with one another and lined by a layer of endothelium; they pass in the spongy substance of the flat cranial bones.

105

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 lateral sulcus (cisterna fossae lateralis cerebri) lies in the lateral sulcus of the cerebrum. All subarachnoid spaces widely communicate with one another and at the foramen magna are continuous with the subarachnoid space of the spinal cord. Moreover, they communicate directly with the ventricles of the brain by the median aperture of the fourth ventricle, or Magendie's foramen

106 (apertura mediana ventriculi quarti) which opens into the cerebellomedullar cisterna and by the lateral apertures of the fourth ventricle, or Luschka’s foramina (aperturae laterales ventriculi quarti). The subarachnoid spaces contain cerebral vessels which lire protected from compression by means of connective-tissue arachnoid trabeculae and the surrounding fluid. Pacchionian bodies, or arachnoid granulations (granulatiories arachnoideales) are characteristic property of the structure of the arachnoid. These are rounded grey-pink protrusions of the arachnoid tissue into the cavity of the venous sinuses or into the adjoining blood lacunae. They are arranged in groups and are particularly developed along the distance of the superior sagittal sinus on the superior border of the hemisphere, but are also encountered along the length of the other sinuses. They are present both in children and in adults but enlarge and become greater in number at old age. Growing in size, the granulations exert pressure on the cranial bones and form depressions on their inner surface which are known in osteology as granular pits (foveolae granulares). Cerebrospinal fluid drains through the Pacchionian bodies into the blood stream by filtratilon.

THE PIA MATER

The pia mater of the brain (pia mater encephali) is in intimate contact with the brain and dips into all sulci and

107 fissures on its surface; it contains blood vessels and vascular plexuses. There is a perivascular slit communicating with the subarachnoid space between the membrane and the vessels. The pia mater is also supplied with numerous nerves originating from the sympathetic trunk and running in attendance to the vessels.

THE CEREBROSPINAL FLUID

The cerebrospinal fluid (liquor cerebrospinalis) filling the cerebral and spinal subarachnoid spaces and the ventricles of the brain differs sharply from the other body fluids. Only the endo- and the perilymph of the internal ear and the aqueous humour are identical to it. The cerebrospinal fluid is secreted by the choroid plexuses whose epithelial lining has the character of glandular epithelium. The apparatus producing the cerebrospinal fluid possesses the property of allowing some substances to pass into the fluid but being impermeable to others (the haemato-encephalic, or blood-brain barrier), which is important for the protection of the brain from harmful effects. Owing to its specific features the cerebrospinal fluid is therefore not only a mechanical protective device for the brain and the vessels lying on its base, but is a special internal medium which is necessary for proper functioning of the central organs of the nervous system. The space containing the cerebrospinal fllrid is closed. Fluid drains from it by filtration mainly into the venous system through the Pacchionian bodies

108 and partly into the lymphatic system through the nerve sheaths with which the meninges are continuous.

SCHEMATICAL REPRESENTATION OF THE CONDUCTING TRACTS OF THE NERVOUS SYSTEM

According to the direction of impulse conveyance, two large groups of conducting tracts, afferent and efferent, can be distinguished. The afferent pathways compose the middle link, i.e. the conductor for an analyzer, and some of them will therefore be discussed together with the corresponding analyzers (see “The Sensory Organs”).

THE AFFERENT CONDUCTING TRACTS

In view of the fact that the organism receives stimuli both from the external and the internal environment, tracts exist which carry impulses from the receptors of external and from those of internal stimulations.

THE CONDUCTING TRACTS OF THE SKIN ANALYZER

The afferent fibers of the skin analyzer bring to the cerebral cortex tactile stimuli and the sense of stereognosis, pain, and thermal stimulations.

109

The Conducting Tracts of Skin Tactile Sense (The Sense of Touch)

The ganglio-spino-thalamo-cortical tract. The receptor is in the skin. The conductor consists of three neurons. The cell body of the first neuron is in the spinal ganglion, which is an aggregation of cells of the peripheral neurons concerned with all types of sensitivity. The process arising from the cells of this ganglion divides into two branches, the peripheral one runs in the skin nerve to the receptor, while the central branch passes as a component of the posterior root into the posterior funiculi of the spinal cord and, in turn, separates there into an ascending and descending branches. The terminal ramifications and collaterals of some of the fibres end in the posterior horns of the spinal cord in the substantia gelatinosa (this part of the tract is called the gangliospinal tract [tractus gangliospinalis]). The other ascending fibres do not enter the posterior horns but pass in the posterior funiculi and then in the fasciculus gracilis and fasciculus cuneatus to reach the nucleus gracilis and nucleus cuneatus in the medulla oblongata (this part of the tract is the gangliobulbar tract [tractus gangliobulbaris]). The cell body of the second neuron is in the posterior horns of the spinal cord and in the above-named nuclei of the medulla oblongata. The axons of cells lodged in the posterior horns cross the midplane in the white commissure to become a

110 component of the anterior spinothalamic tract stretching in the lateral funiculus of the opposite side. It must be borne in mind that the fibres of the spinothalamic fasciculi cross not at the level of the entry of the corresponding posterior root into the spinal cord but higher by two or three segments. This is of essential importance in clinical practice because in unilateral injury to this fasciculus the disorders of skin sense on the contralateral side occur not on the level of the injury but below it. This fasciculus reaches the thalamus through the brain stem. On the way it establishes connections with the motor nuclei of the brain stem and the cranial nerves along which head reflexes occur in stimulation of the skin, e.g. movements of the eye in stimulation of the skin on the hand. The axons of the second link cells situated in the nuclei of the medulla oblongata also reach the thalamus along the bulb thalamic tract (tracts bulbothalamicus), which crosses to the opposite side in the medulla oblongata to form the sensalry decussation (decussatio lemniscorum). Thus, for each half of the body the spinal cord contains two tracts, as it were, conveying impulses of touch, namely: (1) uncrossed, in the posterior funiculus of the same side and (2) crossed, in the lateral funiculus of the opposite side. That is why in unilateral lesion of the spinal cord tactile sense may be undisturbed because the corresponding fasciculus on the healthy side remains intact. The cell body of the third neuron is in the thalamus. The axons of the cell extend in the thalamocortical tract to the

111 cerebral cortex into the postcentral gyrus (areas 1, 2, and 3) and the superior parietal lobule (areas 5 and 7), where the cortical end of the skin analyzer is situated. The sense of pain and the sense of touch are marked by diffuse localization in the cerebral cortex, which explains their milder disorders in local cortical lesions.

The Conducting Tracts of Three-Dimensional Skin Sense, Stereognosis (The Recognition of Objects by Touch)

Skin sense, like tactile sense conveyed along the fasciculus gracilis and fasciculus cuneatus, has three links: (1) the spinal ganglia; (2) the nucleus gracilis and nucleus cuneatus in the medulla oblongata; (3) the thalamus and, finally, the skin analyzer nucleus in the superior parietal lobule (areas 5 and 7).

The Conducting Tracts of Pain and Temperature Sense

The cell body of the first neuron is in the spinal ganglion whose cells are connected by peripheral processes with the skin and by central processes with the posterior horns of the spinal cord (nuclei propria) in which the cell body of the second neuron (gangliospinal tract) is situated. The axon of the second neuron passes to the opposite side as a component of the white commissure and ascends in the lateral spinothalamic tract to the thalamus. It should be pointed out that this tract

112 separates, in turn, into two parts: an anterior part conducting the sense of pain, and a posterior part conveying temperature sense. The thalamus contains the cell body of the third neuron whose process extends in the thalamocortical tract to the brain cortex and terminates in the postcentral gyrus (the cortical end of the skin analyzer). Some authors believe that the sense of pain is appreciated not only in the cortex but also in the thalamus in which all types of sense acquire an emotional colouring. Pain and temperature impulses from the parts or organs of the head are conveyed by the corresponding cranial nerves (fifth, seventh, ninth, and tenths, pairs). Since the fibres of the second neuron of the conducting pathways extending from the exteroceptors cross to the other side, the impulses of pain, temperature and partly those of tactile sense are brought to the postcentral gyrus from the opposite side. It should therefore be borne in mind that affection of the first or second neuron before (below) the level of the crossing causes sense disorders on the side of the affection. Pain and temperature sense is disturbed on the side contralateral to the affection when damage occurs to the second neuron above the crossing or if the third neuron is damaged.

113 THE CONDUCTING TRACTS FROM RECEPTORS OF INTERNAL STIMULI

The conducting pathways from receptors of internal stimuli may be divided into those from the motor apparatus (of the body proper), i.e. from proprioceptors (L proprius one's own) composing the conductor of the motor analyser, and pathways from the visceral and vascular receptors, i.e. interceptors. The second group is the conductor of the interoceptive analyser.

THE CONDUCTING TRACTS OF THE MOTOR ANALYZER

The motor analyzer appreciates deep proprioceptive sense, namely, musculoarticular sense, vibratory sensibility, and the sense of pressure and weight (gravitation). The main type of proprioceptive sensibility is the musculo-articular sense, i.e. impulses arising with changes in the extent of tendon stretching and muscle tension. Owing to these impulses man is aware of the spatial position of his body and its part (which is particularly important in space flights in which the state of weightlessness occurs). The conducting pathways of the motor analyzer are the ganglio-bulbo-thalamo-cortical and the anterior and posterior spinocerebellar tracts.

114 The ganglio-bulbo-tbalamo-cortical tract. The receptors are in the bones, muscles, tendons, and joints, i.e. in the body proper, hence the name proprioceptors. The conductor consists of three neurons. The cell body of the first neuron is in the spinal ganglion. Its axon ramifies to form a peripheral branch running in the muscular nerve to the proprioceptor, and a central branch passing in the posterior roots into the fasciculus gracilis and fasciculus cuneatus in the posterior columns of the spinal cord and then to the medulla oblongata. In the medulla they terminate in the nucleus gracilis and nucleus cuneatus (tractus ganglio-bulbaris). In these nuclei are lodged the bodies of the second neurons. The axons of the neurons stretch in the bulbothalamic tract to the lateral thalamic nuclei in which the third link begins. The axons of the cells of the thalamic nuclei pass through the internal capsule into the cortex of the precentral gyrus, in which the cortical end of the motor analyzer is situated (areas 4 and 6). Nerve impulses are conveyed to the brain cortex by the proprioceptive tracts described (having first passed in the spinal nerves). Impulses from the muscles of the lower limbs and lower half of the trunk are conducted along the fasciculus gracilis and those from the muscles of the upper limbs and upper part of the trunk along the fasciculus cuneatus. Proprioceptive fibres from the muscles of the head pass along the cranial nerves: from the eye muscles and the muscles of mastication, along the trigeminal (fifth pair) nerve; from the muscles of facial expression along the seventh pair; and from

115 the tongue, the muscles of the pharynx and the former visceral apparatus, along the ninth, tenth, eleventh, and twelfth pairs. In loss of deep (proprioceptive) sense, the patient is not aware of the spatial position of the parts of his body or of the changes in the position; the movements are no longer precise or cooperative, disorders of coordination (ataxia) develop. As distinct from cerebellar (motor) ataxia it is termed sensory. Some pathways of proprioceptive sensibility do not reach the cortex. Subconcious proprioceptive impulses are directed into the cerebellum which is the most important centre of proprioceptive sense.

Proprioceptive Tracts to the Cerebellum

Sensory subconscious impulses from the motor apparatus (bones, joints, muscles and tendons) reach the cerebellum through spinal, proprioceptive pathways, the most important among which are the anterior and posterior spinocerebellar tracts. 1. The posterior spinocerebellar (Flechsig's) tract (tractus spinocerebellaris posterior [Flechsig]). The cell body of the first neuron is in the spinal ganglion, the axon separates into a peripheral branch running in the muscular nerve to the receptor located in some part of the motor apparatus, and a central branch which as a component of the posterior root penetrates the posterior columns of the spinal cord and surrounds the thoracic nucleus of the posterior horns with its

116 terminal rami and collaterals. The thoracic nucleus contains the cells of the second neuron whose axons form the posterior spinocerebellar tract. As it is shown by its name, the thoracic nucleus is pronounced best in the thoracic segment between the level of the last cervical and that of the second lumbar vertebra. The posterior cerebellar tract, on reaching as a component of the lateral funiculus the medulla oblongata on its own side, passes in the inferior cerebellar peduncles to the cortex of the vermis. On its way in the spiral cord and medulla oblongata it does not cross over to the other side and is therefore called the direct cerebellar tract. On entering the cerebellum, however, most of its fibres cross in the vermis. 2. The anterior (Gowers') spinocerebellar tract (tractus spinocerebellaris anterior [Gowers']) has the first neuron in common with the posterior tract. The cells of the second neurons are in the posterior horn. Their axons form the anterior spinocerebellar tract and stretch in the anterior parts of the lateral white column on their side and on the opposite side to which they cross through the white commissure. The tract ascends through the medulla oblongata and the pons to the superior medullary velum where it again crosses to the other side. Then the fibres enter the cerebellum through its superior peduncles and terminate in the cortex of the vermis. As a result the whole tract forms two decussations owing to which proprioceptive sensibility is conveyed to the same side from which it had arrived.

117 Thus, both cerebellar tracts connect each half of the spinal cord with the corresponding (on the same side) half of the cerebellum. In addition to these tracts, the nuclei gracilis and cuneatus located in the medulla oblongata also send proprioceptive impulses to the cerebellum. The processes of cells lodged in these nuclei reach the cerebellum through its inferior peduncles. All pathways conveying deep (subconscious) sense terminate in the vermis, i.e. in the old part of the cerebellum, the palaeocerebellum.

THE INTEROCEPTIVE ANALYZER

The interoceptive analyzer, in contrast to other analyzers, does not possess a compact and morphologically strictly demarcated conducting pathway although it maintains its specificity along its entire distance. Its receptors, termed interoceptors, are scattered in all organs of vegetative life (the viscera, vessels, smooth muscles and glands of the skin, etc.). The conductor is formed of afferent fibres of the vegetative nervous system passing in the sympathetic, parasympathetic, and animal nerves and then in the spinal cord and brain to the cortex. Part of the conductor of the interoceptive analyzer is formed by afferent fibres running in

118 the cranial nerves (fifth, seventh, ninth, and tenth) and carrying impulses from the organs of vegetative life situated in the area innervated by each of these nerves. The afferent pathway formed by them consists of three links: the cells of the first link lie in the ganglia of these nerves (the trigeminal ganglion, ganglion of the facial nerve, and inferior ganglion of the glossopharyngeal nerve); the cells of the second neuron are in the nuclei of these nerves (the nucleus of the spinal tract of the trigeminal nerve, nucleus of tractus solitarius of the seventh, ninth, and tenth nerves). The fibres emerging from these nuclei cross to the opposite side and run to the thalamus. Finally, the cells of the third link are located in the thalamus. The vagus nerve, which is the main component of parasympathetic innervation, makes up a considerable part of the interoceptive analyzer conductor. The afferent pathway passing in it is also broken up into three links: the cells of the first neuron are in the inferior ganglion of the vagus nerve, those of the second neuron are in the nucleus of tractus solitarius. The vagal fibres arising from this nucleus pass to the opposite side together with the processes of the second neurons of the glossopharyngeal nerve, intersect with the fibres of the other side, and ascend on the brain stem. At the level of the superior quadrigeminal bodies they join the second neurons of the skin analyzer (medial lemniscus) and reach the thalamus in which the cells of the third neurons are located. The processes of these cells pass through the posterior third of the posterior

119 limb of the internal capsule to reach the lower part of the postcentral gyrus. Apart of the cortical end of the interoceptive analyzer connected with the cranial parasympathetic nerves and the area which they innervate is situated here. Afferent pathways from the organs of vegetative life also run in the posterior roots of the spinal nerves. In this case the cells of the first neurons are in the spinal ganglia. A powerful collector of the afferent pathway extending from the organs of vegetative life passes through the greater and lesser splanchnic nerves. Different groups of splanchnic nerve fibres ascend in the spinal cord, in its posterior and lateral funiculi. The afferent fibres of the posterior funiculi convey interoceptive impulses which reach the cerebral cortex by way of the thalamus. The afferent fibres of the lateral funiculi terminate in the nuclei of the brain stem, cerebellum, and thalamus (posterior ventral nucleus). Thus, the thalamus contains the cells of the third neurons of the entire conductor of the interoceptive analyzer related both to sympathetic and to parasympathetic innervation. As a consequence closure of the interoceptive reflex arcs occurs in the thalamus and “outflow” to the efferent pathways is possible. Closure of individual reflexes may take place also at other, lower levels. This explains the automatic, subconscious activity of organs controlled by the vegetative nervous system. The cortical end of the interoceptive analyzer is located not only in the postcentral gyrus (which is pointed out above) but also in the premotor zone where afferent fibres running from

120 the thalamus terminate. Interoceptive impulses arriving along the splanchnic nerves also reach the cortex of the pre- and postcentral gyri in the zones of musculocutaneous sense. These zones may possibly be the first cortical neurons of the vegetative nervous system efferent pathways concerned with cortical regulation or the vegetative functions. From this standpoint these first cortical neurons may be regarded as analogues of Betz's pyramidal cells, which are the first neurons of the pyramidal tracts. The limbic system consists of a set of structures situated on the medial surface of the cerebral hemispheres and the base of the brain. These are the gyrus cinguli, the amygdaloid nucleus (corpus amygdaloideum), the region of septum pellucidum, and the hippocampus. The limbic system contributes to the maintenance of the equilibrium of the organism's internal environment (homeostasis) and regulation of vegetative functions. It is therefore also termed the “visceral brain”. As is seen from what was said above the interoceptive analyzer resembles the exteroceptive analyzers structurally and functionally, but the area of its cortical end is much smaller than that of the cortical ends of the exteroceptive analyzers. This explains its “coarseness”, i.e. less finer and less precise differentiations in relation to consciousness. Intimate overlapping of the pathways and zones of representation of the animal and vegetative organs takes place at all levels of the central nervous system: in the spinal cord,

121 cerebellum, thalami, and cerebral cortex. Visceral and somatic afferent impulses can be addressed to one and the same neuron “serving” both vegetative and somatic functions. All this ensures the cooperation of the animal and vegetative parts of the single nervous system. The highest integration of the animal and vegetative functions is accomplished in the cerebral cortex, especially in the premotor zone. We have discussed here afferent pathways marked by definite specialization of neurons conducting specific impulses (tactile, proprioceptive, interoceptive). Together with the conducting tracts from the organs of vision, hearing, taste, and olfaction they form the specific afferent system. In addition to this system, there is an afferent system represented by the reticular formation related to non-specific structures. The reticular formation appreciates absolutely all impulses – pain, light, sound, etc. The specific impulses from every organ of sense arrive along special conducting systems to the cortex of the corresponding analyzers; in the reticular formation, in contrast, there is no specialization of neurons, one and the same neurons appreciate different impulses and transmit them to all layers of the cortex. The reticular formation constitutes, therefore, the second afferent system.

