Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/874

Rh 824 ANATOMY [SKELETON sinuses are partially closed in front by a pair of small bony plates called sphenoidal spongy bones, or bones of Berlin. Behind the pituitary fossa is a pair of processes called posterior clinoid, from which the bone slopes back to the basi-occipital; this slope is called the dors-am sellce, and on it rests the pons Varolii. From the posterior part of each side of the body the great wings, or all-sphenoids, pass outwards and upwards to the sides of the skull, and each sends off a plate-like process to enter into the formation of the outer wall of the orbit. From the anterior part of each side of the body the lesser wings, orbito-sphenoids, pass outwards, and assist in forming the roof of each orbit; each orbito-sphenoid ends internally in a knob-like process called anterior clinoid, and at its root is a foramen called optic, which transmits the second nerve, or nerve of sight, into the orbit. From the great wings on each side, close to its junction with the body, a pair of pterygoid processes, called internal and external, project downwards, and the internal process ends in a slender hook termed the hamular process. The ali-sphenoid is pierced by foramina called rotundum, ovale, and spinosum, the two former of which transmit divisions of the fifth cranial nerve, the last an artery to the membranes of the brain; between the orbito- and ali-sphenoids is a fissure which transmits the third, fourth, sixth, and first divisions of the fifth cranial nerve into the orbit; and at the root of the pterygoid processes is the vidian canal, for the transmission of a nerve of the same name. The Ethmoid, or Sieve-like bone (Fig. 7, and Plate XIII, fig. 5), is situated between the two orbital platesof the frontal, and in front of the body of the sphenoid. It is cuboidal in shape, and is composed of a central portion and two lateral masses, which are connected together by a thin horizontal plate pierced with holes like a sieve, and called cribriform. This cribriform plate forms a part of the floor of the cranial cavity ; on it rest the two olfactory bulbs, and the branches of the nerves of smell, called olfactory or first cranial nerves, pass from the bulbs through the holes in this plate into the nose. The central portion of the bone is a mesial perpendicular plate, mes-ethmoid, and forms a part of the septum which subdivides the nose into the right and left nostrils. Each lateral mass consists of an external smooth plate, os planum, which assists in forming the inner wall of the orbit ; and an internal convoluted part, called superior and middle spongy bones or turbinals, which enter into the formation of the outer wall of the nostril. These turbinals are associated with the distribution of the nerves of smell; in the toothed whales, where there are no olfactory nerves, the turbinals are absent, whilst in some mammals, as the crested seal, they assume a highly convoluted form. The lateral masses are hollowed out into air-sinuses, called ethmoidal cells, which communicate with the nostrils and svith corresponding sinuses in the sphenoid and frontal bones. The Frontal, or bone of the Forehead (Figs. 6 and 7, and Plate XIII. ), consists originally of a right and left lateral half, united by the frontal suture in the middle line of the fore head. As a rule, this suture disappears in early life, and a single greatly curved bone is formed. The bone is convex forwards, to form the rounded forehead, and presents two eminences, the centres of ossification of the bone ; at the root of the nose is an elevation called glabella, extending outwards, from which, on each side, is the supra-ciliary ridge, corresponding to the position of the eyebrow. In the crania of some races, e.g., the Australian, the forward projection of the glabella and supra-ciliary ridges is con siderable ; and in the well-known skull from the valley of the Neander it has reached a remarkable size. These ridges and the glabella mark the position of the air-sinuses iu the frontal bone. The upper border of each orbit, which ends internally and externally in a process of bone called angular, forms the lower boundary of the forehead. The cerebral surface of the bone is deeply concave, for the reception of the frontal lobes of the brain ; the concavity is deepened by the backward projection of two thin plates of bone which form the roofs of the orbits, which plates are separated from each other by the deep notch in which the ethmoid bone is lodged ; along the margins of this notch may be seen the openings into the frontal air-sinuses. The Parietal bones, two in number (Figs. G and 7, and Parietal Plate XIV.), form the greater part of the side wall of the skull, and mount upwards to the vertex, where they unite together along the line of the sagittal suture. Each bone possesses about the centre of its outer surface an eminence, the centre of ossification of the bone, with which a hollow on the cerebral surface, lodging a convolution of the parietal lobe of the brain, corresponds. The bone is quadrilateral in form. Three of its margins are strongly denticulated, for junction with the occipital, frontal, and corresponding parietal ; the fourth is scale-like, for union with the tem poral, and forms the squamous suture ; near the upper margin on the cerebral surface is a groove for the lodgment of the superior longitudinal venous sinus. The anterior inferior angle articulates with the ali-sphenoid, and is marked by a groove for the meningeal artery; the posterior inferior is grooved for the lateral venous sinus, and articu lates with the mastoid of the temporal. The Temporal bones, two in number (Figs. 6 and 7. Tempoi and Plate XIV.), are placed at the side and base of the skull, and are remarkable for containing in their interior the organs of hearing. Each bone consists originally of four subdivisions a squamoso-zygomatic, a tympanic, a petro-mastoid, and a styloid which in course of time fuse together to form an irregular-shaped bone. The squamous part of the squamoso-zygomatic is a thin plate which forms that part of the side of the skull familiarly known as the &quot; temple.&quot; The zygoma extends horizontally forwards as a distinct arched process, to join the malar or cheek-bone. At the root of the zygoma is a smooth fossa, called glenoid, which receives the condyle of the lower jaw, and assists in forming the temporo-maxillary joint. The tympanic portion forms in the foetus a ring, which enlarges subsequently into a curved plate that forms the wall of the external auditory meatus, or passage into the tympanum or middle ear. The tympanic and squamoso-zygomatic parts of the bone fuse together; but a fissure, called Glaserian, situated behind the glenoid fossa, marks their original separation ; in this fissure the slender process of the malleus (one of the bones of the tympanum) is lodged. The petro-mastoid or periotic part of the temporal contains the organ of hearing, and is complicated in its internal anatomy. It extends forwards and inwards along the floor of the skull, and forms on the exterior of the skull the large nipple-shaped mastoid pro cess. This process is rough on its outer surface, for the attachment of muscles, and is hollowed out internally into the mastoid cells or air-sinuses, which communicate with the tympanum or middle ear. The petrous-temporal is distinguished by its stony hardness, and has the form of a three-sided pyramid. Its apex lies in relation to the side of the body of the sphenoid ; its base corresponds to the tympanic cavity and external meatus ; its under surface is rough, and forms a part of the under surface of the skull ; its anterior and posterior surfaces are smooth and in rela tion to certain parts of the brain. The petrous part of the bone is traversed by a canal which transmits the internal carotid artery and sympathetic nerve into the cranial cavity ; in its posterior surface is a passage, internal meatus, down which the seventh cranial nerve proceeds ; at the bottom of the meatus the auditory part of that nerve enters the internal ear, whilst the part of the nerve