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. 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.
[Illustration: