The internal surface ([Fig. 18]) is concave dorsoventrally, following the outline of the foramen magnum. It is convex from side to side. It is smooth except at its outer margin, which is rough for articulation with the mastoid portion of the temporal bone. The cranial face of the jugular process (f) shows a rough concavity for the reception of the bulla tympani. Mediad of the jugular process is a notch (jugular notch) (g) which when the bones are articulated forms part of the boundary of the jugular foramen. Mediad of this notch is a foramen which forms one end of the hypoglossal canal. It passes dorsocaudad into the cranial cavity and transmits the hypoglossal nerve. Dorsad of the hypoglossal canal is the cranial opening of the condyloid canal, which passes caudad and opens just craniad of the dorsal end of the condyle. It transmits a vein. The outer border of this portion is rough for articulation with the petrous and mastoid portions of the temporal.
The squamous portion (c) has the form of a sector of a circle whose arc is a little more than ninety degrees. The central angle of the sector is truncated and bounds the foramen magnum dorsally. The arc of the sector forms the dorsal margin of the bone, while along the radii it passes into the lateral portions. Its dorsal portion is thick and porous; its ventral portion near the foramen magnum is thin and compact.
The external surface ([Fig. 17]) is marked by a prominent ridge, the lambdoidal ridge (h), parallel with the dorsal border and near to it. The narrow portion of the outer surface that lies dorsad of the ridge forms an angle of about ninety degrees with the remainder of the surface.
A median crest (i) extends ventrad from the middle of the lambdoidal ridge toward the foramen magnum; this is the external occipital crest. At its junction with the lambdoidal ridge it is elevated into a tubercle, the external occipital protuberance (j).
The inner surface ([Fig. 18]) presents depressions for the convolutions of the cerebellum.
The dorsal border is thick and rough for articulation with the parietals and interparietal. The ventral border abuts on the foramen magnum and is thin and smooth.
Interparietal Bone. Os interparietale
([Fig. 19]).—This is a small triangular bone lying between the parietals, with its apex directed craniad, and its base in contact with the squamous portion of the occipital.
Its dorsal surface is arrow-shaped and has its posterior border notched. It is marked by a median crest (part of the sagittal crest) which is continued craniad from the middle of the lambdoidal crest.
The ventral surface is irregularly triangular, smooth, and concave. The three borders are rough for articulation with the parietals and occipital.