The most visible feature of the squamous part of the occipital bone when examining the inferior view of the skull is a ridge of bone (the external occipital crest), which extends downward from the external occipital protuberance toward the foramen magnum. The inferior nuchal lines arc laterally from the midpoint of the crest.
Immediately lateral to the foramen magnum are the lateral parts of the occipital bones, which contain numerous important structural features.
On each anterolateral border of the foramen magnum are the rounded occipital condyles (Fig. 8.23). These paired structures articulate with the atlas (vertebra CI). Posterior to each condyle is a depression (the condylar fossa) containing a condylar canal, and anterior and superior to each condyle is the large hypoglossal canal. Lateral to each hypoglossal canal is a large, irregular jugular foramen formed by opposition of the jugular notch of the occipital bone and jugular notch of the temporal bone.
Laterally in the posterior part of the base of the skull is the temporal bone. The parts of the temporal bone seen in this location are the mastoid part of the petromastoid part and the styloid process (Fig. 8.23).
The lateral edge of the mastoid part is identified by the large cone-shaped mastoid process projecting from its inferior surface. This prominent bony structure is the point of attachment for several muscles. On the medial aspect of the mastoid process is the deep mastoid notch, which is also an attachment point for a muscle.
Anteromedial to the mastoid process is the needle-shaped styloid process projecting from the lower border of the temporal bone. The styloid process is also a point of attachment for numerous muscles and ligaments.
Finally, between the styloid process and the mastoid process is the stylomastoid foramen.
The cranial cavity is the space within the cranium that contains the brain, meninges, proximal parts of the cranial nerves, blood vessels, and cranial venous sinuses.
The calvaria is the dome-shaped roof that protects the superior aspect of the brain. It consists mainly of the frontal bone anteriorly, the paired parietal bones in the middle, and the occipital bone posteriorly (Fig. 8.24).
Sutures visible internally include: the coronal suture, between the frontal and parietal bones, the sagittal suture, between the paired parietal bones, and the lambdoid suture, between the parietal and occipital bones.
Visible junctions of these sutures are the bregma, where the coronal and sagittal sutures meet, and the lambda, where the lambdoid and sagittal sutures meet.
Other markings on the internal surface of the calva include bony ridges and numerous grooves and pits.
From anterior to posterior, features seen on the bony roof of the cranial cavity are: a midline ridge of bone extending from the surface of the frontal bone (the frontal crest), which is a point of attachment for the falx cerebri (a specialization of the dura mater that partially separates the two cerebral hemispheres); at the superior point of the termination of the frontal crest the beginning of the groove for the superior sagittal sinus, which widens and deepens posteriorly and marks the position of the superior sagittal sinus (an intradural venous structure); on either side of the groove for the superior sagittal sinus throughout its course, a small number of depressions and pits (the granular foveolae), which mark the location of arachnoid granulations (prominent structures readily identifiable when a brain with its meningeal coverings is examined; the arachnoid granulations are involved in the reabsorption of cerebrospinal fluid); and on the lateral aspects of the roof of the cranial cavity, smaller grooves created by various meningeal vessels.
The floor of the cranial cavity is divided into anterior, middle, and posterior cranial fossae.