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 aquseductus vestibuli and aquse- 

 ductus cochlea. 



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 fossse. 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 fossae for the olfactory 

 bulbs. The ethmoidal foramen perforates the cranial wall at the outer side of these 

 fossse. Further back the central part of the surface is shghtly 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 fossse. The middle fossa 

 (Fossa cranii media) is the widest part of the cavity. It extends backward to the 

 spheno-occipital 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 meduUae oblongatse) 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 hjrpoglossal 

 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 

 wall is crossed obhquely 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 premaxillse and max- 

 illae, 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 



