ix.] OF THE DOG. 123 



3. At a very short distance behind this is a more irregular 

 oval opening, between the orbitosphenoid and the ali- 

 sphenoid. This is the sphenoidal or orbital fissure, or 

 foramen lacerum anterius. It leads into the orbit, and 

 allows the exit of the motor nerves of the eyeball, or third, 

 fourth, and sixth cranial nerves, and also the first division 

 of the trigeminal or fifth nerve. 



4 and 5. The alisphenoid near its base is perforated by- 

 two foramina : the anterior small and somewhat round, the 

 posterior larger and oval ; these are the foramen rotunditm 

 and the foramen ovate, and transmit respectively the second 

 and third divisions of the fifth nerve. 



6. Between the alisphenoid and the exoccipital is a large 

 space, almost entirely filled by the bony capsule of the organ 

 of hearing, the periotic bone. In front of the inner end of 

 this bone is an opening {foramen lacerum medium basis 

 cranii\ through which the internal carotid artery sometimes 

 enters the cranial cavity. 



7. Near the middle of the inner surface of the periotic 

 is the meatus auditorius interims, into which the seventh 

 and eighth nerves enter : the former (the facial nerve) 

 passes through the bone and emerges on the other side 

 (by the stylo-mastoid foramen] ; the latter, the auditory, is 

 distributed to the internal organ of hearing within the 

 periotic bone. 



A deep depression seen above the internal auditory 

 meatus, and of nearly the same size, is not a foramen but a 

 fossa, lying within the concavity of the superior semicircular 

 canal. It lodges the flocculus, a small process of the cere- 

 bellum. 



8. Between the periotic and the exoccipital an irregular 

 space is left (fat foramen lacerum poster his], through which 

 the glossopharyngeal, pneumogastric, and spinal accessory 



