Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/873

Rh SKULL.] ANATOMY 823 skull; in front of the basal part of the sphenoid is the ethmoid; mounting upwards in front of the ethmoid is the FIG. (5. Profile of the skull. Fr, frontal bone; Pa, parietal; SO, supra-occipital; Sq, squamous-temporal ; MT, mastoid-temporal ; Ty, tympanic; St, styloid- temporal ; As, ali-sphenoid ; E, os planum of ethmoid ; L, lachrymal ; N, nasal; MX, superior maxilla; Ma, malar; Mn, mandible; iih, basi-hyal; th, thyro-hyal; ch, cerato-hyal ; em, external meatus;, coronal suture; Is, lambdoidal suture; ts, squamoua suture. frontal, which forms the forehead, and closes in the front of the cranial box; forming the vault and side walls are the two parietal bones ; completing the side walls, and extending for a short distance along the side of the floor, are the two temporal bones; the vertex of the skull is at the junction of the two parietal bones with each other. R- FIG. 7. Section through the skull immediately to the right of the mesial plane. The lettering as in Fig. 6, with, in addition, 110, basi-occipital; EO, ex-occipital; FT, petrous-temporal ; BS, basi-sphcnoid; PS, prc-sphenoid (the letters are placed in the sphenoidal sinus); OS, orbito-sphenoid; ME, mes-ethmoid; SC, septal cartilage of nose; V, vomer; PI, palate; Pt, pterygoid of sphenoid; fs, frontal sinus r Pf, pituitary fossa; fin, foramen magnum; a, angle; and s, Kymphysis of lower jaw. The fourteen bones of the face, which are situated below and in front of the cranium, enter into the formation of the walls of cavities which open on the front of the face ; thus they complete, along with the frontal, sphenoid, and ethmoid, the walls of the two orbits in which the eye-balls are lodged ; along with the ethmoid and sphenoid, the walls of the nostrils ; and they form the osseous walls of the mouth. As a general rule, the cranial bones are expanded, and plate- like in form. The outer surface of each bone assists in forming the exterior of the cranium, and not unfrequently is marked by ridges or processes for the attachment of muscles. The inner surface, again, is smooth, and pitted with depressions, in which the convolutions of the brain are lodged, and also marked by grooves for the lodgment of dilated veins called blood sinuses, and of arteries termed meningeaL The two surfaces of a cranial bone, dense in structure, are called its tables, outer and inner, and are separated from each other by bone, looser and more spongy in its texture, called diploe. In some localities, more especially in certain of the bones which form the walla of the nostrils, the diploe disappears, and comparatively wide interspaces separate the two tables which contain air, and are called air-sinuses. The margins of the bones are denticulated, and it is by the interlocking of the denticula- tions of adjacent bones that they are jointed together, the joints being named sutures. The bones are pierced by holes or foramina, and similar holes exist between the adjacent margins of some of the bones. These foramina are mostly situated in the floor of the skull, and transmit arteries into the cranial cavity to supply the brain and the inner table with blood, and veins and nerves out of the cavity. The largest of these holes is called foramen magnum. It lies in the occipital bone, immediately above the ring of the atlas; through it the spinal marrow becomes continuous with the brain, and the vertebral arteries pass to supply the brain with blood. The Occipital, or bone of the Back of the Head (Figs. 6 Occipital. and 7, and Plate XIII.), consists of four originally distinct pieces fused into a curved plate-like bone. Its subdivisions are arranged around the foramen magnum the basilar part, basi-occipital, in front ; the condyloid parts, ex-occipi- tals, one on each side ; and the tabular part, or supra-occipital, behind. The anterior surface of the supra-occipital is sub divided into four fossse, in the two upper of which are lodged the occipital lobes of the cerebrum, in the two lower the cerebellum; the upper and lower pairs of fossas are separated by a groove for the lodgment of the lateral venous sinus. The posterior surface is marked by a pro tuberance and by curved lines for the attachment of muscles; by its margin the supra-occipital articulates with the parietal and temporal bones. Each ex-occipital has on its under surface a smooth condyle for articulation with the atlas ; in front of the condyle is a foramen which transmits the last or ninth cranial nerve, called hypoglossal, and behind it a foramen for the transmission of a vein sometimes exists. The basi-occipital articulates and, in the adult skull, is fused with the body of the sphenoid (Fig. 7). The upper surface of the basi-occipital is grooved for the lodgment of the medulla oblongata. Sometimes the part of the supra-occipital situated above the protuberance and upper curved line ossifies as an inde pendent bone, called interparietal. In some mammals, as the sheep, the existence of an interparietal in the young skull is the rule and not the exception. The Sphenoid or Wedge-shaped bone (Fig. 7, and Plate Sphenoid. XII.), lies at the base of the skull; it articulates behind with the occipital; in front it is jointed to the ethmoid and frontal, and by its lateral processes or wings to the frontal, parietal, and temporal bones. From its position, therefore, it binds together all the bones of the cranium, and, more over, articulates with many of those of the face. For con structive purposes it is the most important bone of the head. It consists of a centrum or body, with which four pairs of processes are connected. The body has a deep depression on its upper surface, compared in shape to a Turkish saddle, in which is lodged the pituitary body; hence it is called 2iituitary fossa. In front of this fossa is a ridge which marks the place of union of the pre- and post-sphenoidal subdivisions of the body of this bone; the body is grooved laterally for the internal carotid artery and the cavernous blood sinuses, and it is hollowed out in its interior to form the sphenoidal air-sinuses: these air-