82 THE SKELETON OF THE HORSE 
the crest is a depression for the hemisphere of the cerebellum. Behind this are the 
internal acoustic meatus and the openings of the aqueeductus vestibuli and aquie- 
ductus cochle. 
The roof and lateral walls are marked by digital impressions and vascular 
grooves. 
The ventral wall or floor (Basis cranii interna) (Fig. 53) may be regarded as 
forming three fosse. The anterior fossa (Fossa cranii oralis) 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 fosse for the olfactory 
bulbs. The ethmoidal foramen perforates the cranial wall at the outer side of these 
fosse. Further back the central part of the surface is slightly elevated, and is 
flanked by shallow depressions which support the olfactory tracts. 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 fossee. The middle fossa 
(Fossa cranii media) is the widest part of the cavity. It extends backward to the 
spheno-ocecipital and petrosal crests, thus corresponding to the postsphenoid. In its 
middle is the hypophyseal fossa in which the hypophysis cerebri lies. On either side 
are two grooves: the medial one transmits the cavernous sinus and the ophthalmic, 
third, and sixth nerves to the foramen orbitale; the lateral one leads to the foramen 
rotundum, and lodges the maxillary nerve. Lateral to the grooves is a depression 
for the piriform lobe of the cerebrum. The posterior fossa (Fossa cranii posterior) 
corresponds to the basilar part of the occipital bone. It contains the medulla 
oblongata, pons, and cerebellum. In front is a median depression (Fossa pontis) 
for the pons. The surface behind this (Fossa medullze oblongatie) is concave trans- 
versely and slopes gently downward to the foramen magnum; it supports the 
medulla oblongata. On either side are the foramen lacerum and the hypoglossal 
foramen. 
The anterior or nasal wall (Fig. 33) 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. 
THE NASAL CAVITY 
The nasal cavity (Cavum nasi) is a longitudinal passage which extends through 
the upper part of the face. It is divided into right and left halves by a median 
septum nasi. The lateral walls are formed by the maxilla, premaxilla, the per- 
pendicular part of the palatine, and the turbinate and ethmoid bones in part. This 
yall is crossed obliquely by the lacrimal canal and groove for the naso-lacrimal 
duct, and its posterior part is perforated by the sphenopalatine foramen. The 
dorsal wall or roof.is formed by the frontal and nasal bones, which form a median 
prominence at their junction. It is concave from side to side, and nearly straight 
longitudinally, except in the posterior part, where it curves downward. The ven- 
tral wall or floor is formed by the palatine processes of the premaxillee and max- 
ille, and the horizontal parts of the palate bones. It is wider but considerably 
shorter than the roof. It is concave transversely, and nearly horizontal from 
before backward, except in the posterior third, where there is a slight declivity. 
The anterior part presents a median groove for the cartilage of the septum, and a 
furrow for the vomero-nasal organ (of Jacobson) on either side. Posteriorly there 
is a median elevation, the nasal crest, to which the vomer is attached. Lateral to 
the palatine process of the premaxilla is the palatine fissure. 
The septum nasi osseum is formed by the perpendicular plate of the ethmoid 
