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Gross Anatomy of a Skull

Gross Anatomy of a Skull

GROSS ANATOMY OF A

The head consists of the skull, face, scalp, (eye socket) and the superior margin of the teeth, brain, cranial nerves, meninges, special external acoustic meatus (auditory canal) are sense organs, and other structures such as horizontal. This is called the orbitomeatal plane blood vessels, lymphatics, and fat. It is also the (Frankfurt plane). site where food is ingested and air is inspired THE in adults is formed by and expired (Fig. 101; Fig. 102). Diseases of eight : important structures in the head form the bases 1. A frontal (os frontale), of many medical, dental, and surgical specialties 2. Paired parietal bones (os parietale), - dentistry, maxillofacial surgery, neurology, 3. Paired temporal bones (os temporale), neuroradiology, neurosurgery, neuropsychiatry, 4. An (os occipitale), oral surgery, maxillofacial surgery, ophthalmology, 5. A (os sphenoidale), oral surgery, otology, psychiatry, and rhinology. 6. An (os ethmoidale). The skull is the skeleton of the head; a series of Most of these bones are largely flat, curved, bones form its two parts, the neurocranium and and united by fibrous interlocking sutures. During facial skull. The neuro-cranium ('brain box' or childhood, some bones are united by hyaline cranial vault) provides a case for the brain and cartilage (synchondroses) - between the occipital cranial meninges (membranes covering the and sphenoid bones. A number of irregular brain), proximal parts of the cranial nerves, and bones form the framework of the face and cranial blood vessels. The term cranium (means skull) base. is sometimes restricted to a skull without the 1. THE - specifically its mandible. The cranium has a domelike roof-the squamous (flat) part - forms the skeleton of the - skullcap (Fig. 103) - and a floor or cranial forehead, articulating inferiorly with the nasal base (basicranium) consisting of the ethmoid and zygomatic bones (Fig. 105). In the fetal bone and parts of the occipital and temporal skull, the frontal suture separates the two halves bones (Fig. 104). The facial skeleton consists of of the frontal bone, and they remain separate the bones surrounding the mouth and nose and until approximately 6 years of age. The smooth, contributing to the orbits (eye sockets, orbital broad, convex plate of bone called the frontal cavities). In the anatomical position, the skull is squama. The frontal articulates with twelve oriented so that the inferior marain of the orbit bones: the sphenoid, the ethmoid, the two

89 Fig. 105.Frontal bone (lat. view) Fig. 106. Frontal bone (ant. view) Fig. 107. (internal view) parietals, the two nasals, the two maxillæ, the in most people is related to a distinctly depressed two lacrimals, and the two zygomatics. In fetal area («bridge of the nose»). The frontal bone also , a metopic suture divides the halves of articulates with the lacrimal, ethmoid, and the frontal squama. In most people the halves of sphenoid bones, and a horizontal portion of bone the frontal bone begin to fuse during infancy and (the orbital part [plate] of the frontal bone) forms the suture between them is usually not visible both the roof of the orbit and part of the floor of after 6 years of age. The frontal bone forms the the . The frontal bone consists of thin roof of the orbits (eye sockets). Just superior two portions - a vertical portion, the squama, to and parallel with each supraorbital margin is a corresponding with the region of the forehead; bony ridge, the superciliary arch, which overlies and an orbital or horizontal portion, which the (Haimori). This arch is more enters into the formation of the roofs of the pronounced in males. Between these arches orbital and nasal cavities. there is a gently, rounded, median elevation called SQUAMA (SQUAMA FRONTALIS) contents of the . In most people the skin over the the external surface, which is convex and glabella is hairless. The slight prominences of usually exhibits, in the lower part of the middle the forehead on each side, superior to the line, the remains of the frontal or metopic superciliary arches, are called frontal suture. In infancy this suture divides the bone eminences (tubers). The supraorbital into two, a condition which may persist foramen (occasionally a notch), which transmits throughout life. The border of the squama is the supraorbital vessels and nerve, is located in thick, strongly serrated, bevelled at the expense the medial part of the supraorbital margin. The of the inner table above, where it rests upon frontal bone articulates with the two parietal the parietal bones, and at the expense of the bones at the . It also articulates outer table on either side, where it receives the with the nasal bones at the frontonasal suture. lateral pressure of those bones; this border is At the point where this suture crosses the continued below into a triangular, rough surface, internasal suture in the median plane, there is which articulates with the great wing of the an anthropological landmark called the . sphenoid. On either side of this suture, about 3 This depression is located at the root of the nose, cm. above the supraorbital margin, is a rounded where it joins the cranium (Fig. 106). The frontal elevation, the frontal eminence (tuber bone also articulates with the zygomatic, lacrimal, frontale). These eminences vary in size in ethmoid, and sphenoid bones. Bruising of the different individuals, are occasionally skin over a superciliary arch causes tissue fluid unsymmetrical, and are especially prominent in and blood to accumulate in the surrounding con­ young skulls. The surface of the bone above them nective tissue, which gravitates into the upper is smooth, and covered by the galea eyelid and around the eye. This results in swelling aponeurotica. Below the frontal eminences, and and a 'black eye.' In some adults the separation separated from them by a shallow groove, are line persists as the metopic suture in the midline two arched elevations, the superciliary arches. of the glabella, the smooth, slightly depressed They are prominent medially, and are joined to area between the superciliary arches. The nasion one another by a smooth elevation named the glabella. They are larger in the male than in the

