The Cranial Cavity

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.

The roof and lateral walls are marked by digital impressions and vascular grooves.

The inferior wall or floor (Basis cranii interna) may be regarded as forming three fossæ. The anterior fossa (Fossa cranii anterior) supports the frontal and olfactory parts of the cerebrum. It is formed chiefly by the presphenoid, and lies at a higher level than the middle fossa. In front the fossa is divided medially by the crista galli, lateral to which are the deep ethmoidal or olfactory fossæ for the olfactory lobes. The ethmoidal or internal orbital foramen perforates the cranial wall at the outer side of these fossæ. Further back the central part of the surface is slightly elevated, and is flanked by shallow depressions which support the olfactory peduncles. Posteriorly is a bony shelf which covers the entrance to the optic foramina; the edge of this shelf and the posterior borders of the orbital wings of the sphenoid may be taken as the line of demarcation between the anterior and middle fossæ. The middle fossa (Fossa cranii media) is the widest part of the cavity. It extends backward to the internal spheno-occipital and petrosal crests, thus corresponding to the postsphenoid. In its middle is a small fossa, the sella turcica, in which the pituitary body, or hypophysis cerebri, is situated. On either side are two grooves for nerves; the inner one transmits the ophthalmic, third, and sixth nerves to the foramen lacerum orbitale; the outer one leads to the foramen rotundum, and lodges the maxillary nerve. External to the grooves is a depression for the pyriform lobe of the cerebrum. The posterior fossa (Fossa cranii posterior) corresponds to the basilar part of the occipital bone. It contains the medulla, pons, and cerebellum. In front is a median depression (Fossa pontis) for the pons. The surface behind this is concave transversely and slopes gently downward to the foramen magnum; it supports the medulla. On either side are the foramen lacerum basis cranii and the hypoglossal foramen.

The anterior or nasal wall is formed by the cribriform plate of the ethmoid, which separates the cranium from the nasal cavity. It is perforated by numerous foramina for the passage of the olfactory nerve-bundles.