Squama occipitalis

The squama of the occipital bone, situated above and behind the foramen magnum, is curved from above downward and from side to side.

External surface
The external surface is convex and presents midway between the summit of the bone and the foramen magnum a prominence, the external occipital protuberance (or inion).

Extending lateralward from this on either side are two curved lines, one a little above the other. The upper, often faintly marked, is named the highest nuchal line, and to it the galea aponeurotica is attached.

The lower is termed the superior nuchal line. That part of the squama which lies above the highest nuchal lines is named the planum occipitale and is covered by the Occipitalis muscle. That below, termed the planum nuchale, is rough and irregular for the attachment of several muscles.

From the external occipital protuberance, an often faintly marked ridge or crest, the median nuchal line, descends to the foramen magnum and affords attachment to the ligamentum nuchæ. Running from the middle of this line across either half of the nuchal plane is the inferior nuchal line.

Several muscles are attached to the outer surface of the squama, thus the superior nuchal line gives origin to the Occipitalis and Trapezius, and insertion to the Sternocleidomastoideus and Splenius capitis. Into the surface between the superior and inferior nuchal lines the Semispinalis capitis and the Obliquus capitis superior are inserted, while the inferior nuchal line and the area below it receive the insertions of the Recti capitis posteriores major and minor.

The posterior atlantoöccipital membrane is attached around the postero-lateral part of the foramen magnum, just outside the margin of the foramen.

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

The upper two fossæ 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 cruciate eminence is the internal occipital protuberance.

From this protuberance the upper division of the cruciate 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 cruciate 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 (torcular Herophili), and its position is indicated by a depression situated on one or other side of the protuberance.