122 THE SECOND AFFERENT SYSTEM OF THE BRAIN, THE RETICULAR FORMATION

The reticular formation is a complex of structures situated in the central parts of the brain stem and distinguished by the following morphological features. 1. The neurons of the reticular formation differ from the other neurons in structure: their dendrites ramify poorly, while the axons, in contrast, separate into an ascending and descending branches which give off very many collaterals. As a result an axon can have contact with a great number of nerve cells (an axon 2 cm in length may communicate with 27 500 cells). 2. The nerve fibres extend in different directions and resemble on microscopy a network, on grounds of which Deiters (a hundred years ago) called it the reticular (L reticulum little net) formation. 3. The reticular formation consists of cells differing in size (giant, large, moderate-sized, and small) and shape (polygonal, spindle-shaped, spherical, oval). The macro-cell neurons of the reticular formation are so arranged that their dendrites and axon collaterals ramify in a plane perpendicular to the axis of the brain stem. The collaterals of the specific conducting tracts spread in the same direction. On grounds of this, some authors believe the reticular formation to be a series of neuropilic segments.

123 The dendrites of cells lying in the medial part of the reticular formation of the stem run longitudinally, those of cells in the lateral part run laterally, stretching toward the main sources of afferentation. 4. The cells of the reticular formation are at places scattered and at places form nuclei the discernment of which was initiated by Bechterew when he described the reticular nucleus of the tegmentum of the pons (nucleus reticularis tegmenti pontis). Ninety-six nuclei have been described to date. The exact region of distribution of the reticular formation has not yet been determined. According to the physiological data, it is situated along the whole length of the brain stem and occupies the central position in the medulla oblongata, pons, midbrain, the sub- and hypothalamic areas, and even in the medial parts of the thalami. Here it tapers to end as a keel, the rostral end. Reports have recently appeared that the reticular formation is present even in the cerebral cortex. Connections of the reticular formation. The reticular formation is connected with all parts of the central nervous system and the following connections are therefore distinguished: (1) reticulopetal connections passing: (a) from all afferent tracts of the brain stem; (b) from the cerebellum; (c) from the vegetative subcortical centres; (d) from the cortex of all cerebral lobes;

124 (2) reticulofugal connections passing: (a) to the cerebral cortex; (b) to the cranial nerve nuclei; (c) to the cerebellum; (d) to the spinal cord (the reticulospinal fasciculus in the medial part of the anterior column); (3) reticulo-reticular connections (ascending and descending) between different nuclei of the reticular formation itself. Function. Although the reticular formation was described in the 19th century (M.Lenhossek, O.Deiters, V.Bechterew), attention was drawn to it only in the last decades when electrophysiological investigations led to the development of Sechenov's theory of central inhibition and demonstration of the inhibiting effect of the medulla oblongata. It is now believed that the reticular formation is a “generator of energy” and that it regulates processes in other parts of the central nervous system, including the cerebral cortex. This function is ensured by the above mentioned two- way (reticulopetal and reticulofugal) connections of the reticular formation with the different parts of the brain and spinal cord. For instance, connection of the reticular formation cells with the cranial nerve nuclei provides for the switching of impulses from these cells in different directions as a result of which these nuclei take part in the accomplishment of many complex unconditioned-reflex acts. The reticular formation,

125 therefore, coordinates all complex reflex acts in which many muscles participate in a variety of combinations (articulation, phonation, swallowing, respiration, as well as vomiting, coughing, etc.). In this event, the reticular formation itself is a complex reflex centre ensuring the relative maintenance of the automatism of respiration and cardiac activity. An especially important fact is that the reticular formation produces a generalized non-specific activating effect on the entire cerebral cortex (Anokhin); the conducting tracts ascending from the reticular formation to all the lobes of the brain are responsible for this. That is why it is termed also the ascending activating reticular system. Being connected by the collaterals of the axons of its cells with all specific afferent tracts passing through the brain stem, the reticular formation receives impulses from them and carries non-specific information to the cerebral cortex. As a result two afferent systems pass through the brain stem to the cortex: one is specific and is composed of all specific sensory conducting path-ways carrying impulses from all receptors (extero-, intero-, and proprioceptors) and terminating on the bodies of cells, predominantly those of the fourth cortical layer; the other, non-specific, system is formed by the reticular formation and terminates in the dendrites of all the cortical layers. The interaction of both these systems causes the final response of the cortical neurons. The functional interrelationship between the reticular formation and the brain cortex is supplemented and mediated

126 by the system of humoral regulation. Recent data provide evidence that the reticular formation cells themselves are hypersensitive to the effect of some humoral factors, to adrenaline in particular. The results of these studies indicate that the interrelationship between the reticular formation nuclei and the parts of the brain located at a higher level should be regarded today as a complex of neural and humoral connections responsible for the analysis and synthesis of nerve impulses reaching the cortex by way of the afferent tracts. Such is the modern concepts of the two afferent systems of the brain. In view of this importance of the reticular formation and its influence on the cerebral cortex, some authors in other countries exaggerate its role and believe that this formation, being located in the central parts of the brain, constitutes a special “centriencephalic system” concerned with the function of consciousness and integration. The tendency to bring the highest level of integration from the cerebral cortex down to the subcortex is absolutely unfounded and is antievolutional because the highest level of the brain, i.e. the pallium (cortex), and not the stem attains highest development during evolution. This tendency is at variance with the materialistic idea of nervism and reflects the freudian theory which is an idealistic theory of the principal role of the subcortex and not the cortex. The structure and function of the reticular formation remain unclarified to date and are the subjects of further research.

127 THE DESCENDING MOTOR TRACTS

The descending motor tracts extend from the brain cortex (the corticonuclear and cerebrospinal tracts, or the pyramidal system), from the subcortical nuclei of the forebrain (the extrapyramidal system) and from the cerebellum.

THE CEREBROSPINAL (PYRAMIDAL) TRACT, OR THE PYRAMIDAL SYSTEM

The cell body of the first neuron of the pyramidal system is in the precentral gyrus of the cerebral cortex (Betz's pyramidal cells). The axons of these cells descend through the corona radiata into the internal capsule (the genu and the anterior two thirds of its dorsal part), then into the crus cerebri (its middle part), and finally into the pars basilaris of the pons and into the medulla oblongata. In the medulla some of the fibres of the pyramidal system communicate with the nuclei of the cranial nerves. This part of the pyramidal system passing through the genu of the internal capsule and connecting the cerebral cortex with the nuclei of the cranial nerves is called the corticonuclear tract (tractus corticonuclearis). (The fibres of the corticonuclear tract are connected with the nuclei of the cranial nerves not directly but through internuclear neurons.) Some fibres of the tract cross to the opposite side, the others remain on their own side. The axons of the cells located in the cranial nerve nuclei (the cell bodies of the second neurons)

128 terminate as components of the respective nerves in the striated muscles innervated by these nerves. The other part of the pyramidal system extending in the anterior two thirds of the dorsal part of the internal capsule is responsible for connection with the nuclei of the spinal nerves. It descends to the anterior horns of the spinal cord and is therefore called the cerebrospinal tract (tractus corticospinalis [pyramidalis]). This tract passes in the brain stem to form pyramids in the medulla oblongata. In the pyramids some of the fibres of the cerebrospinal tract cross over to the other side (decussatio pyramidum) and descend in the lateral white column of the spinal cord to form the lateral cerebrospinal (pyramidal) tract (tractus corticospinalis s. pyramidalis lateralis). The remaining fibres of the cerebrospinal tract, which do not cross over to the opposite side descend in the anterior white column and form the anterior cerebrospinal (pyramidal) tract (tractus corticospinalis s. pyramidalis anterior). The fibres of this tract also cross gradually to the opposite side in the white commissure along the whole distance of the spinal cord as a result of which the entire cerebrospinal tract is crossed. As a consequence the cortex of each hemisphere innervates the muscles on the opposite side of the body. The motor and sensory decussations in the different paris of the brain (decussatio pyramidum, commissura alba, decussatio lemniscorum, etc.) are, according to Pavlov,

129 adjustments of the nervous system responsible for the maintenance of innervation in injury to the brain in some area on one of its sides. The axons forming the cerebrospinal (pyramidal) tract come in contact with the motor cells of the anterior horns of the spinal cord. This is where the second neuron originates. (The fibres of the cerebrospinal tract are also connected with the anterior roots by means of internuncial neurons.) The axons of cells located in the anterior horns pass in the anterior roots and then stretch as components of the muscle nerves to the striated muscles of the trunk and limbs innervated by the spinal nerves. The corticonuclear and the cerebrospinal tracts form, therefore, a single pyramidal system concerned with conscious control of the whole skeletal musculature. The pyramidal system is particularly developed in man because of his erect posture and conscious use of the motor apparatus in the process of labour activity.

THE DESCENDING TRACTS OF THE FOREBRAIN SUBCORTICAL NUCLEI, THE EXTRAPYRAMIDAL SYSTEM

As is mentioned above, the pyramidal system arises in the brain cortex (the fifth layer, Betz's pyramidal cells). The extrapyramidal system is formed of subcortical structures. Its components are corpus striatum, thalamus, subthalamic (Luys') nucleus, nucleus ruber, substantia nigra, and the white matter

130 tracts connecting them. The extrapyramidal system differs from the pyramidal system in development, structure, and function. It is the oldest motor-tonic apparatus phylogenetically and is already found in fishes that still have only the pallidum (palaeostriatum); the putamen (neostriatum) has already appeared by that time in the amphibia. In this developmental stage, when there is still no pyramidal system, the extrapyramidal system is the highest part of the brain which appreciates stimuli arriving from the receptor organs and sends impulses to the muscles through the automatic mechanisms of the spinal cord. Relatively, simple movements (automatized) occur as a result. With the gradual development of the forebrain and its cortex m mammals, a new kinetic system develops. This is the pyramidal system, which corresponds to the new type of motor acts associated with the ever increasing specialization of small groups of muscles. As a result the following two systems fully develop in man. 1. The pyramidal system is phylogenetically yonger and is represented by screen cortical centres concerned with man's voluntary movements in which certain small groups of muscles take part. (Paralyses are encountered in injury to the pyramidal system.) Cortical activity manifested by movements based on conditioned reflexes is also accomplished through the pyramidal system. 2. The extrapyramidal system is older phylogenetically and is composed of subcortical nuclei. In man it plays a subordinate role and enacts higher unconditioned reflexes,

131 maintaining the tonus of the musculature and regulating automatically its activity (involuntary automatic innervation of the body muscles). This automatic regulation of muscles is effected due to the connection of all the components of the extrapyramidal system with one another and with the nucleus ruber from which extends the descending motor tract to the anterior horns of the grey matter of the spinal cord. This is the rubrospinal tract (tractus rubrospinalis). It arises in the cells of the red nucleus, crosses to the opposite side through the median plane at the level of the anterior quadrigeminal bodies to form the ventral tegmental decussation (decussatio ventralis tegmenti Foreli), descends through the brain stem into the lateral white column of the spinal cord, and terminates among the motor neurons of the anterior horns of grey matter. Thus, the extrapyramidal system exerts its action on the spinal cord through the red nucleus, which is a most important part of the system. Some parts of the extrapyramidal system are functionally interdependent. The corpus striatum, for instance, has an inhibiting effect on the pallidum and in damage to it disinhibition of the pallidum occurs, which explains the appearance of involuntary movements. In general, lesions of the extrapyramidal system are attended by rigidity of muscles and by various involuntary movements resulting from impaired automatic regulation and tonus of muscles. The descending cerebellar tracts are closely related to the activity of the extrapyramidal system.

132

THE DESCENDING MOTOR TRACTS OF THE CEREBELLUM

The cerebellum takes part in controlling the spinal cord motor neurons (muscle coordination, maintenance of balance, maintenance of muscle tonus. and the overcoming of inertia and force of gravity). This is accomplished through the cerebellorubrospinal tract (tractus cerebellorubrospinalis). The cell body of the first link of this tract lies in the cerebellar cortex (Purkinje cells). The axons of the cells terminate in the cerebellar dentate nucleus and, possibly, in other nuclei of the cerebellum. The second link arises here. The axons of the second neurons pass through the superior cerebellar peduncles to the midbrain and terminate in the red nucleus, in which the cells of the third link are located. As components of the rubrospinal (Monakow's) tract (tractus rubrospinalis) the axons of these cells, after changing over in the anterior horns of the spinal cord (the fourth link), reach the skeletal muscles.

THE DESCENDING TRACTS OF THE CEREBRAL CORTEX TO THE CEREBELLUM

The cerebral cortex, which is in charge of all body processes, also governs the cerebellum, the most important proprioceptive centre concerned with body movement. This is achieved due to the presence of a special tract descending from

133 the cortex of the brain to that of the cerebellum, the corticopontocerebellar tract (tractus corticopontocerebella- ris). The first link of this tract consists of neurons whose cell bodies are located in the cerebral cortex while the axons descend to the nuclei of the pons nuclei (proprii) pontis. These neurons form separate bundles termed, according to the different lobes of the brain, the frontopontine (tractus frontopontinus), occipitopontine (tractus occipitopontinus), and parietopontine (tractus parietopontinus) tracts. The pontine nuclei give rise to the second neuron whose axons form the pontocerebellar tract (tractus pontocerebellaris) passing to the opposite side of the pons and reaching the cerebellar cortex (neocerebellum) as components of the middle cerebellar peduncles. A connection is thus established between the cortex of the brain and the cerebellar hemispheres. (Each cerebral hemisphere is connected with the contralateral cerebellar hemisphere.) Both these parts of the brain are younger than the other parts and are interrelated in development. The more the cerebral cortex and hemispheres are developed, the more are developed the cortex and hemispheres of the cerebellum. In view of the fact that the connections between these parts of the brain run through the pons, the degree of the development of the last named is also determined by the development of the cerebral cortex.

