COMPARATIVE ANATOMY 15 



circular canals. The position of the horizontal semicircular 

 canal is indicated by a ridge on the lateral side of the cranium 

 between the posterior orbital process and the post-otic process. 

 The post-otic process forms the posterior-lateral corner of the 

 cranium. What is the function of the semicircular canals .f* 

 They will be dissected when the sense organs are studied. 



7. Fenestrae. — These are the large openings on the ventral 

 surface, one on either side of the keel of the rostrum. In life 

 they are closed by membranes. 



8. Olfactory Capsules. — These large openings on the 

 anterior end of the cranium, lateral to the fenestrae, are for the 

 organs of smell. During life they are closed, but the thin 

 anterior walls are usually destroyed in preparation of the 

 cranium. 



9. Principal Foramina. — The large hole through the 

 median wall of each orbit is the optic foramen, for passage of the 

 optic nerve. In the posterior ventral corner of the orbit is a 

 group of five foramina, the largest of which is for the exit of the 

 fifth and seventh cranial nerves; the others are for the third and 

 sixth nerves and for blood vessels. The row of foramina 

 extending along either side of the dorsal surface is for passage of 

 branches of the fifth and seventh nerves. The small foramen 

 posterior to the anterior fontanelle markc the location of the 

 epiphysis, a small dorsal outgrowth of the brain. On the pos- 

 terior end of the cranium are four large foramina; the lateral 

 ones are for the exit of the ninth cranial nerves, the medial ones 

 for the passage of the tenth cranial nerves. The foramen in 

 the mid-ventral line, behind the orbits, is for the entrance of the 

 internal cartoid arteries. 



Examine a cranium that has been bisected longitudinally. 

 Identify the structures previously mentioned, and in addition 

 locate the foramen for the passage of the eighth nerve to the 

 ear, the cavity for the brain, and the depression on the floor of 

 the cranium in which the pituitary gland rests. 



Visceral Skeleton of the Shark 



For this study use the mounted skeletons. A knowledge of 

 the visceral arches of the shark Is essential for a clear under- 

 standing of the modifications that occur in higher vertebrates. 