90 female, and their degree of prominence depends internal surface of the squama is concave and to some extent on the size of the frontal air presents in the upper part of the middle line a sinuses; prominent ridges are, however, vertical groove, the sagittal sulcus, the edges occasionally associated with small air sinuses. of which unite below to form a ridge, the frontal The squama and the zygomatic processes are crest. This sulcus lodges the superior sagittal very thick, consisting of diploic tissue contained sinus, while its margins and the crest afford between two compact laminae. The diploic attachment to the falx cerebri. The crest ends tissue is absent in the regions occupied by the below in a small notch which is converted into a frontal air sinuses. Beneath each superciliary foramen, the foramen cecum. This foramen arch is a curved and prominent margin, the varies in size in different subjects, and is supraorbital margin, which forms the upper frequently impervious. When open, it transmits boundary of the base of the orbit, and separates a vein from the nose to the superior sagittal sinus. the squama from the orbital portion of the bone. On either side of the middle line the bone The lateral part of this margin is sharp and presents depressions for the convolutions of the prominent,affording to the eye, in that situation, brain, and numerous small furrows for the considerable protection from injury; the medial anterior branches of the middle meningeal part is rounded. At the junction of its medial and vessels. Several small, irregular fossæ may also intermediate thirds is a notch, sometimes be seen on either side of the sagittal sulcus, for converted into a foramen, the supraorbital the reception of the arachnoid notch or foramen, which transmits the supra­ granulations.ORBITAL OR HORIZONTAL orbital vessels and CN V-1. A small aperture in PART (PARS ORBITALIS). This portion the upper part of the notch transmits a vein from consists of two thin triangular plates, the orbital the diploë to join the supraorbital vein. The plates, which form the vaults of the orbits, and supraorbital margin ends laterally in the are separated from one another by a median gap, , which is strong and the ethmoidal notch. The posterior borders of prominent, and articulates with the zygomatic the orbital plates are thin and serrated, and bone. Running upward and backward from this articulate with the small wings of the sphenoid. process is a well-marked line, the temporal line, The orbital portion is thin, translucent, and which divides into the upper and lower composed entirely of compact bone; hence the temporal lines,continuous, in the articulated facility with which instruments can penetrate the skull, with the corresponding lines on the parietal cranium through this part of the orbit; when the bone. The area below and behind the temporal frontal sinuses are exceptionally large they may line forms the anterior part of the , extend backward for a considerable distance into and gives origin to the temporalis muscle. the orbital portion, which in such cases also Between the supraorbital margins the squama consists of only two tables. The inferior surface projects inferiorly to a level inferior that of the of each orbital plate is smooth and concave, and zygomatic processes. This portion is known as presents, laterally, under cover of the zygomatic the nasal part and presents a rough, uneven process, a shallow depression, the lacrimal interval, the nasal notch, which articulates on fossa, for the lacrimal gland. Near the nasal part either side of the middle line with the nasal bone, is a depression, the fovea trochlearis,or and laterally with the frontal process of the occasionally a small trochlear spine, for the maxilla and with the lacrimal. The term nasion attachment of the cartilaginous pulley of the is applied to the middle of the frontonasal suture. obliquus oculi superior muscle. The superior From the center of the notch the nasal process surface is convex, and marked by depressions projects downward and forward beneath the for the convolutions of the frontal lobes of the nasal bones and frontal processes of the maxillæ, brain, and faint grooves for the meningeal and supports the bridge of the nose. The nasal branches of the ethmoidal vessels. The process ends below in a sharp spine, and on ethmoidal notch separates the two orbital either side of this is a small grooved surface plates. It is quadrilateral, and filled, in the which enters into the formation of the roof of the articulated skull, by the of the corresponding nasal cavity. The spine forms part ethmoid. The margins of the notch present of the septum of the nose, articulating in front several half-cells which, when united with with the crest of the nasal bones and behind with corresponding half-cells on the upper surface of the perpendicular plate of the ethmoid. The 