134 Therefore, the three pairs of cerebellar peduncles are responsible for its versatile connections. The cerebellum receives impulses from the spinal cord and medulla oblongata through the inferior peduncles, from the cortex of the cerebral hemispheres through the middle peduncles; the superior peduncles contain the main efferent tract along which the cerebellar impulses are conveyed to the cells of the anterior horns of the spinal cord. The connection of the cerebral hemispheres with the cerebellar hemispheres, i.e. with its new part (neocerebellum) is crossed, while the vermis, i.e. the old part of the cerebellum (palaeocerebellum), is connected with the spinal cord in a straight manner, i.e. homolaterally. In summary of the discussion of the conducting tracts of the nervous system, the following general conclusion can be made. The conducting tracts of the brain can be subdivided into three main groups: projection, commissural, and association tracts. The projection tracts are divided into centripetal, or afferent, and centrifugal, or efferent, according to the direction in which the signalization is conveyed along them. The centripetal tracts conduct information in the ascending direction, from the receptors to the brain formations, while the centrifugal tracts convey it in the opposite, descending direction, from the brain structures to the effector organs. As a result the organs and parts of the body are projected in the brain, as it were, hence the term projection tracts.

135 The commissural tracts are responsible for the paired activity of the endbrain. Information passes along them from one hemisphere to the other and an interrelationship is thus established between them. The cerebral commissures connecting one hemisphere with the other are the anatomical substrate of this interhemispheric relationship. Hence, the name commissural tracts. The association tracts are the anatomical substrate of the joint (associated) activity of the cerebral hemispheres and connect different cortical areas of the same hemisphere. Short and long association bundles of nerve fibres are distinguished. The short ones connect adjacent gyri and are called intralobular tracts, whereas the long tracts link remotely located areas of the hemisphere and are termed interlobular. The projection, commissural, and association conducting tracts take part in the formation of the body's integral behaviour reactions. The commissural and association tracts unite the cortex for integrated activity.

136 THE VEGETATIVE (AUTONOMIC) PART OF THE NERVOUS SYSTEM

The fundamental qualitative difference in the structure, development and action of the smooth and striated musculature has been pointed out above. The skeletal muscles take part in the organism's reaction to environmental factors and respond by rapid and purposeful movements to changes in the environment. The smooth musculature is located in the viscera and vessels and works slowly but rhythmically, thus ensuring the course of vital processes in the body. These functional differences are linked with the difference in innervation: the skeletal musculature receives motor impulses from the animal, or somatic part of the nervous system, whereas the smooth musculature receives them from the vegetative, or autonomic part. The vegetative nervous system controls the activity of all organs concerned with the vegetative functions of the body (nutrition, respiration, excretion, reproduction, and fluid circulation) and accomplishes trophic innervation (Pavlov). The trophic function of the vegetative nervous system is responsible for the nutrition of the tissues and organs in conformity to their functioning under certain environmental conditions (adaptational-trophic function). It is general knowledge that changes in the state of the higher nervous activity affect the function of the viscera and vice versa, changes in the organism's internal environment

137 cause an effect on the functional state of the central nervous system. The vegetative nervous system intensifies or weakens the function of the specifically working organs. This regulation is of a tonic character and the vegetative system alters therefore the tonus of the organ. Since one and the same nerve fibre acts only in one direction and is incapable of simultaneously increasing and reducing the tonus, the vegetative nervous system is accordingly separated into two parts, or systems: the sympathetic and parasympathetic systems. The sympathetic part is mainly concerned with trophic functions. It is responsible for intensification of oxidation processes, nutrient consumption, and respiration and increases the rate of cardiac activity and the supply of oxygen to the muscles. The parasympathetic system carries a protective role: constriction of the pupil in bright light, inhibition of cardiac activity, evacuation of the cavitary organs. Comparison of the areas of distribution of the sympathetic and parasympathetic innervation discloses, ftrstly, the predominant role of one vegetative part over the other. The urinary bladder, for instance, receives mostly parasympathetic innervation, and division of the sympathetic nerves causes no essential changes in its activity; the sweat glands, the pilary muscles of the skin, the spleen, and the suprarenals are supplied only with sympathetic innervation. Secondly, in organs with double vegetative innervation, interaction of the sympathetic and parasympathetic nerves in the form of a definite

138 antagonism is encountered. Stimulation of the sympathetic nerves causes dilatation of the pupil, constriction of the vessels, an increase in the rate of cardiac contractions, and inhibition of intestinal peristalsis; stimulation of the parasympathetic nerves, in contrast, leads to constriction of the pupil, dilatation of the vessels, diminution of the heart beat rate, and intensification of peristalsis. The “antagonism” of the sympathetic and parasympathetic systems, however, should not be considered static, as an opposition of their functions. There are reciprocally acting systems and the relations between them alter dynamically in the difierent phases of the functioning of this or that organ; they can act both as antagonists and as synergists. Antagonism and synergism are two aspects of a single process. The normal functions of our organism is ensured by the coordinated action of these two parts of the vegetative nervous system. This coordination and regulation of functions is brought about by the cerebral cortex. The sympathetic and parasympathetic parts are distinguished in the vegetative system mainly according to the physiological and pharmacological data, but morphological distinctions due to their structure and development also exist. We shall therefore first characterize the morphological features of the vegetative nervous system as compared to those of the somatic nervous system. We describe firstly the centres of the vegetative nervous system.

139 The somatic nerves emerge from the brain stem and spinal cord segmentally for the whole length of these structures. The segmental character is also maintained partly on the periphery. The vegetative nerves emerge only from some of the parts (foci) of the central nervous system. Four such foci exist. 1. The mesencephalic part located in the midbrain (the accessory, or Yakubovitch's nucleus, nucleus accessorius, and the unpaired median nucleus oi the third pair of cranial nerves). 2. The bulbar part located in the medulla oblongata and pons (the nuclei of the seventh, ninth, and tenth pairs of cranial nerves). Both parts are united under the term cranial part. 3. The thoracolumbar part situated in the lateral horns of the spinal cord for the distance of the C8, Th1-L3 segments. 4. The sacral part located in the lateral horns of the spinal cord for the distance of the S2-S4 segments. The thoracolumbar part belongs to the sympathetic system, the cranial and sacral parts to the parasympathetic system. Certain authors assume that vegetative centres may also exist in the cervical segment of the spinal cord and relate them to the spinal nucleus of the accessory nerve. Higher vegetative centres dominate over the foci; these centres are not simply sympathetic or parasympathetic but are concerned with the regulation of both parts of the vegetative nervous system. They are suprasegmental and are situated in the brain stem and pallium as follows.

140 1. In the metencephalon: the vasomotor centre on the floor of the fourth ventricle; the cerebellum to which regulation of some of the vegetative functions (vasomotor reflexes, skin trophics, the rate of wound healing, etc.) is attributed. 2. In the midbrain: the grey matter of the aqueduct of Sylvius. 3. In the diencephalon: the hypothalamus (tuber cinereum). 4. In the endbrain: the striated body. The hypothalamic region is the most essential in vegetative regulation; it is one of the oldest parts of the brain though older and phylogenetically Jyounger structures are distinguished in it. The nuclei of the hypothalamic area are connected through the hypothalamo-hypophyseal fasciculus with the hypophysis to form the hypothalamo-hypophyseal system. This system, acting by means of the hypophyseal incretions, is a regulator of all the endocrine glands. The hypothalamic region regulates the activity of all organs of vegetative life by uniting and coordinating their functions. The vegetative and somatic functions of the whole body are united in the cerebral cortex (Danilevsky and Bechterew), in the premotor zone in particular. According to Pavlov, the cortex is a complex of the cortical ends of the analysers and it receives stimuli from all organs, the organs of vegetative life among others, producing

141 an effect on them through its efferent systems, those of the vegetative nervous system included. Consequently, a two-way connection exists between the cortex and the viscera, i.e. a cortico-visceral connection. As a result, all vegetative functions are subordinate to the cerebral cortex which directs all body proccesses. The vegetative nervous system is thus not an independently functioning autonomic structure, as was contended before Pavlov, it is a special part of the integrate nervous system to whose highest parts, the cerebral cortex among others, it is subordinate. Therefore, like in the somatic part of the nervous system, a central and peripheral part can be distinguished in it. The central part are the above described foci and centres in the spinal cord and brain. The peripheral part is composed of the nerve ganglia, nerves, plexuses, and peripheral nerve endings. Reports have appeared recently claiming that the vegetative ganglia possess their own afferent innervation through which they are controlled by the central nervous system. The reflex arc differs markedly. The cell body of the sensory neuron, both of the' somatic and of the vegetative nervous systems, is located in the spinal ganglion (ganglion spinale) in which afferent pathways, both from organs of somatic life and those of vegetative life, gather and which is therefore a mixed somatic vegetative ganglion.

142 The cell body of the internuncial neuron of the vegetative nervous system, as distinct from that of the somatic system, is located in the lateral horns of the spinal cord. The axon of a somatic internuncial neuron arises from the cells of the posterior horn and terminates within the boundaries of the spinal cord among the cells of its anterior horns.The internuncial neuron of the vegetative system, in contrast, does not terminate in the spinal cord but passes from it to nerve ganglia lying on the periphery. On emerging from the spinal cord the axon of the internuncial neuron runs into the ganglia of the sympathetic trunk (ganglia trunci sympathici) related to the sympathetic part of the vegetative nervous system (these are ganglia of the first order, they form the sympathetic trunk), or the fibres do not terminate in these ganglia but stretch to the intermediate ganglia (ganglia intermedia) lying closer to the periphery between the sympathetic trunk and an organ (e.g. the mesenteric ganglia). These are ganglia of the second order and are also related to the sympathetic nervous system. Finally, the fibres may reach, without interruption, ganglia lying either near to an organ (paraorganic ganglia, e.g. the ciliary, optic ganglia, and others) or within the organ (intraorganic, intramural ganglia); both are ganglia of the third order and are called the terminal ganglia (ganglia terminalia). They are related to the parasympathetic part of the vegetative nervous system. All fibres which stretch to the ganglia of the first, second, or third order and are axons of an internuncial neuron are called

143 preganglionic fibres (rami preganglionares). They are covered with myelin. The third, effector, neuron of the somatic reflex arc is located in the anterior horns of the spinal cord, whereas the effector neuron of the vegetative reflex arc was brought out of the central into the peripheral nervous system in the process of development, closer to the working organ, and is located in the vegetative nerve ganglia. This positioning of the effector neurons on the periphery is responsible for the main sign of the vegetative nervous system, namely the double-neurone structure of the efferent peripheral pathway: the first is the internuncial neuron whose body lies in the vegetative nuclei of the cranial nerves or in the lateral horns of the spinal cord while its axon runs to a ganglion; the second is the efferent neuron with the body located in the ganglion and the axon reaching the working organ. The effector neurons of the sympathetic nerves arise in the ganglia of the sympathetic trunk (ganglia of the first order) or in the intermediate ganglia (ganglia of the second order); the effector neurons of the parasympathetic nerves originate in the para- or intraorganic ganglia, the terminal ganglia (ganglia of the third order). Since there is synapsis of the internuncial and efferent neurones in these ganglia, the indicated difference between the sympathetic and parasympathetic parts of the vegetative nervous system is linked exactly with these neurons. The axons of the efferent vegetative neurons are almost devoid of myelin, they are nonmedullated (grey). They

144 constitute the postganglionic fibres (rami postganglionares). The postganglionic fibres of the sympathetic nervous system which arise from the ganglia of the sympathetic trunk diverge in two directions. Some pass to the viscera and form the visceral part of the sympathetic system. Other fibres form the communicating branches (rami communicantes grisei) connecting the sympathetic trunk with the somatic nerves. As components of these nerves, the fibres reach the somatic organs (the motor apparatus and the skin) in which they innervate the smooth muscles of the vessels and nerves, and the glands. The sum total of the described efferent vegetative fibres stretching from the ganglia of the sympathetic trunk to the organs of the soma form the somatic part of the sympathetic system. Such structure provides for the functioning of the vegetative nervous system which regulates the metabolism of all parts of the organism in conformity with the continuously changing environmental conditions and with the activity (work) of the organs and tissues. In accordance with this most universal function associated with all parts, all organs and tissues of the body and not simply with separate organs and tissues, the vegetative nervous system is characterized morphologically also by universal, generalized distribution in the body, and penetrates all organs and tissues. Therefore, the sympathetic nervous system innervates not only the viscera but also the soma in which it is responsible for the metabolic and trophic processes.

145 As a result each organ according to Pavlov is under a triple nervous control, in view of which he distinguished three types of nerves: (1) functional, concerned with the function of the given organ; (2) vasomotor, responsible for the rough flow of blood to the organ, and (3) trophic, regulating the assimilation of nutrients from the blood brought to the organ. The visceral part of the sympathetic system contains all these three types of nerves for the viscera, whereas the somatic part of this system contains only vasomotor and trophic nerves. As to the functional nerves for the organs of the soma (the skeletal musculature, etc.) these pass as components of the somatic nervous system. The main distinction of the efferent part of the vegetative nervous system from the efferent part of the somatic nervous system consists therefore in the fact that the somatic nerve fibres on emerging from the central nervous system, pass to the working organ without interruption, whereas the vegetative fibres are interrupted on their way from the brain to the working organ in one of the ganglia of the first, second or third order. As a consequence, the efferent tract of the vegetative system breaks up into two parts of which it is actually composed: the preganglionic myelinated (medullated) fibres, rami preganglionares, and postganglionic devoid of myelin (non-medullated) fibres, rami postganglionares. The presence of ganglia in the efferent part of the reflex arc is a characteristic sign of the vegetative nervous system, distinguishing it from the somatic system.

146 The nerves also possess certain characteristic features. The afferent pathways of the vegetative nervous system do not possess the character of macroscopically visible. nerves and their fibres pass as components of other nerves (the greater and lesser splanchnic nerves, the posterior roots, etc.). The sympathetic system in this case is marked by the fact that the sensory innervation associated with it can spread .extensively and, consequently, the sympathetic system can be regarded as a system of collateral innervation.

THE SYMPATHETIC NERVOUS SYSTEM

The sympathetic nervous system occurs historically as a segmental part and therefore in man it also has a segmental structure.

THE CENTRAL PART OF THE SYMPATHETIC NERVOUS SYSTEM

The central part of the sympathetic system is located in the lateral horns of the spinal cord between the level of C7 and

Th1-L3 in the intermediolateral nucleus (nucleus intermediolateralis). It gives rise to fibres innervating the smooth muscles of the viscera and the sensory organs (eyes), and the glands. Vasomotor, pilomotor, and perspiration centres are also located here. It is considered (and has been verified by

147 clinical experience) that different parts of the spinal cord cause a trophic effect and have influence on thermoregulation and metabolism.

THE PERIPHERAL PART OF THE SYMPATHETIC NERVOUS SYSTEM

The peripheral part of the sympathetic system is firstly formed of two symmetrical right and left sympathetic trunks (truncus sympathicus dexter and sinister) stretching on either side of the spine from the base of the skull to the coccyx where the caudal ends of both trunks meet to form a single common ganglion. Each sympathetic trunk is composed of a series of nerve ganglia of the first order connected by longitudinal interganglionic branches (rami interganglionares) that consist of nerve fibres. In addition to the ganglia of the sympathetic trunk (ganglia trunci sympathici), the intermediate ganglia mentioned above are also constituents of the sympathetic system. It has also been found that beginning from the level of the superior cervical ganglion the sympathetic trunk contains elements of the parasympathetic and even those of the somatic nervous system. The processes of cells located in the lateral horns of the thoracolumbar part of the spinal cord emerge from it through the anterior roots and, on separating from them, pass in the white communicating branches (rami communicantes albi) to the sympathetic trunk. Here they join by means of synapsis

148 with the cells of the sympathetic trunk ganglia or pass through the ganglia without interruption and reach one of intermediate ganglia. This is the preganglionic pathway. From the ganglia of the sympathetic trunk or (if there was no interruption) from the intermediate ganglia arise non-medullated fibres of the postganglionic pathways and pass to the blood vessels and viscera. Since the sympathetic system has a somatic part, it is connected with the spinal nerves providing innervation of the soma. This connection is brought about by the grey communicating branches (rami communicantes grisei) which are a segment of postganglionic fibres stretching from the sympathetic trunk ganglia to a spinal nerve. As components of the grey communicating branches and the spinal nerves the postganglionic fibres spread in the vessels, glands, and smooth muscles of the skin of the trunk and limbs, as well as in the striated muscles for whose nutrition and tonus they are responsible. Thus, the sympathetic nervous system is connected with the somatic system by two types of communicating branches, grey and white. The white communicating branches (medullated) are the preganglionic fibres. They stretch from the centres of the sympathetic nervous system through the anterior roots to the ganglia of the sympathetic trunk. Since the centres are situated at the level of the thoracic and upper lumbar segments, the white communicating branches are also present only in the area between the level of the first thoracic and that

149 of the third lumbar spinal nerves. The grey communicating branches, the postganglionic fibres, provide for the vasomotor and trophic processes in the soma; they connect the sympathetic trunk with the spinal nerves for its entire length. The cervical part of the sympathetic trunk is also connected with the cranilll nerves. All the plexuses of the somatic nervous system contain therefore fibres of the sympathetic system in their bundles and nerve trunks, which emphasizes the unity of these systems.

