Squamous part of occipital bone

Internal surface

The internal surface is deeply concave and divided into four fossae by a cruciate eminence.

The upper two fossae are triangular and lodge the occipital lobes of the cerebrum; the lower two are quadrilateral and accommodate the hemispheres of the cerebellum.

At the point of intersection of the four divisions of the cruciform eminence is the internal occipital protuberance.

From this protuberance the upper division of the cruciform eminence runs to the superior angle of the bone, and on one side of it (generally the right) is a deep groove, the sagittal sulcus, which lodges the hinder part of the superior sagittal sinus. To the margins of this sulcus the falx cerebri is attached.

The lower division of the cruciform eminence is prominent and is named the internal occipital crest; it bifurcates near the foramen magnum and gives attachment to the falx cerebelli. In the attached margin of this falx is the occipital sinus, which is sometimes duplicated.

In the upper part of the internal occipital crest, a small depression is sometimes distinguishable; it is termed the vermian fossa since it is occupied by part of the vermis of the cerebellum. Transverse grooves, one on either side, extend from the internal occipital protuberance to the lateral angles of the bone; those grooves accommodate the transverse sinuses, and their prominent margins give attachment to the tentorium cerebelli.

The groove on the right side is usually larger than that on the left and is continuous with that for the superior sagittal sinus.

Exceptions to this condition are, however, not infrequent: the left may be larger than the right or the two may be almost equal in size.

The angle of union of the superior sagittal and transverse sinuses is named the confluence of the sinuses, and its position is indicated by a depression situated on one or other side of the protuberance.