In front of the foramen magnum is the under surface of the basilar process, which, by reason of the superior thickness of its an terior extremity, is not so oblique as it appears on its upper surface. There is a slight tuber cle on the middle line to which is fixed the middle constrictor of the pharynx, and behind it, on both sides, a transverse line for the su perior constrictor, between which and the foramen are depressions caused by the recti capitis antici majores et minores.
The superior angle of this bone is applied on the junction of the two parietal, and the serrated borders which extend from it to the lateral angles are articulated to the posterior borders of the same bones. The upper angle itself and more than half of the borders pro ceeding from it, overlap the parietals, but in the remainder of their extent the latter bones overlap the occipital; in each case the arrange ment being the same as that which exists be .tween the parietal and the frontal bones.
From the lateral angles to the jugular pro cesses, a rough but not denticulated border articulates it to the posterior border of the mastoid portion of the temporal bone. Im mediately in front of the jugular process is the fossa jugularis, which forms, in common with the temporal bone, the foramen jugu/are or foramen lacer= posticum in basi cranii, through which emerge the jugular vein, the pneumo-gastric, glosso-pharyngeal, and spinal accessory nerves. The rest of the border from the fossa jugularis to the anterior angle is in apposition with the petrous portion of the temporal bone, but the quantity of cartilage between them is too large to admit of there being any fixed articulation at this part.
The anterior angle itself is truncated and presents a rough quadrilateral surface, which articulates, and, indeed, consolidates itself, at an early period of life, with the basilar process and body of the sphenoid bone. This union is so complete and so similar to the union which takes place beween the several elements of the bones of the cranium, that Sammering and Meckel have described the two as one bone, under the name of os basilare or os spheno occipitale.
The connexions of this bone are few and simple, being, in its superior half, with the parietals ; in its inferior half, with the tem porals ; at its anterior extremity, with the sphenoid ; and, by its condyles, with the atlas.
At birth this bone is separable into four dis tinct portions, one being in front, one behind, and one on each side of the foramen magnum, the border of which is, consequently, not then completed. The anterior and two lateral por tions are formed by the extension of ossific matter from one point of deposit in each; but that posterior to the foramen is produced from many points, in the number of which ana tomists are not agreed. The ossification com mences in the lower part, at some distance from the foramen, by one point on each side of the median line ; and before they have completely approached each other, two ana logous deposits appear in the upper part, which coalesce before the upper and lower pieces are joined. This occurs during the fourth month, at which time the inferior and broad part dis plays on each side another point of ossification on a level with the spot where the process first commenced; in the fifth month the whole of these are consolidated into one piece. It often happens, however, that other deposits are formed, especially in the upper part ; and frequently they refuse to merge into the others, continuing then to be distinct through life as separate small bones having their own serrated margins to articulate with the adjoining. struc tures.
The lateral pieces (those which comprehend the condyles, and lateral and jugular pro-. cesses) commence their formation about the fourth month ; and the anterior piece is the last in the order of development.
The tenzporal bone (os temporum ; Germ. das Schlufenbein.) One is situated on each side of the sphenoid and lower half of the occipital bone ; they complete the base of the cranium and form the inferior part of the sides of the vault.
For the purposes of description it is usually divided into three portions; one, strong and compact, in the base and between the middle and posterior fossm, the petrous ; a second, tumid and less dense, behind the ear, the mastoid ; and a third rising from the former two, thin and scaly, situated in the temple, the squamous.