THE SYMPATHETIC TRUNK

Each of the two sympathetic trunks is subdivided into four parts: cervical, thoracic, lumbar (or abdominal), and sacral (or pelvic). The cervical part stretches from the base of the skull to the neck of the first rib; the sympathetic trunk lies behind the carotid arteries on the deep muscles of the neck. It has three cervical sympathetic ganglia: superior, middle, and inferior. The superior cervical ganglion (ganglion cervicale superius) is the largest ganglion of the sympathetic trunk and is about 20 mm in length and 4-6 mm in breadth. It lies on the level of the second and partly the third cervical vertebrae behind the internal carotid artery and medial to the vagus nerve. The middle cervical ganglion (ganglion cervicale medium) is small and is usually located at the intersection of

150 the inferior thyroid artery with the carotid artery. Often it is absent or separated into two small ganglia. The inferior cervical ganglion (ganglion cervicale injerius) is quite large and is situated behind the initial part of the vertebral artery; it is often fused with the first and sometimes also with the second thoracic ganglion to form a common inferior cervical ganglion (ganglion cervicothoracicum s. ganglion stellatum). Certain authors describe four cervical ganglia of the sympathetic trunk which are linked with the development of the segmental arteries, namely, superior, middle, inferior, and stellate ganglia. The cervical ganglia send nerves to the head, neck, and chest. These can be divided into an ascending group passing to the head, a descending group stretching to the heart, and a group running to the organs of the neck almost immediately from the site of origin. The nerves for the head arise from the superior and inferior cervical ganglia and separate into a group of nerves that penetrate the cranial cavity and another group of nerves that reach the head from outer surface. The first group is represented by the internal carotid nerve (n. caroticus internus) arising from the superior cervical ganglion, and the vertebral branch of the inferior cervical ganglion (n. vertebralis) branching off from the inferior cervical ganglion. Both nerves pass in attendance to arteries of the same names and form plexuses around them, namely the internal carotid plexus (plexus caroticus internus) and the

151 vertebral plexus (plexus vertebralis). Together with the arteries the nerves enter the cranial cavity where they anastomose with one another and send branches to the cerebral vessels, the meninges, the hypophysis, the trunks of the third, four.th, fifth, and sixth pairs of cranial nerves and to the tympanic nerve. The internal carotid plexus (plexus caroticus internus) is coritlnuous with the cavernous plexus (plexus cavernosus) which surrounds the internal carotid ar.tery in the part passing through the cavernous sinus. The branches of the plexus extend on the internal carotid artery itself and on its ramifications. Among the branches of the artery is the deep petrosal nerve (nervus petrosus profundus), which joins the greater superficial petrosal nerve (n. petrosus major) to form the nerve of the pterygoid canal (n. canalis pterygoidei) stretching through the pterygoid canal to the sphenopalatine ganglion (ganglion pterygopalatinum). The second, external, group of the sympathetic nerves of the head consists of two branches of the superior cervical ganglion, the external carotid nerves (nervi carotici externi), which form plexuses around the external carotid artery and then pass in attendance to its ramifications on the head. The plexus sends a small ramus to the otic ganglion (ganglion oticum); the facial plexus (plexus facialis) gives off a branch accompanying the facial artery and passing to the submandibular ganglion. Through rami included in the plexuses around the carotid artery and its branches, the superior cervical plexus

152 sends fibres to the vessels (vasoconstrictors) and the glands of the head (sweat, lacrimal, mucous, and salivary), as well as to the smooth muscles of the hair and to the muscle which dilates the pupil, m. dilatator pupillae. The pupilodilator centre, called the ciliospinal centre (centrum ciliospinale), is in the spinal cord at the level between the seventh cervical and second thoracic segments. The organs of the neck receive nerves from all three cervical ganglia; besides, some nerves arise from the interganglionic areas of the cervical part of the sympathetic trunk and still others from the plexuses of the carotid arteries. The rami of the plexuses follow the course of the branches of the external carotid artery and are known by the same name; they approach the organs together with the arterial branches as a consequence of which the number of sympathetic plexuses is equal to the number of the arterial branches. Among the nerves arising from the cervical part of the sympathetic trunk mentior should be made of the pharyngeal branches (rami laryngopharyngei) of the superior cervical ganglion, part of which pass with the superior laryngeal nerve (a branch of the vagus nerve) to the larynx and part descend to the lateral pharyngeal wall where together with the branches of the glossopharyngeal, vagus, and superIor laryngeal nerves form the pharyngeal plexus (plexus pharyngeus). The descending group of branches of the cervical sympathetic trunk segment is formed, by the cardiac branches of the superior, middle, and inferior cenvical ganglia (nervi

153 cardiaci cervicales superior, medius and interior). They descend into the thoracic cavity and together with the cardiac branches of the sympathetic thoracic ganglia and branches of the vagus nerve contribute to the formation of the cardiac plexuses (see section dealing with innervation of the heart). The thoracic part of the sympathetic trunk lies in front of the necks of the ribs and is covered anteriorly by pleura. It consists of 10 to 12 ganglia of a more or less triangular shape. The thoracic part is characterizerl by the presence of white communicating branches (rami communicantes albi) which connect the anterior roots of the spinal nerves with the sympathetic trunk ganglia, The branches of the thoracic part are as follows: (1) the cardiac branches (nervi cardiaci thoracici) arise from the superior thoracic ganglia and participate in the formation of the cardiac plexus (plexus cardiacus) (the cardiac plexuses are described in detail in the section dealing with the heart); (2) the grey communicating branches (rami communicantes grisei) which are non-medullated fibres supplied to the intercostal nerves (the somatic part of the sympathetic system); (3) the pulmonary branches (rami pulmonales) pass to the lungs to form the pulmonary plexus (plexus pulmonalis); (4) the aortic branches (rami aortici) form a thoracic aortic plexus (plexus aorticus thoracicus), partly on the oesophagus – oesophageal plexus (plexus esophageus), and on the thoracic duct (the vagus nerve also contributes to the formation of these plexuses); (5) the greater and lesser splanchnic nerves (nervi splanchnici major and

154 minor); the greater splanchnic nerve originates as several roots from the fifth to ninth thoracic ganglia, which then pass medially to the level of the ninth thoracic vertebra where they fuse into one common trunk which is transmitted through the space between the muscular bundles of the diaphragmatic crura into the abdominal cavity in which it becomes a component of the coeliac plexus (plexus celiacus); the lesser splanchnic nerve arises from the tenth and eleventh thoracic ganglia, penetrates the diaphragm together with the greater splanchnic nerve or is separated from it by a few muscular bundles, and also becomes a component of the coeliac plexus. Vasoconstricting fibres pass in the splanchnic nerves, which is confirmed by drastic overfilling of the intestinal vessels with blood when these nerves are divided; the splanchnic nerves also contain fibres inhibiting motor activity of the stomach and intestine and fibres conducting sensations from the viscera (the afferent fibres of the sympathetic system). The lumbar, or abdominal part of the sympathetic trunk consists of four, sometimes of three, ganglia. Both sympathetic trunks come closer to each other in the lumbar part than in the thoracic part as a result of which the ganglia lie on the anterolateral surface of the lumbar vertebrae on the medial border of the psoas major muscle. White communicating branches are sent only to the superior two or three lumbar nerves. Along its entire distance the abdominal part of the sympathetic trunk sends off a great number of branches which,

155 together with the greater and lesser splanchnic nerves and the abdominal segments of the vagus nerves, form the largest unpaired coeliac plexus (plexus celiacus). Numerous spinal ganglia (C5-L3) also take part in its formation. The coeliac plexus lies on the anterior semicircumference of the abdominal aorta behind the pancreas and surrounds the initial parts of the coeliac trunk (truncus celiacus) and the superior mesenteric artery. It occupies an area between the renal arteries, the suprarenal glands, and the aortic opening of the diaphragm and includes the paired ganglion of the coeliac artery – coeliac ganglion (ganglion celiacum), and sometimes the unpaired ganglion of the superior mesenteric artery – superior mesenteric ganglion (ganglion mesentericum superius) lying under the root of this artery. The coeliac plexus also gives off some smaller paired plexuses to the diaphragm,suprarenals, and kidneys as well as the testicular (ovarian) plexus (plexus testicularis [ovaricus]) extending along the course of the arteries of the same name. There are also a series of unpaired plexuses which pass to some organs along the walls of arteries whose names they are given. Among these is the superior mesenteric plexus (plexus mesentericus superior) which supplies the pancreas, the small intestine and the large intestine to half the length of the transverse colon, and the ovary. The second main source of innervation of the abdominal organs is the plexus on the aorta – aortic plexus (plexus aorticus abdominalis), formed by two trunks arising from the

156 coeliac plexus and branches running from the lumbar ganglia of the sympathetic trunk. The aortic plexus gives rise to the inferior mesenteric plexus (plexus mesenteric us inferior) for the transverse, descending, and sigmoid colon, and the upper part of the rectum (the superior rectal plexus, plexus rectalis superior). At the origin of the inferior mesenteric plexus lies the inferior mesenteric ganglion (ganglion mesentericum inferius) whose postganglionic fibres pass to the pelvis as components of the hypogastric nerves. The aortic plexus is continuous with the unpaired hypogastric plexus (plexus hypogastricus superior) which bifurcates at the promontory of the sacrum and is in turn continuous with the pelvic plexus (plexus hypogastricus inferior s. plexus pelvinus). Fibres derived from the superior lumbar segments are functionally vasomotor (vasoconstrictor) in relation to the penis and, motor in relation to the uterus and the sphincter urethrae muscle. The sacral, or pelvic, part usually has four ganglia. Lying on the anterior surface of the sacrum along the medial margin of the anterior sacral foramen, both trunks gradually converge to terminate as one common unpaired ganglion impar on the anterior surface of the coccyx. The ganglia of the pelvic part, like those of the lumbar part, are connected both by small longitudinal and transverse trunks. From the ganglia of the sacral part of the sympathetic trunk arise some branches which join the branches of the inferior mesenteric plexus to form a lamina stretching from the

157 sacrum to the urinary bladder; this is the pelvic plexus (plexus hypogastricus inferior s. plexus pelvinus). It has its own small ganglia (ganglia pelvina). Several parts are distinguished in the pelvic plexus: (1) anteroinferior part in which are distingished a superior portion innervating the urinary bladder, the vesical plexus (plexus vesicalis) and an inferior portion supplying the prostatic gland, the prostatic plexus (plexus prostaticus), the seminal vesicles and ductus deferens, the plexus of the vas deferens (plexus deferentialis), and the cavernous bodies, cavernous nerves of the penis (nervi cavernosi penis); (2) posterior part of the plexus supplies the rectum, the middle and inferior rectal plexuses (plexus rectales medii and inferiores). A third, middle part, is distinguished, in addition, in females; its inferior portion sends branches to the uterus and vagina, the uterovaginal plexus (plexus uterovaginalis) and the cavernous bodies of the clitoris, the cavernous nerves of the clitoris (nervi cavernosi clitoridis), while the superior portion gives off branches to the uterus and ovaries.

THE PARASYMPATHETIC SYSTEM

The parasympathetic system develops historically as a suprasegmental part and its centres are therefore located both in the spinal cord and in the brain.

158 THE CENTRES OF THE PARASYMPATHETIC SYSTEM

The central part of the parasympathetic system consists of the cranial and the spinal, or sacral, part. Certain authors claim that the parasympathetic centres are located in the spinal cord not only in the region of the sacral segments but also in its other segments, particularly in the thoracolumbar segment between the anterior and posterior horns, in the intermediate zone. The centres give rise to the efferent fibres of the posterior horns which cause dilation of the vessels and inhibition of perspiration and of the contraction of the smooth muscles of hairs on the trunk and limbs.

THE CRANIAL PART

The cranial part consists, in turn, of centres lodged in the midbrain (mesencephalic part) and in the rhombencephalon, namely in the pons and medulla oblongata (the bulbar part). 1. The mesencephalic part is represented by the accessory nucleus (nucleus accessorius) of the oculomotor nerve (Yakubovich's nucleus) and by the median unpaired nucleus which are responsible for the innervation of the smoothnuscles of the eye (m. sphincter pupillae and m. ciliaris). 2. The bulbar part is represented by the superior salivary nucleus (nucleus salivatorius superior) of the facial

159 nerve (the sensory root of the facial nerve, to be more precise), the inferior salivary nucleus (nucleus salivatorius inferior) of the glossopharyngeal nerve, and the dorsal nucleus (nucleus dorsalis) of the vagus nerve (see the corresponding nerves).

THE SACRAL PART

The parasympathetic centres lie in the spinal cord, in the intermediolateral nucleus (nucleus intermediolateralis) of the lateral horn at the level of the second to fourth sacral segments

THE PERIPHERAL PART OF THE PARASYMPATHETIC SYSTEM

The peripheral part of the cranial parasympathetic system consists of the following structures: (1) preganglionic fibres passing in the third, seventh ninth, and tenth pairs of cranial nerves (according to Mitchell, also in the first and eleventh pairs): (2) terminal ganglia lying close to the organs namely, the ciliary, sphenopalatine, submandibular, and optic ganglia, and (3) postganglionic fibres which either stretch independently, e.g. the short ciliary nerves arising from the ciliary ganglion, or pass in some other nerves e.g. postganglionic fibres originating from the otic ganglion and running in the auriculotemporal nerve. Certain authors claim that the parasympathetic fibres also emerge from different

160 segments of the spinal cord through the dorsal roots and pass to the walls of the trunk and the limbs. The peripheral part of the sacral parasympathetic system consists of fibres which run in the anterior roots of the second, third, and fourth sacral nerves, then in their anterior branches forming the sacral plexus (somatic plexus) and finally enter the true pelvis. In the pelvis they leave the plexus and as the pelvic splanchnic nerves (nn. splanchnici pelvini) pass to the pelvic plexus (plexus hypogastricus inferior) together with which they innervate the pelvic organs (the rectum with the sigmoid colon, the urinary bladder, and the external and internal genitalia). Stimulation of the pelvic splanchnic nerves causes contraction of the rectum and bladder (m. detrusor urinae) with relaxation of their sphincter muscles. The fibres of the sympathetic hypogastric plexus, in contrast, delay the evacuation of these organs; they stimulate uterine contractions, while the pelvic splanchnic nerves inhibit it. The pelvic splanchnic nerves also contain vasodilator fibres (nn. erigentes) for the cavernous bodies of the penis and clitoris which are responsible for erection. The parasympathetic fibres arising from the sacral segment of the spinal cord extend to the pelvic plexuses not only in the erigentes and pelvic splanchnic nerves but also in the pudendal nerve (the preganglionic fibres). According to certain data, the pudendal nerve is a complex nerve containing, in addition to somatic fibres, vegetative (sympathetic and parasympathetic) fibres that form part of the inferior hypogastric plexus. The sympathetic fibres arising from the

161 ganglia of the sacral segment of the sympathetic trunk as postganglionic fibres join the pudendal nerve in the true pelvis and pass through the inferior hypogastric plexus to the pelvic organs. The intramural nervous system also belongs to the parasympathetic nervous system. The walls of some hollow organs contain nerve plexuses of small ganglia (terminal) with ganglionic cells and non-medulated fibres; this is the gangliono-reticular, or intramural system. Leontovich discovered a diffuse nervous network in some tissues (“Leontovich's network”). The intramural system is particularly developed in the digestive tract where it is represented by several plexuses. 1. The myenteric (Auerbach's) plexus (plexus myentericus Auerbachii) lies between the longitudinal and circular muscles of the digestive tube. 2. The submucous (Meissner's) plexus (plexus submucosus Meissneri) is located in the submucous tissue. It is continuous with the plexus of the glands and villi. The “Leontovich's network” is to the periphery of the above-named plexuses. The plexuses receive nerve fibres from the sympathetic and parasympathetic systems. In the intramural plexuses the preganglionic fibres of the parasympathetic systems are switched over to the postganglionic fibres.

