This cavity incloses the brain, with its membranes and vessels. It is
relatively small and is ovoid in shape.
The =superior wall= or =roof= (Calvaria) is formed by the
supraoccipital, interparietal, parietal, and frontal bones. In the
middle line is the internal sagittal crest, which joins the crista galli
in front, and furnishes attachment to the falx cerebri. Posteriorly the
crest is continued by the sharp anterior margin of the =tentorium
osseum=, which projects downward and forward into the cavity, and gives
attachment to the tentorium cerebelli by its sharp lateral edges. Behind
this the roof is grooved centrally for the middle lobe or vermis of the
cerebellum. Transverse grooves pass from the base of the tentorium
osseum to the parieto-temporal canals. The anterior part of the roof is
hollowed by the frontal sinus. The occipital part is very thick and
strong.
The =lateral walls= are formed by the occipital, parietal, temporal, and
frontal bones, and in part by the orbital wings of the sphenoid. Each is
crossed obliquely by the =petrosal crest=, which concurs with the
projecting margin of the parietal bone and the tentorium osseum in
dividing the cavity into cerebral and cerebellar compartments. Behind
the crest is a depression for the lateral lobe (hemisphere) of the
cerebellum. Below this are the internal auditory meatus and the openings
of the aquæductus vestibuli and aquæductus cochleæ.
[Illustration: