IX.] OF THE DOG. 103 



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 third 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 rotunduni 

 and the foramen ovale., 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 pcriotic. 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 infernus, into which the seventli 

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

 passes through the bone and emerges on the other side 

 (by the stylo-mastoid forameti) \ 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 (the foramem lacerum posterius), through which 

 the glossopharyngeal, pneumogastric, and spinal accessory 



