THE DOGFISH 29 



on each side looking into the brain cavity, the median pair having 

 the keel of the rostrum between them ; the lateral pair lie each in 

 the posterior wall of the nasal capsule and permit the egress of the 

 olfactory nerve on that side from the brain cavity. 



Exercise 34. Draw the dorsal view. 



Study the lateral aspect of the skull. Note the great extent of 

 the orbit and identify the foramina through which the cranial 

 nerves find passage into it. Three of these foramina are much 

 larger than the others. They are the following : the optic foramen, 

 the large hole for the passage of the optic nerve, which lies near 

 the middle of the lower margin of the orbit ; the trigemino-facial 

 foramen, at the hinder end of the orbit, for the passage of the 

 trigeminus and facial nerves; the ophthalmic foramen, near the 

 upper border toward the forward end of the orbit, for the exit 

 of the superficial ophthalmic branches of the trigeminus and 

 facial nerves. The smaller foramina are the following: the mi- 

 nute trochlear foramen, directly above the optic foramen and be- 

 tween it and the ophthalmic foramen; the oculomotor foramen, 

 directly anterior to the trigemino-facial foramen ; the minute ab- 

 ducens foramen, below and in the lip of the trigemino-facial fora- 

 men ; the two hyomandibular foramina, below the trigemino-facial 

 foramen, for the passage of the hyomandibular branches of the 

 facial nerve. Besides these foramina several other small ones are 

 present which are occupied by blood vessels. 



Exercise 35. Draw a side view of the cranium and carefully label all 



the features. 

 Exercise 36. Draw a ventral view of the cranium. 



The Visceral Skeleton. This consists of a series of seven visceral 

 arches, which lie beneath the cranium and the anterior end of the 

 vertebral column and surround the pharynx and the mouth. Each 

 arch is a paired structure consisting of a right and a left side which 

 meet in the midventral plane ; each side is also made up of a dorsal 

 and a ventral half. 



The visceral arches fall into two groups : an anterior group, con- 

 sisting of two arches,— the first or mandibular arch and the second 



