| Squamous part of occipital bone | |
|---|---|
 Human skull seen from above (parietal bones removed). Squamous part is shown in red. | |
 Occipital bone at birth, seen from below. (Squamous part is top half, portion aboveforamen magnum, shown in yellow.) | |
| Details | |
| Identifiers | |
| Latin | squama occipitalis |
| TA98 | A02.1.04.010 |
| TA2 | 565 |
| FMA | 52860 |
| Anatomical terms of bone | |
Thesquamous part of occipital bone is situated above and behind theforamen magnum, and is curved from above downward and from side to side.
The external surface is convex and presents midway between the summit of the bone and theforamen magnum a prominence, theexternal occipital protuberance andinion.
Extending lateralward from this on either side are two curved lines, one a little above the other. The upper, often faintly marked, is named thehighest nuchal line, and to it theepicranial aponeurosis is attached.
The lower is termed thesuperior nuchal line. That area of the squamous part, which lies above the highest nuchal lines is named theoccipital plane(planum occipitale) and is covered by theoccipitalis muscle. That below, termed thenuchal plane, is rough and irregular for the attachment of several muscles.
From the external occipital protuberance, an often faintly marked ridge or crest, themedian nuchal line, descends to the foramen magnum and affords attachment to thenuchal ligament. Running from the middle of this line across either half of the nuchal plane is theinferior nuchal line.
Several muscles are attached to the outer surface of the squamous part, thus the superior nuchal line gives origin to theoccipitalis andtrapezius muscles, and insertion to thesternocleidomastoid andsplenius capitis muscles. Into the surface between the superior and inferior nuchal lines thesemispinalis capitis and theobliquus capitis superior are inserted, while the inferior nuchal line and the area below it receive the insertions of therectus capitis posterior major andminor.
Theposterior atlantooccipital membrane is attached around the postero-lateral part of the foramen magnum, just outside the margin of the foramen.
The internal surface is deeply concave and divided into four fossae by thecruciform eminence.
The upper two fossae are triangular and lodge theoccipital lobes of thecerebrum; the lower two are quadrilateral and accommodate the hemispheres of thecerebellum.
At the point of intersection of the four divisions of the cruciform eminence is theinternal 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, thesagittal sulcus, which lodges the hinder part of thesuperior sagittal sinus. To the margins of this sulcus thefalx cerebri is attached.
The lower division of the cruciform eminence is prominent and is named theinternal occipital crest; it bifurcates near the foramen magnum and gives attachment to thefalx 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 thevermis 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 thetransverse sinuses, and their prominent margins give attachment to thetentorium cerebelli.
The groove on the right side is usually larger than that on the left and is continuous with that for thesuperior 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 theconfluence of the sinuses, and its position is indicated by a depression situated on one or other side of the protuberance.
This article incorporates text in thepublic domain frompage 129 of the 20th edition ofGray's Anatomy(1918)