162 BRIEF REVIEW OF THE VEGETATIVE INNERVATION OF ORGANS

INNERVATION OF THE EYE

Convergence and accommodation of the visual apparatus occur in response to definite visual stimuli arriving from the retina. Convergence of the eyes, the bringing of their visual axes together to be fixed on the object examined, occurs by reflex due to associated contraction of the eyeball muscles. This reflex, necessary for binocular vision, is linked with accommodation of the eye. Accommodation, the property of the eye to see objects clearly at different distances from it, depends on contraction of the smooth muscles (the ciliary muscle and the sphincter of the pupil). In view of the fact that the activity of the smooth muscles of the eye occurs simultaneously with the contraction of its striated muscles, we shall discuss the vegetative innervation of the eye together with the somatic innervation of its motor apparatus. The afferent pathway from the eyeball muscles (proprioceptive sensitivity) are, according to some authors, the somatic nerves innervating these muscles (the third, fourth, and sixth cranial nerves) and according to others, the ophthalmic nerve (the first division of the trigeminal nerve). The centres of innervation of the eyeball muscles are the nuclei of the third, fourth, and sixth pairs. The efferent pathway

163 are the third, fourth, and sixth cranial nerves. Convergence of the eye, as it is pointed out above, is accomplished by simultaneous contraction of the muscles of both eyes. It should be borne in mind that isolated movement of one eyeball does not occur altogether. Both eyes always take part in any voluntary and reflex movements. Associated movement of the eyeballs (the gaze) is produced by a special system of fibres connecting the nuclei of the third, fourth, and sixth nerves to one another; it is called the medial longitudinal bundle. The medial longitudinal bundle arises in the cerebral peduncle from Darkshevich's nucleus. It is connected to the nuclei of the third, fourth, and sixth nerves by means of collaterals and descends on the brain stem into the spinal cord where it evidently terminates in the cells of the anterior horns of the superior cervical segments. Due to this, the movements of the eyes are combined with movements of the head and neck. The smooth muscles of the eyes, i.e. the ciliary muscle and the sphincter of the pupil, responsible for accommodation, are supplied with parasympathetic innervation; the dilator of the pupil receives nerves from the sympathetic system. The oculomotor and ophthalmic nerves are the afferent pathways of the vegetative system. Efferent parasympathetic innervation. The preganglionic fibres pass from Yakubovich's nucleus (mesencephalic fart of the parasympathetic nervous system) in the oculomotor nerve and in the root o this nerve reach the

164 ciliary ganglion in which they terminate. The ciliary ganglion gives rise to the postganglionic fibres which through the short ciliary nerves (nn. ciliares breves) reach the ciliary muscle and the circular muscle of the iris (sphincter of the pupil). Function: contraction of the pupil and accommodation of the eye to vision at a long and short distance. Efferent sympathetic innervation. The preganglionic fibres arise from the cells of the intermediolateral nucleus, the lateral horns of the last cervical and two upper thoracic segments (C8-Th2, centrum ciliospinale), emerge through two superior thoracic white communicating branches, pass in the cervical segment of the sympathetic trunk, and terminate in the superior cervical ganglion. The postganglionic fibres pass in the internal carotid nerve into the cranial cavity and enter the internal carotid and ophthalmic plexuses; after that some of the fibres penetrate into the communicating branch which is connected with the nasociliary and the long ciliary nerves, while others pass to the ciliary ganglion through which they extend without interruption into the short ciliary nerves. The sympathetic fibres passing in the long ciliary nerves, as well as those passing in the short ciliary nerves, reach the radial muscle of the iris (the dilator of the pupil). Function: dilation of the pupil and constriction of the eye vessels.

165 INNERVATION OF THE LACRIMAL AND SALIVARY GLANDS

The afferent pathway for the lacrimal gland are the lacrimal nerve (a branch of the ophthalmic nerve which is the first division of the trigeminal nerve), for the submandibular and sublingual glands the lingual nerve (a branch of the mandibular nerve which is the third division of the trigeminal nerve), and the chorda tympani (a branch of nervus intermedius, formerly called the sensory root of the facial nerve). The auriculotemporal and glossopharyngeal nerves are the afferent pathways for the parotid gland. Efferent parasympathetic innervation of the lacrimal gland. The centre is in the upper part of the medulla oblongata (Bechterew) and is connected with the nucleus of the nervus intermedius (the superior salivary nucleus). The preganglionic fibres extend as components of nervus intermedius and then of greater superficial petrosal nerve to the sphenopalatine ganglion. This ganglion gives rise to the postganglionic fibres, which as components of the maxillary nerve and then of its branch, the zygomatic nerve, reach the lacrimal gland through connections with the lacrimal nerve. Efferent parasympathetic innervation of the submandibular and sublinguai glands. The preganglionic fibres extend from the superior salivary nucleus as components of nervus intermedius, then of the chorda tympani and the lingual nerve to the submandibular ganglion from which the

166 postganglionic fibres arise and reach the glands in the lingual nerve. Efferent parasympathetic innervation of the parotid gland. The preganglionic fibres pass from the inferior salivary nucleus as components of the glossopharyngeal nerve, then of the tympanic and the lesser superficial petrosal nerve to the otic ganglion. Here arise the postganglionic fibres and extend in the auriculotemporal nerve to reach the gland. Function: stimulation of the secretion of the lacrimal and the salivary glands mentioned above; dilation of the vessels of the glands. Efferent sympathetic innervation of all the glands named above. The preganglionic fibres originate in the lateral horns of the superior thoracic segments of the spinal cord and terminate in the superior cervical ganglion. The postganglionic fibres arise in this ganglion and reach the lacrimal glands as components of the internal carotid plexus the parotid gland in the external carotid plexus, and the submandibular and sublingual glands through the external carotid plexus and then through the facial plexus. Function: inhibition of saliva secretion (dryness in the mouth); mild stimulation of lacrimation.

INNERVATION OF THE HEART

The afferent pathways extend from the heart in the vagus nerve and in the cardiac branches of the middle and inferior cervical and the thoracic sympathetic ganglia. The

167 sense of pain is conducted along the sympathetic nerves, all the other afferent impulses along the parasympathetic nerves. Efferent parasympathetic innervation. The preganglionic fibres arise in the dorsal vegetative nucleus of the vagus nerve and pass in it and in its cardiac branches and cardiac plexuses (see “Innervation of the Heart”), to the ganglia located in the heart (the Russian scientist V.V. Nikolaev was the first to determine this in 1893), and to the ganglia of the pericardial fields. Postganglionic fibres arising from these ganglia extend to the heart muscle. Function: inhibition and suppression of cardiac activity. Constriction of the coronary arteries. In 1866 I.F. Tsion discovered the “cardiosensitive” nerve passing centripetally as a component of the vagus nerve. It is concerned with reduction of arterial pressure and is therefore termed the depressor nerve. Efferent sympathetic innervation. The preganglionic fibres originate in the lateral horns of the spinal cord of the upper four or five thoracic segments, pass in the corresponding white communicating branches and then through the sympathetic trunk to the five upper thoracic and three cervical ganglia. These ganglia give rise to the postganglionic fibres, which as components of the cardiac branches of the superior, middle, and inferior cervical and the thoracic ganglia reach the heart muscle. According to certain authors, the pathway is interrupted only in the stellate ganglion. In G.F. Ivanov's representation, the cardiac branches of the ganglia contain

168 preganglionic fibres which switch over to postganglionic fibres in the cells of the cardiac plexus. Function: (1) stimulation of cardiac activity (a fact established by Pavlov in 1888 who called the sympathetic nerve the “stimulating” nerve), (2) increase of the heart beat rate (determined by Tsion in 1866), and (3) dilation of the coronary vessels.

INNERVATION OF THE LUNGS AND BRONCHI

The afferent outflow from the visceraI pleura occurs along the pulmonary branches of the thoracic sympathetic trunk, from the parietal pleura along the intercostal and phrenic nerves, and from the bronchi along the vagus nerve. Efferent parasympathetic innervation. The preganglionic fibres originate in the dorsal vegetative nucleus of the vagus nerve and extend as components of this nerve and its pulmonary branches to the ganglia of the pulmonary plexus and to ganglia arranged along the course of the trachea and bronchi and in the lungs. The postganglionic fibres stretch from these ganglia to the muscles and glands of the bronchial tree. Function: constriction of bronchi and bronchioles, stimulation of mucus excretion, dilation of vessels. Efferent sympathetic innervation. The preganglionic fibres emerge from the lateral horns of the spina cord of the superior thoracic segments (Th2-Th6) and pass through the corresponding white communicating branches and the sympathetic trunk to reach the stellate or upper thoracic

169 ganglia. From the last named originate the postganglionic fibres which reach the bronchial muscles and blood vessels as components of the pulmonary plexus. Function: dilation of bronchi,. Constriction and ,sometimes dilation of vessels.

INNERVATION OF THE GASTRO-INTESTINAL TRACT (TO THE LEVEL OF THE SIGMOID COLON), PANCREAS, AND LIVER

The afferent outflow from these organs occurs along fibres which are constituents of the vagus and the lesser and greater splanchnic nerves, the hepatic and coeliac plexuses, and the thoracic and lumbar spinal nerves, and, according to some authors, also of the phrenic nerve. The sympathetic nerves conduct the sense of pain from these organs; the vagus nerve conducts the other afferent impulses as well as the sense of nausea and hunger from the stomach. Efferent parasympathetic innervation. Preganglionic fibres from the dorsal vegetative nucleus of the vagus nerve pass as components of the last named to the terminal ganglia in the tissues of the organs discussed. In the intestine these are the cells of the intestinal plexuses (the myenteric and submucous plexuses). From the ganglia the postganglionic fibres run to the smooth muscles and glands. Function: stimulation of peristalsis of the stomach, relaxation of the pyloric sphincter, stimulation of peristalsis of the intestine and gall bladder. The vagus nerve

170 contains fibres which stimulate and inhibit secretion. Efferent parasympathetic innervation is also concerned with dilation of vessels. Efferent sympathetic innervation. The preganglionic fibres emerge from the lateral horns of the fifth to twelfth spinal segments and then pass in the corresponding white communicating branches into the sympathetic trunk and then without interruption, as components of the greater splanchnic nerves (between the sixth and ninth ganglia), reach the intermediate ganglia contributing to the formation of the coeliac and mesenteric plexuses. Here arise the postganglionic fibres ,which ,as components of the coeliac and superior mesenteric plexuses extend to the liver, pancreas, the small and large intestine and to the level of the middle of the transverse colon; the left half of the transverse colon and the descending colon are innervated by the inferior mesenteric plexus. The plexuses mentioned supply the muscles and glands of the organs discussed. Function: inhibition of gastric, intestinal and gall bladder peristalsis, constriction of the blood vessels, and inhibition of glandular secretion. It should be added that the movement of the gastric and intestinal contents, can also be delayed because the sympathetic nerves cause active contraction of the pyloric, intestinal and other sphincters.

171 INNERVATION OF THE SIGMOID COLON, RECTUM, AND URINARY BLADDER

The afferent pathways run as components in the inferior mesenteric, superior and inferior hypogastric plexuses, and in the pelvic splanchnic nerves. Efferent parasympathetic innervation. The preganglionic fibres arise in the lateral horns of the second to fourth sacral spinal segments and emerge as components of the corresponding anterior roots of the spinal nerves. They then pass as the pelvic splanchnic nerves to the intraorganic ganglia of the parts of the large intestine discussed and to the ganglia located around the urinary bladder. All these ganglia give rise to the postganglionic fibres, which reach the smooth muscles of the organs. Function: stimulation of peristalsis of the sigmoid colon and rectum, relaxation of the sphincter and internus muscle, contraction of the detrusor urinae, and relaxation of the sphincter vesical muscle. Efferent sympathetic innervation. The preganglionic fibres pass from the lateral horns of the lumbar spinal cord through the corresponding anterior roots into the white communicating branches, run without interruption through the sympathetic trunk, and reach the inferior mesenteric ganglion. In this ganglion arise the postganglionic fibres, which then extend as components of the hypogastric nerves to the smooth muscles of the organs discussed. Function: inhibition of peristalsis of the sigmoid colon and rectum and contraction of

172 the sphincter and internus muscle. The sympathetic nerves in the urinary bladder cause relaxation of the detrusor urinae and contraction of the sphincter urethrae muscle.

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.

173 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

174 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

175 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

176 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.

177 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

178 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.

179 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

180 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

181 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

182 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

183 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

184 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

185 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

186 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

187 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

188 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

189 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

190 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

191 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

192 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.

193 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

194 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:

195 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

196 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

197 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

198 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

199 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

200 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”).

SPINAL NERVES

The spinal nerves (nn. spinales) are located in regular order (neuromeres) corresponding to the myotomes (myomeres) of the trunk and alternate with the segments of the spine; every nerve is attended by a corresponding area of skin (dermatome).

201 Man has 31 pairs of spinal nerves: 8 pairs of cervical, 12 pairs of thoracic, 5 pairs of lumbar, 5 pairs of sacral and 1 pair of coccygeal nerves. Every spinal nerve branches off from the spinal cord in two roots: the dorsal or posterior (sensory) root, and the ventral or anterior (motor) root. Both roots are joined in one trunk, or funiculus, which passes from the spine through an intervertebral orifice. Near and somewhat externally of the place where the roots join, the posterior root forms the ganglion spinale or ganglion intervertebrale in which the anterior motor root does not participate. Since both roots are joined the spinal nerves are mixed nerves; they contain sensory (afferent) fibres from the cells of the spinal ganglia, motor (efferent) fibres from the cells of the anterior horn, and also vegetative fibres from the cells of the lateral horns emerging from the spinal cord as part of the anterior root. In the opinion of certain authors vegetative fibres are also contained in the posterior root. The vegetative fibres which pass through the roots into the animal nerves. ensure such processes in the soma as trophics, vasculomotor reactions, etc. On emerging from the intervertebral orifice every cerebrospinal nerve divides, according to two parts of the myotome (dorsal and ventral), into two branches: (1) the posterior, dorsal branch (ramus dorsalis) for the autochthonous muscles of the back developing from the dorsal part of the myotome and the skin covering it;

202 (2) the anterior, ventral branch (ramus ventralis) for the ventral wall of the trunk and the limbs, developing from the ventral parts of the myotomes. Besides this, another two kinds of branches arise from the cerebrospinal nerve: (3) the communicating branches (rami communicantes) to the sympathetic trunk for innervating the internal organs; (4) the meningeal branch (ramus meningeus) passing back through the intervertebral orifice for innervating the membranes of the spinal cord.

THE POSTERIOR BRANCHES OF THE SPINAL NERVES

The posterior branches (rami dorsales) of all the cerebrospinal nerves pass backward between the transverse processes of the vertebrae, curving around their articular processes. With the exception of the first cervical, fourth and fifth sacral and coccygeal branches, they all divide into the medial branch (ramus medialis) and lateral branch (ramus lateralis) which supply the skin of the back of the head, the posterior surface of the neck and back and the deep dorsal muscles. The posterior branch of the first cervical nerve, n. suboccipitalis, emerges between the occipital bone and the atlas and then divides into branches supplying mm. recti capitis

203 major and minor, m. semispinalis capitis, mm. obliqui capitis. N. suboccipitalis does not give off branches to the skin. The posterior branch of the second cervical nerve, the greater occipital nerve (n. occipitalis major) coming out between the posterior arch of the atlas and the second vertebra, pierces the muscles and, having become subcutaneous, innervates the occipital part of the head. Rami dorsales of the thoracic nerves divide into medial and lateral branches giving rise to branches running to the autochthonous muscles; the skin branches of the superior thoracic nerves originate only from rami mediales, while those of the inferior thoracic nerves, from rami laterales. The cutaneous branches of the three upper lumbar nerves pass to the superior part of the gluteal region under the name of gluteal branches (of the posterior primary rami of lumbar nerves) (nn. clunium superiores); the cutaneous branches of the sacral nerves are called gluteal branches (of the posterior primary rami of sacral nerves) (nn. clunium medii).

THE ANTERIOR BRANCHES OF THE SPINAL NERVES

The anterior branches (rami ventrales) of the spinal nerves innervate the skin and muscles of the ventral wall of the body and both pairs of limbs. Since the skin of the lower abdomen participates in the development of the external sexual

204 organs, the skin cvering them is also innervated by the anterior branches. Except for the first two, the anterior branches are much larger than the posterior branches. The anterior branches of the spinal nerves preserve their original metameric structure only in the thoracic segment (nn. intercostales). In the other segments connected with the limbs in whose development the segmentary character is lost, the nerves arising from the anterior spinal branches intertwine. This is how nervous plexuses are formed in which exchange of fibres of different neoromeres takes place. A complex redistribution of fibres occurs in the plexuses: the anterior branch of every spinal nerve sends its fibres into several peripheral nerves and, consequently, each of them contains fibres of several segments of the spinal cord. It is therefore understandable that lesion of a nerve arising from the plexus is not attended by disturbed function of all the muscles receiving Innervation from the segments which gave origin to this nerve. Most of the nerves emerging from plexuses are mixed; this is why the clinical picture of the lesion is made up of motor disorders, sensory disorders and vegetative disorders. Three large plexuses are distinguished: cervical, brachial and lumbosacral.