91 the ethmoid, complete the ethmoidal air cells. inferior temporal lines, cross the middle of the Two grooves cross these edges transversely; lateral surfaces of the parietal bones. The they are converted into the anterior and superior temporal line indicates an attachment posterior ethmoidal canals by the ethmoid, and of temporal fascia, the inferior temporal line open on the medial wall of the orbit. The anterior marks the superior limit of temporal muscle. canal transmits the nasociliary nerve and anterior The parietal bones articulate with each other in ethmoidal vessels, the posterior, the posterior the median plane at . The ethmoidal nerve and vessels. In front of the median plane of the body passes through the ethmoidal notch, on either side of the frontal sagittal suture. The inverted V-shaped suture spine, are the openings of the frontal air between the parietal bones and the occipital bone sinuses. These are two irregular cavities, which is called . The parietal bones extend posterosuperiorly, and laterally for a articulate with each other in the median plane at variable distance between the two tables of the sagittal suture. The median plane of the body skull. They are separated from one another by a passes through the sagittal suture. The point thin bony septum, which often deviates to one or where the parietal and occipital bones join is a other side, with the result that the sinuses are useful reference point called the . It can rarely symmetrical. Absent at birth, they are be felt as a depression in some people. In usually fairly well-developed between the seventh addition to articulating with each other and the and eighth years, but only reach their full size frontal and occipital bones, the parietal bones after puberty. They vary in size in different articulate with the temporal bones and the greater persons, and are larger in men than in women. wings of sphenoid bone. The parietal articulates They are lined by mucous membrane, and each with five bones: the opposite parietal, the occipital, communicates with the corresponding nasal frontal, temporal, and sphenoid.The parietal cavity by means of a passage called the bones form, by their union, the sides and roof of . The frontal bone is ossified the cranium. Each bone is irregularly in membrane from two primary centers: one quadrilateral in form, and has two surfaces, four for each half, which appear toward the end of borders, and four angles. The parietal bone is the second month of fetal life, and one superiorly divided into two parts, upper and lower, by an each supraorbital margin. From each of these anteroposterior suture. centers ossification extends superiorly to form THE EXTERNAL SURFACE is convex, smooth, the corresponding half of the squama, and and marked near the center by the parietal backward to form the orbital plate. The spine is eminence (tuber parietale), which indicates the ossified from a pair of secondary centers, on point where ossification commenced. Crossing either side of the middle line; similar centers the middle of the bone in an arched direction are appear in the nasal part and zygomatic two curved lines, the superior and inferior processes. At birth the bone consists of two temporal lines. The former gives attachment pieces, separated by the frontal suture, which is to the temporal fascia, and the latter indicates usually obliterated, except at its lower part, by the upper limit of the muscular origin of the the eighth year, but occasionally persists temporalis muscle. Superiorly these lines the throughout life. It is generally maintained that the bone is covered by the galea aponeurotica. development of the frontal sinuses begins at the Inferiorly them it forms part of the temporal fossa, end of the first or beginning of the second year, and affords attachment to the temporalis muscle. but Onodi's researches indicate that At the back part and close to the upper or sagittal development begins at birth. The sinuses are of border is the , which transmits considerable size by the 7 or 8 year, but do not a vein to the superior sagittal sinus, and attain their full proportions until after puberty. sometimes a small branch of the occipital artery. It is not constantly present, and its size varies. 2. THE PARIETAL BONES THE INTERNAL SURFACE is concave. It The two parietal bones form large parts of the presents depressions corresponding to the walls of the calvaria (Fig. 107). On the outside of cerebral convolutions, and numerous furrows for these smooth convex bones, there are slight the ramifications of the middle meningeal vessel. elevations near the center called parietal They run superoposteriorly from the sphenoidal eminencies. Two curved lines, the superior and angle, and from the central and posterior part of the squamous border. Along the upper margin is 92

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