MAMMALIA 



263 



body, guarded by a posterior prominence, the posterior clinoid 

 process. The presphenoid completes the floor of the brain 

 case, uniting in front with the ethmoid. On the cranial surface 

 it presents a transverse groove for the optic chiasma, and 

 the groove passes into the optic foramen on each side. The 

 alisphenoids are attached on each side of the basisphenoid 

 and come at their outer margins in contact with the parietals 

 and frontals. Each has a descending pterygoid process 

 to support the bone of that name, and each presents two 

 foramina. The posterior is the foramen ovale and transmits 



Pnmaxilla 



Lachrymal 

 Nasal foramen 



Orbito- 

 sphen id 



.Alisphenoid 



.Supraoccipital 



Palatine process 

 - ojpremaxilla 



Basl-ocdp. 

 For, lac.post. 

 Flocc.fossa 

 Int.aud.for. 

 Pe'riotic 



FIG. 131. Skull of dog. Internal view of longitudinal section. The ' carti- 

 lage' bones are dotted, and together give an idea of the embryonic 

 chondr ocranium . 



the mandibular branch of V. The anterior is the foramen 

 rotundum for the maxillary branch of V. The carotid artery 

 enters the brain through a foramen behind the posterior 

 margin of the alisphenoid : this is the foramen lacerum medium. 

 The orbitosphenoids are similarly fused with the presphenoid 

 and are small wings which come into relationship with the 

 frontals. Between the orbitosphenoid and the alisphenoid 

 is the foramen lacerum anterius the sphenoidal or anterior 

 orbital fissure for the ophthalmic branch of V and the motor 

 nerves of the eye, III, IV, VI. The orbitosphenoid is 

 pierced also, as has been observed, by the optic nerve. The 

 maxillary branch of V usually leaves the skull by the 

 sphenoidal fissure, but in the dog it has a separate foramen. 



