The roots of the cranial nerves are shown on the left side and are designated by number. _I_, Anterior cranial fossa; _II_, middle cranial fossa; _III_, posterior cranial fossa; _a_, ethmoidal fossa; _b_, ethmoidal foramen; _c_, foramen for nasal branch of ophthalmic artery; _d_, orbital wing of sphenoid bone; _e_, optic fossa; _f_, sella turcica; _g_, spheno-occipital crest; _h_, _h′_, dotted line indicating contour of pituitary body; _h″_, slight elevation representing dorsum sellæ; _i_, _k_, grooves for nerves and cavernous sinus; _l_, depression for pyriform lobe of cerebrum; _m_, groove for middle meningeal artery; _n_, depression for pons; _o_, foramen lacerum anterius; _p_, foramen lacerum posterius; _q_, incisura carotica; _q′_, incisura ovalis; _q″_, incisura spinosa; _r_, depression for medulla oblongata; _s_, hypoglossal foramen; _t_, internal auditory meatus; _u_, foramen magnum; _v_, frontal sinus; _w_, zygomatic process of temporal bone; _x_, section of petrous temporal; _y_, section of occipital bone; _z_, crista galli; 1, 1′, 1″, dotted lines indicating position of olfactory tracts and peduncle. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ] 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.