THE CERVICAL PLEXUS

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

205 which are connected by three arching loops and are located laterally of the transverse processes between the prevertebral muscles from the medial side and vertebral (m. scalenus medius, m. levator scapulae and m. splenius cervicis) from the lateral side, 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)

(from CII and CIII), runs to the skin of the lateral part of the occipital region. 2. The great auricular nerve (n. auricularis magnus)

(from CIII), 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) (from CII-III) arises, like the preceding two nerves, from the middle of the posterior edge of m. sternocleidomastoideus and divides into branches which curve around the posterior edge of the sternocleidomastoid muscle and pass over its external surface forward and down under the m. platysma, supplying the skin of the neck. 4. The supraclavicular nerves (nn. supraclaviculares)

(from CIII and CIV) descend under the platysma nearly

206 perpendicularly along the supraclavicular fossa into the skin above the pectoralis major and deltoideus. The muscular branches: 1. Branches to the mm. recti capitis anterior and lateralis, mm. longus capitis and colli, mm. scaleni, m. levator scapulae and, finally, to mm. intertransversarii anteriores.

2. Radix inferior, arising from CII-CIII, 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 forming a cervical loop (ansa ceroicalis) with this branch. 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 (from CIII to CIV) which participate in the innervation of these muscles together with the accessory nerve. The mixed branches:

The phrenic nerve (n. phrenicus) (CIII-CIV) 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 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

207 nerve receives fibres from two inferior cervical ganglia of the sympathetic trunk. 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) and anastomose with the sympathetic plexus of the diaphragm, 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. There is information pointing to more extensive innervation by the phrenic nerve; it is presumed that with its fibres in the thoracic cavity it supplies the heart, lungs, the thymus gland, and in the peritoneal cavity it is connected with the solar plexus through which it innervates certain internal organs.

THE BRACHIAL PLEXUS

The brachial plexus (plexus brachialis) is composed of the anterior branches of four inferior cervical nerves (CV-CVII) and the greater part of first thoracic nerve (ThI); it is often joined by a thin branch from CIV. The brachial plexus passes through the space between the scalenus anterior and scalenus medius into the supraclavicular fossa higher and behind the subclavian artery. Three thick nerve trunks arise from it; they pass into the axillary fossa and surround a. axillaris from three

208 sides: from the lateral (lateral trunk) and medial (medial trunk) sides and posteriorly of the artery (posterior trunk). Pars supraclavicularis and pars infraclavicularis are usually distinguished in the brachial plexus. There are short and long peripheral branches. The short branches arise from various points in the supraclavicular part of the plexus and supply the cervical muscles partially, as well as the muscles of the shoulder girdle (with the exception of m. trapezius) and the shoulder joint. The long branches arise from the abovementioned three trunks and run the length of the upper limb, innervating its muscles and skin.

The circumflex nerve (n. axillaris) (from CV-CVI) is the thickest nerve among the short branches of the brachial plexus. It passes together with a. circumflexa humeri posterior through the foramen quadrilaterum onto the posterior surface of the surgical neck of the humerus and sends branches to mm. deltoideus, teres minor and to the shoulder joint. Along the posterior edge of the deltoid muscle it gives off a cutaneous branch, n. cutaneous brachii lateralis superior, that innervates the skin of the deltoid and the posterolateral region of the upper arm in its upper part.

The long branches: Among the long branches we can distinguish the anterior branches for the flexors and pronators (nn. musculocutaneus, medianus and ulnaris) and the posterior branches for the extensors and supinators (n. radialis).

209 1. The musculocutaneous nerve (n. musculocutaneus) originates from the lateral trunk of the brachial plexus (from

CV-CVII), pierces m. coracobrachialis and innervates all the anterior muscles of the shoulder: mm. coracobrachialis, biceps and brachialis. On passing between the latter two onto the lateral side of the upper arm, it continues onto the forearm as the lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis) supplying the skin of the radial side of the latter and the skin of the thenar.

2. The median nerve (n. medianus) (CV-CVIII, ThI) arises from the medial and lateral trunks in two roots which embrace the a. axillaris in front. It then passes into the sulcus bicipitalis medialis together with the brachial artery. In the cubital fossa the nerve runs under the m. pronator teres and the sup erficial flexor of the fingers and then between the latter and the m. flexor digitorum profundus and into the sulcus medianus in the middle of the forearm and to the palm. On the upper arm, the median nerve gives off no branches. On the forearm it gives off rami musculares to all the muscles of the anterior flexor group, with the exception of the m. flexor carpi ulnaris and the part of the deep flexor of the fingers nearest to it. One of the branches, the posterior interosseous nerve (n. interosseus [antebrachii] anterior) accompanies the a. interossea anterior on the interosseous membrane and innervates the deep flexor muscles (m. flexor pollicis longus and part of the flexor digitorum profundus), m. pronator quadratus and the radiocarpal joint. The median nerve gives off

210 a thin cutaneous branch, the palmar cutaneous branch (of the median nerve) (ramus palmaris n. mediani) over the radiocarpal joint. This branch supplies a small area of skin on the thenar and the palm. The median nerve passes onto the palm through the canalis carpi together with the tendons of the flexors and divides into three branches, the common plantar digital nerves (nn. digitales palmares communes) which run along the first, second and third metacarpal spaces under the palmar aponeurosis toward the fingers. The first branch innervates the muscles of the thenar with the exception of the m. adductor pollicis and the deep head of the m. flexor pollicis brevis which are innervated by the ulnar nerve. The common palmar digital nerves, in turn, divide into seven proper palmar digital nerves (nn. digitales palmares proprii) which pass to both sides of the thumb, to the index and middle fingers and to the radial side of the ring finger. The same branches also supply the skin on the radial side of the palm, the digital nerves also supply the first and second vermiform muscles. 3. The ulnar nerve (n. ulnaris) which emerges from the medial trunk of the brachial plexus (CVII, CVIII, ThI) passes on the medial side of the upper arm to the posterior surface of the medial epicondyle (it lies under the skin here and this is why it gets hurt so often, causing a prickling sensation in the middle zone of the forearm) and then extends in the sulcus ulnaris and further in the canalis carpi ulnaris where it runs, together with the arteries and veins of the same name, to the palm. On the surface of the retinaculum flexorum it transforms into its

211 terminal branch – the ramus palmaris n. ulnaris. Like the median nerve, the ulnar nerve does not give rise to any branches on the upper arm. The branches of the ulnar nerve on the forearm and hand: Rami articulares to the ulnar joint. Rami musculares for the m. flexor carpi ulnaris and its neighbouring portion of the m. flexor digitorum profundus. Ramus cutaneus palmaris to the skin of the hypothenar. Ramus dorsalis n. ulnaris emerges through the space between the m. flexor carpi ulnaris and the ulnar bone to reach the dorsal surface of the' hand where it divides into five dorsal digital branches, nn. digitales dorsales, for the little and ring fingers and the ulnar side of the middle finger. Ramus palmaris n. ulnaris, the terminal branch of the ulnar nerve, at the level of the os pisiforme divides into the superficial and deep branches, of which the superficial (ramus superficialis) supplies, via a small muscle branch, the m. palmaris brevis and then the skin on the ulnar side of the palm and, on dividing, gives off three nn. digitales palmares proprii to both sides of the little finger and to the ulnar side of the ring finger. Ramus profundus, the deep branch of the ulnar nerve, together with the deep branch of the a. ulnaris passes through the space between the m. flexor and m. abductor digiti minimi and accompanies the deep palmar arch. There it innervates all the muscles of hypothenar, all mm. interossei, the third and

212 fourth mm. lumbricales, and from the muscles of the thenar – m. adductor pollicis and the deep head of the m. flexor pollicis brevis. Ramus profundus ends in a thin anastomosis with n. medianus. 4. The medial cutaneous nerve of the arm (n. cutaneus brachii medialis) arises from the medial trunk of the plexus (from CVIII, ThI) and passes along the axilla medially to the a. axillaris; it usually joins with the perforating branch of the second thoracic nerve (n. intercostobrachialis), and supplies the skin on the medial surface of the upper arm up to the ulnar joint. 5. The medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis) also arises from the medial trunk of the plexus (from CVIII, ThI) and lies in the axilla next to the ulnar nerve; in the upper part of the upper arm it is located medially of the brachial artery next to v. basilica together with which it pierces the fascia and becomes subcutaneous. This nerve innervates the skin on the ulnar (medial) side of the forearm down to the articulations of the hand.

6. The radial nerve (n. radialis) (CV-VIII, ThI), is a continuation of the posterior trunk of the brachial plexus. It passes posteriorly of the brachial artery together with the profunda brachii artery onto the posterior surface of the upper arm, runs around the humerus to be lodged in the canalis humeromuscularis, and then, piercing the lateral intermuscular septum from back to front, emerges into the space between the m. brachioradialis and m. brachialis. Here the nerve divides

213 into the superficial (ramus superficialis) and deep (ramus profundus) branches. Prior to this, the radial nerve gives rise to the following branches: Muscular branches (rami musculares) on the upper arm for extensors – m. triceps and m. anconeus. The last small branch also supplies the capsule of the elbow joint and the radial epicondyle of the upper arm. This is why pain spreads along the entire length of the radial nerve in inflammation of the epicondyle (epicondylitis). The posterior cutaneous nerve of the arm and the lower lateral cutaneous nerve of the arm (nn. cutanei brachii posterior and lateralis inferior) branch out in the skin of the posterior and inferior parts of the posterolateral surface of the upper arm. The posterior cutaneous nerve of the forearm (n. cutaneus antebrachii posterior) arises from. the radial nerve in the canalis humeromuscularis, emerges under the skin at the origin of the m. brachioradialis and spreads over the dorsal surface of the forearm. Muscular branches (rami musculares) pass to the m. brachioradialis and m. extensor carpi radialis longus. The superficial branch of the radial nerve (ramus superficialis) passes to the forearm into the sulcus radialis laterally of the a. radialis, and then in the lower third of the forearm through the space between the radial bone c and the tendon of the m. brachioradialis goes over to the dorsal surface of the hand and supplies with five dorsal branches, the dorsal

214 digital nervel (nn. digitales dorsales) along the sides of the thumb and index finger, and the radial side of the middle finger. These branches usually end at the level of the distal interphalangeal joints. They anastomose with the ramus dorsalis n. ulnaris. Thus every finger is supplied with two dorsal and two palmar nerves passing along both sides. The dorsal nerves arise from the radial nerve and the ulnar nerve, each innervating two and a half fingers. The palmar nerves arise from the median nerve and the ulnar nerve, with the former supplying three and a half fingers (beginning with the thumb), and the latter the remaining one and a half fingers. The deep branch (ramus profundus) passes through the m. supinator and, having supplied it with a branch, continues onto the dorsal surface of the forearm, innervating the m. extensor carpi radialis brevis and all the posterior muscles of the forearm. The continuation of the deep branch, n. interosseus (antebrachii) posterior, descends between the extensors of the thumb to the radiocarpal joint, which it supplies. It can be seen by the route followed by the radial nerve that it supplies all the extensors both of the upper arm and of the forearm and also a radial group of muscles in the forearm. In correspondence with this it also innervates the skin on the extensor surface of the upper arm arid forearm. The radial nerve, which is the continuation of the posterior trunk, functions as if it were the posterior nerve of the hand.

215 THE ANTERIOR BRANCHES OF THE THORACIC NERVES

The anterior primary rami (rami ventrales) of the thoracic nerves (nn. thoracici), are called intercostal nerves (nn. intercostales) because they are located in the intercostal spaces, but the twelfth nerve runs along the lower edge of the twelfth rib (n. subcostalis). Each intercostal nerve gives off a communicating branch to the sympathetic trunk and is at first covered only by the pleura, but then it passes down from the posterior intercostal artery and enters the space between the external and internal intercostal muscles and then passes along the inferior edge of the rib forward. The upper six intercostal nerves reach the edge of the sternum, the lower six penetrate the thickness of the peritoneal wall where in the space between the transverse and the internal oblique muscles they pass to the rectus muscle of the abdomen through its sheath. The twelfth intercostal nerve passing posteriorly obliquely on the m. quadratus lumborum approaches closely with its anterior end the pubic symphysis and ends in the lower part of the rectus muscle and m. pyramidalis. On their way the intercostal nerves give off muscular branches to all the ventral muscles in the walls of the thoracic and peritoneal cavities, and to the muscles of ventral origin on the back: mm. serrati posteriores superiores and inferiores and mm. levatores costarum. They also participate in innervation of the pleura and peritoneum (nn.pl eurales and peritoneales).

216

THE LUMBOSACRAL PLEXUS

The lumbosacral plexus (plexus lumbosacralis) is composed of the anterior branches of the lumbar, sacral and coccygeal nerves. This common plexus is divided into separate parts, or separate plexuses: lumbar, sacral and coccygeal.

The Lumbar Plexus

The lumbar plexus (plexus lumbalis) is formed of the anterior branches of three superior lumbar nerves and the upper part of the fourth superior lumbar nerve, and of a small branch from the twelfth intercostal nerve. The plexus lies in front of the transverse processes of the lumbar vertebrae in the thickness of the m. psoas major and gives rise to a number of branches which emerge partly from under the lateral, partly from under the medial edge of this muscle, while another part of them pierces it and appears on its anterior surface. These branches are as follows. 1. Rami musculares to the m. psoas major and minor, m. quadratus lumborum and mm. intertransversarii laterales lumborum.

2. The iliohypogastric nerve (n. iliohypogastricus) (LI) emerges from under the lateral edge of the m. psoas major and stretches on the anterior surface of the m. quadratus lumborum parallel to the twelfth intercostal nerve. The iliohypogastric

217 nerve is a segmentary nerve and like the latter it passes between the transverse and internal oblique muscles of the abdomen, supplying them with muscular branches, and also innervates the skin of the upper part of the buttocks and the inguinal canal above the level of its superficial orifice.

3. The ilio-inguinal nerve (n. ilio-inguinalis) (LI) is also a segmentary nerve, it emerges from under the lateral edge of the m. psoas major .and runs parallel to and downward of the iliohypogastric nerve, and then directly in the inguinal canal, exits through the superficial inguinal ring and branches out in. the skin of the pubis and scrotum or the labia majora.

4. The genitofemoral nerve (n. genitofemoralis) (LII) runs through the thickness of the m. psoas major onto its anterior surface and divides into two branches, one of which, the femoral branch (r. femoralia), descends forward to Poupart's ligament, passes under it and branches in the skin of the thigh immediately below this ligament. The other, the genital branch (r. genitalia), pierces the posterior wall of the inguinal canal and joins the spermatic cord, supplying the m. cremaster and the membranes of the testicle. 5. The lateral cutaneous nerve of the thigh (n. cutaneus femoris lateralia) (LII, LIII), originates from under the lateral edge of the m. psoas major and passes over the surface of the m. iliacus from top to bottom and then laterally to the spina iliaca anterior superior, where it pierces the abdominal wall and emerges onto the thigh, becomes subcutaneous and

218 descends along the lateral surface of the thigh to the knee, innervating the skin. The skin of the external surface of the thigh is also innervated by the iliohypogastric and subcostal nerves. 6. The femoral nerve (n. femoralis) is the largest branch, of the lumbar plexus (LII, LIII, LIV). It emerges through the lacuna musculorum onto the anterior side of the thigh. It lies laterally of the femoral artery, separating from it with a deep fascia, fasciae latae, and then ramifies into numerous branches. Some of these branches, rami musculares, innervate the m. quadriceps, m. sartorius and m. pectineus, while others, the rami cutanei anteriores, supply the skin of the anteromedial surface of the thigh. One of the cutaneous branches of the femoral nerve, a very long branch, the saphenous nerve, stretches in canalis adductorius laterally of the femoral artery. At the hiatus adductorius the nerve leaves the artery, pierces the anterior wall of the canal and becomes superficial. On the leg the nerve is accompanied by the v. saphena magna. It gives rise to the infrapatellar branch (ramus infrapatellaris) in the skin of the lower part of the leg and to the medial cutaneous nerve of the thigh (rami cutanei cruris mediales) in the skin of the medial surface of the leg down to the medial edge of the foot. In addition to the principal femoral nerve there is a supplementary femoral nerve.

