The Internal or Cerebral Surface

The posterior part or tabular is divided by a crucial ridge into four fossae. The two superior fossae receive the occipital lobes of the cerebrum, and present slight eminences and depressions corresponding to their convolutions. The two inferior, which receive the hemispheres of the cerebellum, are larger than the former, and comparatively smooth; both are marked by slight grooves for the lodgment of arteries.

At the point of meeting of the four divisions of the crucial ridge is an eminence, the internal aecipital protuberance. It nearly corresponds to that on the enter surface, and is perforated by one or more large vascular formina. From this eminence the emperior division of the crucial ridge runs upward to the superior angle of the bone; it presents a deep groove for the superior longitudinal sinus, the margins of which give attachment to the falx cerebri. The interior division, the internal eccipital creat, runs to the posterior margin of the foramen magnum, on the edge of which is becomes gradually lost; this ridge, which is bifurcated below, served for the attachment of the falx cerebelli. It is usually marked by a single groove, which commences at the back part of the foramen magnum and lodges the occipital sinus. Occasionally the groove is double where two sinuses exist. The transverse grooves pass outward to the lateral angles; they are deeply channelled, for the lodgment of the lateral sinuses, their prominent margins affording attachment to the tentorium cerebelli.

At the point of meeting of these grooves is a depression, the torcular Herophili, placed a little to one or the other side of the internal occipital protuberance. More anteriorly is the foramen magnum, and on each side of it, but nearer its anterior than its proterior than its posterior part, the internal openings of the anterior conyloid foramen; the internal openings of the posterior condyloid formina are a little external and posterior to them, rotected by a small arch of bone.

At this part of the internal surface there is a very deep groove in which the posterior condyloid foramen, when it exists, has its termination. This groove is continuous, in the complete skull, with the transverse groove on the posterior part of the bone, and lodges the end of the same sinus, the lateral. In front of the foramen magnum is the basilar process, presenting a shallow depression, the bassilar groove, which slopes from behind, upward and forward, and supports the medulla oblongata and part of the pons Varolii, and on each side of the basilar process is a narrow channel, which, when united with a similar channel on the petrous portion of the temporal bone, forum a groove which lodges the inferior petrosal sinus.