In Mammiferous animals the skull is more compact than that of Reptiles and more diffuse than that of Birds. Its elementary portions unite so as to form a determinate number of bones which are either dovetailed together by the interlacement of crooked processes with which their edges are liberally studded, or flow into each other so as to exhibit no trace of their junction. Its structure is made up of two osseous lamellm, called an inner and an outer table, which are united by an areolar ossifie tissue, termed dipla, that adds greatly to the defensive properties of the skull.
The Cranium (in human anatomy)is a hollovv bone of an ovoid figure ; elongated from be hind forwards; narrower before than behind; compressed on the anterior part of its sides; surmounting the face and spine, and projecting considerably beyond the latter. It contains in its parietes the organs of hearing, and contributes to form the orbits, the nostrils, and the face.
The dome-like upper portion is termed the calvaria, and the lower part is the base. The former presents the synciput in front, the occiput behind, the vertex or bregma, (6pExpca, from flpExco, irrigo,) above, and the temples on the sides.
Placed at the summit of the body and des tined to contain the brain, the skull is pierced at its base by numerous foramina for the trans mission, 1st, of-the nerves which establish the communication between the brain and other" organs ; and 2dly, of the vessels which supply the brain and its membranes.
From the inferior surface of the cranium, be tween its anterior and middle thirds, there de scend two columns which limit posteriorly the boundaries of the face; so that it is anteriorly to these columns that it contributes to form the orbits and the nose, and consequently there the bones. which enter into the composition of the face are fixed to it. Hence the surface of that part is very irregular, presenting, in addition to the foramina, depressions and elevations, sulci and processes indicative of the articulation of bones and the lodgement of other organs. Posteriorly, between its middle and posterior thirds, the base of the cranium overtops the spine, and a great opening there establishes the continuity of the vertebral canal with the inte rior of the skull; and the muscles which move the head and maintain its equipoise being at tached around, but especially behind this open ing, the skull is strongly marked in that direc tion. The intermediate space or middle third
is above the pharynx, offering-, centrally, a plain surface' to form the roof of that cavity, and, laterally, rough surfaces and processes for the attachment of muscles concerned in deglutition, also some of the foramina already referred to, for the transmission of the vessels and nerves of the throat to and from the interior of the skull, as well as the surfaces on _which the lower jaw moves.
The upper surface of the base conforming to the base of the brain, there are larger depres sions on it for the anterior and middle lobes ; a deep pit or cavity for the cerebellum, and in the centre a broad sulcus, which glides into that pit, for the medulla oblongata, as well as strong ridges and processes to afford attachment to the membranous partitions which severally exist between the cerebrum and cerebellum, the he mispheres of the former and the lobes of the latter organ.
Thebones into which the cranium is separable or of which it is immediately formed, are eight, viz. the sphenoid, thefrontal, the ethmoid, the occipital, the two temporal, and the two parietal. The first named bone is so placed as to be in connexion with all the others, and to have them grouped around it ; so that the frontal (F, fig. 370) and ethmoid are in its front, the through the medium of the posterior ethmoidal cells into the superior meatus of the nose. On its upper surface is a deep depression (ephip pium,sella turcicatfossa pituitaris ) for the lodge ment of the pituitary gland. The posterior bor der of this depression presents a crest, the corners of which are slightly tumid, (posterior ephip pial, or clinoid processes,) for the attachment of the tentorium, and this crest is prolonged down wards and backwards under the name of the basilar process, to join the process of the same name of the occipital bone; on each side there is a depression (sulcus caroticus) for the reception of the internal carotid artery, and which also marks the situation of the cavernous sinus. On its under surface may be seen, on the median line, the processus azygos (rostrum), which is wedged into the base of the vomer, and on each side of it a line indicating the articulation of the two plates of which the vomer is formed. Still more out wardly there is a groove which is converted into a canal by the application against it of the in ferior orbitary or sphenoidal process of the pala tine bone.