7. The obturator nerve (n. obturatorius) (LII-LIV) passes through the obturator canal to the thigh and divides into the anterior and posterior branches. The posterior branch

219 (ramus posterior) innervates the m. obturatorius externus, m. adductor magnus and the hip joint; the anterior branch (ramus anterior) supplies the other adductor muscles together with the m. gracilis and m. pectineus, and, besides, it gives rise to a long cutaneous branch (ramus cutaneus) which passes down between the adductor muscles. The cutaneous branch runs under the skin on the medial side of the thigh in its lower half, which it innervates.

The Sacral Plexus

The sacral plexus (plexus sacralis), the most significant of all the plexuses, is composed of the anterior branches of the fourth (lower part) and fifth lumbar nerves and of similar branches of four sacral nerves (SI-SIV), which emerge from the anterior sacral foramen. The proximity of many bundles of the plexus to the sacroiliac joint determines the variety of localization and irradiation of pain in case of the disease of this joint. In joining with one another; the nerves of the plexus form a number of loops whose apices merge at the lower edge of the m. piriformis to form a thick trunk of the sciatic nerve passing through the foramen infrapiriforme from the pelvic cavity. The branches originating from the sacral plexus may be divided into short and long ones. The short ones branch out in the region of the pelvic girdle and the long branches supply the whole lower extremity except for that part which is supplied by branches of the lumbar plexus.

220 The short branches: 1. The muscular branches (rami musculares) for the m. piriformis (from SI and SII), m. obturatorius internus with mm. gemelli and quadratus femoris (from LIV, LV, SI and SII), for mm. levator ani and coccygeus (SIII-SIV).

2. The superior gluteal nerve (n. gluteus superior) (LIV and LV and from SI) emerges through the foramen suprapiriforme from the pelvis together with the artery of the same name and then extends to the m. gluteus medius, m. gluteus minimus and m. tensor fasciae latae.

3. The inferior gluteal nerve (n. gluteus inferior) (LV,

SI, SII), emerges through the foramen infrapiriforme lateral of the artery of the same name; its branches supply the m. gluteus maximus and the capsule of the hip joint.

4. The pudendal nerve (n. pudendus) (SI-SIV) emerges through the foramen infrapinforme together with the a. pudenda intema and, curving around the spina ischiadica, passes back: into the pelvis through the foramen ischiadicum minus. Further the pudendal nerve together with the same artery passes along the lateral wall of the fossa ischiorectalis. Within the limits of the latter it gives off the inferior haemorrhoidal nerves (nn. rectales inferiores) which supply the external sphincter (m. sphincter ani externus) and the skin of the region around the anus. At the level of the tuber ischiadicum at the posterior edge of the diaphragma urogenitale the pudendal nerve divides into the perineal nerves and dorsal nerve of the penis (nn. perinei and n. dorsalis penis) (clitoridis). The first run forward and

221 innervate the m. ischiocavernosus, m. bulbospongiosus and m. transversus perinei superficialis and the skin of the perineum. The terminal branches supply the skin of the posterior side of the scrotum, the scrotal branches (nn. scrotales posteriores), and the labia majora, the labial branches (nn. labiales posteriores). The dorsal nerve of the penis (clitoridis) attends a. dorsalis penis in the thickness of the diaphragma urogenitale, gives off small branches to the m. transversus perinei profundus and m. sphincter uretrae, and passes onto the dorsum penis (or clitoris) where it is distributed in the skin mainly of the glans penis. A large number of vegetative fibres pass in the composition of the pudendal nerve. The long branches: 1. The posterior cutaneous nerve of the thigh (n. cutaneus femoris posterior) (SI, SII, SIII) emerges from the pelvis together with the sciatic nerv and then descends under the m. gluteus maximus onto the posterior surface of the thigh. From its medial side it gives off small gluteal branches which run under the skin of the lower part of the buttock (nn. clunium inferiores) and to the perineum-perineal branches (rami perineales). On the surface pf the posterior thigh muscles it reaches down to the popliteal fossa and gives rise to numerous branches which are distributed in the skin of the posterior surface of the thigh and the calf region. 2. The sciatic nerve (n. ischiadicus) is the largest nerve in the entire body and is actually the direct continuation of the sacral plexus that contains fibres of all its roots. It emerges

222 from the pelvic cavity through the large ischiadic foramen below the m. piriformis and is covered by the m. gluteus maximus. Further down the nerve emerges from under the lower edge of this muscle and descends perpendicularly on the posterior surface of the thigh under the flexors of the leg. In the upper part of the popliteal fossa it usually divides into its two main branches: the medial thicker one – the medial popliteal nerve (n. tibialis) and the lateral, thinner one – the lateral popliteal nerve (n. peroneus [fibularis] communis). Quite often the nerve is already divided into two separate trunks the entire length of the postetrior part of the thigh. Branches of the sciatic nerve: 1. Muscular branches (rami musculares) to the posterior muscles of the thigh: m. semitendinosus, m. semimembranosus and to the long head the m. biceps femoris and also to the posterior part of the m. adductor magnus. The short head of the m. biceps receives a small branch from the peroneal nerve. A small branch also arises from the peroneal nerve to the knee joint.

2. The medial popliteal nerve (n. tibialis) (LIV, LV, SV,

SIII) descends directly through the middle of the popliteal fossa along the tract of the popliteal vessels, then enters the canalis crnropopliteus, accompanying the a. and vv. tibiales posteriores in this canal until it reaches the medial malleolus. Behind the latter the medial popliteal nerve divides into its terminal branches – the lateral and medial plantar nerves (nn. plantares lateralis and medialis) in the planter sulci of the same name. In

223 the popliteal fossa the rami musculares branch out from the medial popliteal nerve to the m. gastrocnemius, m. plantaris, m. soleus and m. popliteus. Several small branches pass to the knee joint. Moreover, in the popliteal fossa the medial popliteal nerve gives rise to a long cutaneous branch the lateral cutaneous nerve of calf of the leg (n. cutaneus surae medialis), which runs down together with the v. saphena parva and innervates the skin of the posteromedial surface of the leg. On the leg the medial popliteal nerve supplies all three deep muscles with small branches: m. tibialis posterior, m. flexor hallucis longus and m. flexor digitorum longus and the posterior side of the talocrural articulation and behind the medial malleolus it gives rise to cutaneous branches, rami calcanei mediales, which pass to the skin of the heel and the medial edge of the foot. The medial plantar nerve (n. plantaris merlialis) together with the artery of the same name passes in the sulcus plantaris medialis along the medial edge of the m. flexor digitorum brevis and supplies this muscle and the muscles of the medial group, with the exception of the m. adductor hallucis and the lateral head of the m. flexor hallucis brevis. The nerve finally ramifies into seven nerves-proper plantar digital nerves (nn. digitales plantares proprii) of which one passes to the medial edge of the big toe and also supplies the first and second mm. lumbricales while the remaining six nerves innervate the skin of the sides of toes facing each other,

224 beginning with the lateral side of the big toe and ending with the medial side of the fourth toe. The lateral plantar nerve (n. plantaris laleralis) runs in the same direction as the artery of the same name in the sulcus plantaris lateralis. It supplies by means of rami musculares all three muscles of the lateral group of the sole and the m. quadratus plantae and divides into two branches, a deep and a superficial one. The deep branch (ramus profundus) runs together with the plantar arterial arch and supplies the third and fourth mm. lumbricales and all the dorsal interossei muscles, and also the m. adductor hallucis and the lateral head of the m. flexor hullucis brevis. The superficial branch (ramus superficialis) gives off branches to the skin of the sole and divides into three proper plantar digital nerves (nn. digitales plantares proprii) running to both sides of the fifth toe and to the side of the fourth toe facing the fifth. In general, the distribution of the medial and lateral plantar nerves corresponds to the direction taken by the ulnar and median nerves of the hand. 3. The lateral popliteal nerve (n. peroneus [fibularis] communis) (LIV, LV, SI, SII), runs laterally of the n. tibialis to the head of the fibula where it pierces the beginning of the peroneus longus muscle and divides into the superficial and deep branches. On its way the lateral popliteal nerve gives rise to the lateral cutaneous nerve of calf of the leg (n. cutaneus surae lateralis) which innervates the skin of the lateral surface of the leg. Below the middle of the leg the lateral cutaneous

225 nerve joins the medial cutaneous nerve to form the sural nerve (n. suralis) which curves around the lateral malleolus from the back giving rise to branches to the skin of the heel (rami calcanei laterales), and then continues under the name of the dorsal lateral cutaneous nerve of the foot (n. cutaneus [pedis] dorsalis lateralis) along the lateral edge of the dorsal surface of the foot, also supplying the skin of this region on the lateral side of the small toe. The superficial branch of the musculocutaneous nerve of the lower limb (n. peroneus [fibularis] superficialis), descends between the mm. peronei into the canalis musculoperoneus superior, giving them muscle branches. On the border between the middle and lower third of the leg it pierces the fascia functioning only as a cutaneous nerve and descends to the middle of the dorsal surface of the foot to divide into two branches. One of them the medial branch of the musculocutaneous nerve of the lower limb (n. cutaneus [pedis] dorsalis medialis) supplies the medial surface of the big toe and the edges of the second and third toes facing each other (nervi digitales dorsales). The other, the lateral branch of the musculocutaneous nerve of the lower limb (n. cutaneus [pedis] dorsalis intermedius) divides into the nn. digit ales dorsales pedis supplying the approximate surfaces of the second to fifth toes. The deep branch of the anterior tibial nerve (n. peroneus [fibularis] profundus), passes in attendance to the a. tibialis anterior, giving branches to the m. tibialis anterior, m.

226 extensor digitorum longus and m. extensor hallucis longus and the ramus articularis to the talocrural joint. The anterior tibial nerve together with the artery attending it emerges onto the dorsal surface of the foot, innervates the short extensor of the toes and then, dividing into two digital branches (nn. digitales dorsales) supplies the skin of the surfaces of the big and second toes facing each other. Part of the sacral plexus which belongs to the animal ne:vous system is made up of preganglionary parasymphathetic fibres which begin in the lateral horns of the second to fourth segments of the spinal cord. These fibres as pelvic splanchnic nerves (nervi splanchnici pelvini) pass to the nerve plexuses of the pelvis that innervate the internal organs of the pelvis: the urinary bladder, the sigmoid colon, the rectum and the internal genital organs.

The Coccygeal Plexus

The coccygeal plexus (plexus coccygeus) is composed of the anterior branches of the fifth sacral and the coccygeal nerves. It gives rise to the thin anococcygeal nerves (nn. anococcygei) which join with the posterior branch of the coccygeal nerve, and branches out in the skin at the top of the coccyx.

227 THE SCIENCE OF THE SENSORY ORGANS (AESTHESIOLOGY)

THE ORGAN OF GRAVITATION AND BALANCE AND THE ORGAN OF HEARING

The organ consists of two analyzers: the analyzers of gravitation (i.e. the sense of gravitational attraction) and balance, and the analyzer of hearing (auditory analyzer). Until recently both analyzers were regarded as one organ of hearing and balance (organum vestibulocochleare).

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.

228 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).

229 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

230 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

231 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

232 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

233 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

234 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

235 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

236 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

237 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

238 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.

239 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

240 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

241 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

242 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

243 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

244 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.

245 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

246 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 GRAVITATION AND BALANCE

The analyzer of gravitation, or the statokinetic analyzer begins in the membranous labyrinth, where its peripheral part is located. The parts of the membranous labyrinth (discussed in describing the auditory analyzer) are related to the statokinetic analyzer, or the analyzer of gravitation. The structure of the statokinetic analyzer (the analyzer of gravitation). The sensory hair cells which the fibres of the vestibular part of the auditory nerve approach from the exterior are located in a layer of squamous epithelium lining the inner surface of the saccule, utricle and the ampullae of the semicircular canals. In the utricle and saccule these sites appear

247 as whitish spots (maculae of the utricle and saccule) (s. maculae staticae), because the sensitive epithelium in them is covered by a jelly-like substance. In the ampullae of the semicircular canals they appear in the form of cristae (cristae ampullares, s. cristae staticae). The epithelium covering the projections of the cristae has sensory cells with pili, which are joined by nerve fibres. The semicircular canals as well as the saccule and utricle may also be stimulated by either acceleration or deceleration of rotary or right-angle movements, by shaking, swinging or any kind of change in the position of the head, as well as by the force of gravity. The stimulus in such instances is tension of sensory hairs or the pressure exerted on them by the jelly-like substance, which stimulates the nerve endings. Thus, the vestibular apparatus and the entire system of conductors connected with it and reaching the cerebral cortex, is the analyzer of the position and movements of the head in space. As a consequence of this, it was named the statokinetic analyzer. The receptor of this analyzer in the form of special hair cells which are stimulated by the flow of endolymph is located in the utricle and saccule (maculae), which regulate static equilibrium, i.e. the balance of the head and, thereby, the body when it is at rest, and in the ampullae of the semicircular canals (cristae), regulating dynamic equilibrium, i.e. the balance of the body moving in space. Although changes in the position and movements of the head are also regulated by other

248 analyzers (particularly by the visual, motor and skin analyzers), the vestibular analyzer plays a very special role. The first neuron of the reflex arc of the statokinetic analyzer lies in the vestibular ganglion. The peripheral processes of the cells of this ganglion advance as part of the vestibular division of the auditory nerve to the labyrinth and communicate with the receptor. Meanwhile, the central processes pass together with the cochlear division of the auditory nerve through the porus acusticus internus into the cranial cavity and further into the brain matter through the cerebellopontile angle. Here the fibres of the first neuron divide into ascending and descending fibres and approach the vestibular nuclei (second neuron), which are situated in the medulla oblongata and pons on the floor of the rhomboid fossa. On each side there are four vestibular nuclei: superior, lateral, medial and inferior. The ascending fibres end in the superior, nucleus, the descending fibres in the remaining three nuclei. The descending fibres and their accompanying nucleus descend very low, through the whole medulla oblongata to the level of the gracile and cuneate nuclei. The vestibular nuclei give rise to fibres running in three directions: (1) to the cerebellum; (2) to the spinal cord, and (3) the fibres which are part of the medial longltudmal fasclculus. The fibres to the cerebellum pass through its inferior peduncle; this path is called the vestibulocerebellar tract. (Some of the fibres of the vestibular nerve without interruption in the vestibular nuclei pass directly into the cerebellum; the

249 vestibular nerve is connected with the flocculonodular lobe, the oldest part of the cerebellum.) There are also fibres running in the opposite direction, from the cerebellum to the vestibular nuclei; as a result, a close connection is established between them, while the nucleus fastigii of the cerebellum becomes an important vestibular centre. The nuclei of the vestibular nerve are connected with the spinal cord through the vestibulospinal tract. It passes in the anterior funiculi of the spinal cord and approaches the cells of the anterior horns along the entire length of the spinal cord. It is the connections with the spinal cord that are responsible for the conduction of the vestibular reflexes to the muscles of the neck, trunk and limbs, and for the regulation of the muscle tonus. The fibres from the vestibular nuclei, comprising part of the medial longitudinal fasciculus, establish contact with the nuclei of the nerves of the eye muscles. As a result vestibular reflexes are accomplished by the eye muscles (compensating for accommodation of the eyes, i.e. keeping them directed at a certain object when the head is moved). This also explains the peculiar movements of the eyeballs (nystagmus) in loss of balance. The vestibular nuclei are connected through the reticular formation with the nuclei of the vagus and glossopharyngeal nerves. This is why dizziness in stimulation of the vestibular apparatus is often attended by a vegetative reaction in the form

250 of a slower pulse beat, a drop in arterial pressure, nausea, vomiting, cold hand and feet, a pale face, cold sweat, etc. Vestibular tracts playa major role in regulating balance and help keep the head in its natural position even when the eyes are closed. A decussated tract is directed from the vestibular nuclei to the thalamus (third neuron) and further to the cerebral cortex for conscious awareness of the head’s position. It is presumed that the cortical end of the statokinetic analyzer is distributed in the cortex of the occipital and temporal lobes. (With the gradual phylogenetic development of animals the function of balance becomes less dependent on the vestibular apparatus.) Adequate training of the vestibular apparatus allows airmen and spacefliers to become adapted to sudden movements and changes in the position of the body during flights.

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.

251 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

252 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

253 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

254 (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

255 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

256 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.

257 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

258 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

259 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

260 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.

261 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 ACCESSORY STRUCTURES OF THE EYE

THE MUSCLES

The motor apparatus of the eyeball consists of six voluntary (striated) muscles: the superior, inferior, medial and lateral. rectus muscles (mm. recti superior, inferior, medialis and lateralis), and the superior and inferior oblique muscles (mm. obliquus superior and inferior). All these muscles, with the exception of the inferior oblique muscle, arise in the depths of the orbit around the optic canal and the adjoining part of the superior orbital fissure from the common tendinous ring (anulus tendineus communis) located here; this ring, shaped as a funnel, invests the optic nerve with the ophthalmic artery, as well as the oculomotor, nasociliary and abducent nerves. The rectus muscles are attached by their anterior ends in front of the equator of the eyeball at four sides of the latter fusing with the sclera by means of tendons. The superior

262 oblique muscle passes through the fibrocartilaginous ring (trochlea) fastened to the trochlear fossa (or spina trochlearis, if it exists) of the frontal bone and then it turns backward and laterally at an acute angle and is attached to the superolateral surface of the eyeball behind the equator. The interior oblique muscle arises from the lateral circumference of the fossa of the lacrimal sac and passes under the eyeball laterally and to the back below the anterior end of the inferior rectus muscle; its tendon is attached to the sclera at the side of the eyeball behind the equator. The rectus muscles rotate the eyeball about two axes: the transverse axis (the superior and inferior rectus muscles) with the pupil directed upward or downward, and the vertical axis (the lateral and medial rectus muscles) with the pupil turning laterally or medially. The oblique muscles rotate the eyeball about the sagittal axis. In rotating the eyeball the superior oblique muscle directs the pupil downward and laterally; contraction of the inferior oblique muscle directs the pupil laterally and upward. It should be noted that all the movements of both eyeballs are coordinated and any movement to any side of one eyeball is simultaneously paralleled by the other eye. When all the muscles are uniformly tensed the pupil looks straight ahead and the lines of vision of both eyes are parallel to each other. This occurs when a person looks far ahead. When objects are examined at a close distance the lines of vision converge in front (convergence of the eyes).

263 THE TISSUE OF THE ORBIT AND TENON’S CAPSULE

The orbit is lined with the periorbit (periorbita) which fuses with the dura mater at the optic canal and the superior orbital fissure. Behind the eyeball is fatty tissue called the fatty body of the orbit (corpus adiposum orbitae) occupying all the space between the organs located in the orbit. The fatty tissue investing the eyeball is separated from it by a connective tissue layer which surrounds the eyeball to form the fascial sheath of the eyeball (vagina bulbi), or Tenon’s capsule. The tendons of the eyeball muscles running to the sites of their attachments in the sclera pass through Tenon’s capsule which provides sheaths for them; these sheaths are continuous with the fasciae of the different muscles.

THE EYELIDS AND CONJUNCTIVA

The eyelids (palpebrae) are a kind of sliding shutters protecting the eyeball anteriorly. The upper eyelid (palpebra superior) is larger than the lower one. It is bounded above by the eyebrow (supercilium), a strip of skin with short hairs lying on the boundary with the forehead. When the eye opens the lower lid descends very slightly under its own weight while the upper lid rises actively due to the contraction of the levator palpebrae superioris muscle connected with it. The free margin

264 of both eyelids is a narrow surface. bounded by the anterior and posterior borders (limbus palpebralis anterior and posterior). Immediately behind the anterior border several rows of short hard hairs grow from the free margin of the eyelid; these are eyelashes (cilia) which serve as a lattice preventing small particles from hurting the eye. Between the free margins of the eyelids is the palpebral fissure (rima palpebrarum) through which one can see the anterior surface of the eyeball when the eyelids are parted. In general, the fissure is almond-shaped; its lateral angle is sharp, the medial angle is rounded and forms what is known as the “lacrimal lake” (lacus lacrimalis). Within the lacus lacrimal is a small pinkish eminence, the lacrimal caruncle (lacrimalis caruncula), is visible, which consists of fatty tissue and tiny sebaceous glands with tender pili. The foundation of each eyelid consists of a dense connective-tissue plate termed the tarsus. In the medial angle of the palpebral fissure is a thickening; this is the medial palpebral ligament (lig. palpebrale mediale) which passes horizontally from both tarsi to the anterior and posterior lacrimal crests in front of and to the back of the lacrimal sac. Another thickening is located at the lateral angle of the palpebral fissure in the form of a horizontal strip; this is the lateral palpebral ligament (lig. palpebrale laterale) which corresponds to the lateral palpebral raphe stretching between the tarsi and the lateral wall of the orbit. Within the tarsi are the Meibomian, or tarsal glands

265 (glandulae tarsales) consisting of longitudinal tubular passages with alveoles on them which secrete a fatty matter (sebum palpebrale) for lubricating the margins of the eyelids. There are usually from 30 to 40 of these glands in the superior tarsus, and from 20 to 30 in the inferior one. The Meibomian glands have punctate openings on the free margin of the eyelid close to the posterior border. Besides the Meibomian there are ordinary sebaceous glands attending the eyelashes. As it is mentioned above, the upper eyelid has its own special muscle which raises it, the levator palpebrae superioris muscle. Posteriorly the tarsi are covered by the conjunctiva which is continuous with the skin at their margins. The conjunctiva of the eye (tunica conjunctiva) covers the whole posterior surface of the eyelids and near the edge of the orbit is reflected onto the eyeball and covers its anterior surface. That part covering the eyelids is called the palpebral part of the conjunctiva (tunica conjunctiva palpebrarum), while the part investing the eyeball is termed the scular part (tunica conjunctiva bulbi). In this way the conjunctiva forms a sac opened in front in the area of the palpebral fissure. The conjuctiva resembles a mucous membrane, although in origin it is a continuation of the skin. On the eyelids it is intimately fused with the tarsi, while for the rest of the length it is joined loosely with the underlying parts until it reaches the edge of the cornea where its epithelial layer is continuous with the corneal epithelium. The sites of junctions between the palpebral and ocular parts of the conjunctiva are called the superior and

266 inferior fornices of the conjunctiva (fornix conjunctivae superior and interior). The superior fornix is deeper than the inferior one. The fornices are the reserve folds of the conjunctiva necessary to provide room for the movement of the eye and eyelids. The same role is played by the plica semilunaris conjunctivae located in the medial angle of the palpebral fissure lateral to the lacrimal caruncle. Morphologically, it is a rudiment of the third eyelid (the “winking membrane”).

THE LACRIMAL APPARATUS

The lacrimal apparatus consists of the lacrimal gland, which excretes tears into the conjunctival sac, and of the lacrimal canaliculi arising from this sac. The lacrimal gland (glandula lacrimalis) consists of lobules and is alveolar-tubular in type; it lies in the lacrimal fossa of the frontal bone. From 5 to 12 ducts (ductuli excretorii) of the lacrimal gland open in the lateral part of the superior fornix into the conjunctival sac; the ducts of the inferior part of the gland pass through its upper part. The lacrimal fluid moistens the surface of the eyeball which is conducive to the winking of the eyelid and then drains into the medial angle of the palpebral fissure to the lacus lacrimalis. When the eyes are shut the fluid runs along the rivus lacrimal is forming between the posterior borders of both eyelids and the eyeball. At the lacus lacrimalis the tears flow into the punctate openings located at the medial end of the

267 eyelids. Two fine lacrimal canaliculi (canaliculi lacrimales) arising from these openings by-pass the lacus lacrimalis and drain separately or by means of a common opening into the lacrimal sac. The lacrimal sac (saccus lacrimalis) is the upper blind end of the nasolacrimal duct which lies in a special bony depression at the medial angle of the orbit. The lacrimal sac from the side of the orbit is covered by a dense fibrous membrane attached to the posterior and anterior lacrimal crests and reinforced by the fibres of the medial palpebral ligament. The bundles of the lacrimal part of the orbicularis oculi muscle arising from the wall of the lacrimal sac may distend it and thus help to suck in tears through the lacrimal canaliculi. The direct continuation of the lacrimal sac downward is the nasolacrimal duct (ductus nasolacrimalis) passing in the bony nasolacrimal canal under the inferior turbinated bone. In conclusion of our description of the eye we shall generalize the data on its structure by recalling the anatomical pathways for the appreciation of light stimuli. Light stimulates the light-sensitive elements located in the retina. Before falling on the retina the rays of light pass through different transparent media of the eyeball: first through the cornea, then through the aqueous humour of the anterior chamber and then through the pupil, which, like the diaphragm of a photocamera, regulates the amount of light rays passing into the depths. In the dark the pupil dilates to let through more rays, and in the light it contracts, on the contrary. This regulation is accomplished by

268 special smooth muscles (the sphincter and dilator of the pupil) innervated by the vegetative nervous system. The rays of light pass further through the light- refracting medium of the eye (lens) which helps the eye to see objects near and far; irrespective of the distance to the object, its image is always imprinted on the retina. This adaptation (accommodation) is ensured by the presence of a special smooth muscle, the ciliary muscle which changes the curvature of the lens and is innervated by parasympathetic nerves. To obtain the same image in both eyes (binocular vision) the lines of vision must converge at one point. This is why, depending on the position of the object, these lines diverge when the objects are far away, and converge when they are near. Such adaptation (convergence) is achieved by the striated muscles of the eyeball (the rectus and oblique muscles) innervated by the third, fourth and sixth pairs of cranial nerves. Regulation of the size of the pupil, as well as accommodation and convergence are closely interconnected, because the activity of the smooth and striated muscles is in agreement due to the coordination of the nuclei of the vegetative and animal nerves and centres located in the mesencephalon and diencephalon, which innervate the above-mentioned muscles. As a result of this coordinated activity, the image of the object falls on the retina and the rays of light that come in contact with it give rise to corresponding stimuli of light-sensitive elements. The nerve elements of the retina form a chain of three neurons. The first link of this chain consists of the light-

269 sensitive cells of the retina (rods and cones) which constitute the receptor of the visual analyzer. The second link is made up of the bipolar cells, and the third of multipolar cells (ganglion n. optici), whose processes continue into the nerve fibres of the optic nerve. As an extension of the brain the nerve is covered by all three meninges forming a sheath for it which fuses intimately with the sclera at the eyeball. Spaces called intervaginal are preserved between the sheaths (spatia intervaginalia) corresponding to the spaces in the brain formed between the meninges. On passing out of the orbit through the optic canal, the optic nerve approaches the inferior surface of the brain, where it partly decussates in the region of the optic chiasma. Only the medial parts of the nerves running from the medial halves of the retina decussate; the lateral parts running from the lateral halves of the retina remain uncrossed. This is why every optic tract (tractus n. optici) arising from the chiasma contains fibres in its lateral part running from the lateral half of the retina of the eye located on its own side, and fibres in the medial part running from the medial half of the contralateral eye. Knowing the character of the chiasma the site of the lesion of the visual tract can be determined by the character of the loss of yision. Thus, for example, when the left optic nerve is affected, the left eye will become blind; in a lesion of the left optic tract or optic centre of each hemisphere, vision is lost in the left halves of the retinae of both eyes, i.e. half-blindness of both eyes (hemianopsia) occurs; in a lesion of the chiasma there is a loss of vision in the medial half of both

270 eyes (in central localization of the lesion) or complete blindness of both eyes (in extensive lesion of the chiasma). Both the crossed and uncrossed fibres of the visual tracts end in two bundles in the subcortical visual centres: (1) in the superior quadrigeminal bodies, and (2) in the thalamus pulvinar and lateral geniculate body. The first bundle ends in the superior quadrigeminal body lodging the visual centres which are connected with mesencephalic nerve nuclei innervating the striated muscles of the eyeball and the smooth muscles of the iris. It is due to this connection that in response to certain light stimuli a corresponding convergence and accommodation (pupillary reflex) of the visual apparatus occurs. The other bundle ends in the pulvinar of the thalamus and in the lateral geniculate body where the bodies of the hew (fourth) neurons are located. The axons of the latter pass through the dorsal part of the posterior genu of the internal capsule and then form the optic radiation (radiato optica) in the white matter of the cerebral hemispheres, which reaches the cortex of the occipital lobe of the brain. The described conducting pathways from, the receptors of light to the cortex, beginning with the bipolar cells (second link of the nerve elements of the retina), constitute the conductor of the visual analyzer. Its cortical end is the cerebral cortex lying along the banks of the calcarine sulcus (area 17). The light stimuli falling on the receptor located in the retina are converted into nerve impulses which pass the length of the conductor to the cortical

271 end of the visual analyzer, where they are perceived as visual sensations.

ORGAN OF TASTE

The importance of the sense of taste (“chemical sense”) consists in recognizing the merits of food. In mammals (and in man) the localization of taste buds is even more limited. They are mainly located on the tongue, although are also encountered on the palate, arches and epiglottis. In man most of the buds are located in the vallate and folliate papillae (papillae vallatae and foliatae), a much less number in the fungiform papillae and finally some. of them occur on the soft palate, on the posterior surface of the epiglottis and on the medial surface of the arytenoid cartilages. The buds contain the taste cells which constitute the receptor of the taste analyzer. Its conductor is comprised of the conducting tracts from the receptors of taste consisting of three links. The first neuron is contained in the ganglia of the afferent nerves of the tongue. The nerves conducting the sense of taste in man are: (1) the chorda tympani of the facial nerve (the first two thirds of the tongue), (2) the glossopharyngeal nerve (the posterior third of the tongue, the soft palate and palatal arches), and (3) the vagus nerve (epiglottis). The location of the first neuron: 1. The ganglion of the facial nerve (ganglion geniculi). The peripheral processes of the cells of this ganglion run as part

272 of the chorda tympani to the anterior two thirds of the tongue mucosa where they come into contact with the taste receptor. The central processes pass as part of the sensory root of the facial nerve (n. intermedius) into the medulla oblongata. 2. The inferior ganglion of the ninth pair. The peripheral fibres of the cells of this ganglion run as part of the glossopharyngeal nerve to the mucous membrane of the posterior third of the tongue, where they come into contact with the receptors. The central processes pass as part of this nerve into the pons. 3. The inferior ganglion of the vagus nerve. As a part of the superior laryngeal nerve the peripheral processes of the cells of this ganglion approach the receptors located in the epiglottis. The central processes, as part of the vagus nerve, pass to the medulla oblongata. All the described taste fibres end in the medulla oblongata and the pons, in the nucleus of the tractus solitarii nn. intermedii, glossopharyngei and vagi, where the second neuron is situated. The gustatory part of the nuclei tractus solitarius is connected with all the motor nuclei of the medulla oblongata which are concerned with chewing and swallowing, and also with the spinal cord (control of respiration, coughing and vomiting). The processes of the second neurons ascend from the medulla oblongata and pons to the thalamus, where the third link begins to extend to the cortical end of the taste analyzer. The taste analyzer lies in the cortex of the parahippocampal

273 gyrus close to the anterior end of the temporal lobe, in the uncus and in the cornu Ammonis near to the olfactory centres. According to other data, it lies in the cortex of the operculum where the sensory fibres of the trigeminal and vagus nerves terminate. The clinical data is more in favour of the second hypothesis. The chemical stimulus in the receptor is transformed into a nerve impulse which is transmitted along the conductor to the cortical end of the analyzer where it is perceived in the form of various taste sensations.

THE ORGAN OF SMELL

The developed nasal cavity, as we see it in the normal adult, lodges the olfactory organ, and serves, at the same time, as the upper respiratory tract. On entering the nasal cavity with the air during respiration, fragrant substances stimulate the specific sensory elements of the organ of smell. These sensory elements, the olfactory cells, constitute the receptor of the olfactory analyzer which is situated in the olfactory region (regio olfactoria) that is, in the nasal mucosa, in superior nasal concha and the contralateral area of the nasal septum. The olfactory cells form the first neurons of the olfactory tract; their axons as a component of the fila olfactoria pass through the openings in the cribriform plate of the ethmoid bone into the olfactory bulb where they terminate in the olfactory glomeruli. This is where the second neurons (mitral cells) originate: their axons pass as part of the olfactory tract

274 and terminate in the cells of the grey matter of this tract, the olfactory pyramid (trigonum olfactorium), the anterior perforated substance, and the septum lucidum. The greater part of the fibres pass to the cortex of the parahippocampal gyrus, to the uncus, where the cortical end of the olfactory analyzer is located. In addition to the new cortex (neocortex) the old cortex (archicortex) also receives the olfactory impulses, i. e. they are brought to the cornu ammonis which was the old pallium that developed under the influence of the olfactory receptor. An efferent tract originates from the cornu ammonis and passes to the subcortical olfactory centres located in the midbrain in: (1) the habenula in the epithalamus, where the olfactory impulses are correlated with other somatosensory centres, and (2) the tuber cinereum and mamillary bodies of the hypothalamus where the olfactory impulses come in contact with the sensory systems of the viscera, including the gustatory system. This efferent tract as a component of the fornix crosses partly in the hippocampal commissure and terminates as the anterior column of the fornix in the mamillary bodies. Bundles run from the latter to the thalamus and the mamillothalamic tract (tract of Vicq d’Azyr), and from there to the cortex.

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