262 THE DOG-FISH 



1. The vestibiile is a laterally compressed sac, lying in the 



lower and inner part of the auditory capsule. It is 

 partially divided into two ; contains irdlky calcareous 

 concretions ; and communicates with the dorsal sur- 

 face of the skull by the aqueductus vestibuli, which 

 is a persistent relic of the involution of the dorsal 

 integument from which the ear is developed. 



Pass a seeker down the aqueductus vestibuli into the 

 vestibule. 



2. The semicircular canals are three in number. They 



open at both ends into the vestibule, and each is 

 dilated at one end into an oval ampulla, which re- 

 ceives a branch of the auditory nerve. 

 i. The anterior vertical canal has its ampulla at 



the anterior end close to the orbit, 

 ii. The posterior vertical canal is the longest of 

 the three, and forms almost a complete circle. 

 Its ampulla is at its posterior and inferior end, 

 on the ventral wall of the skull, 

 iii. The horizontal canal lies in the wing-like 

 lateral process of the auditory capsule, behind 

 the orbit : its ampulla is at its anterior end, 

 close to that of the anterior vertical canal. 



3. The auditory nerve enters the auditory capsule on its 



inner side, and at once divides into branches, which 

 can be traced to the vestibule and to the ampullae 

 of the semicircular canals. 



D. The Mucous Canals. 



These are long tubes opening by one end on the surface of 

 the head, while the other end is closed and dilated into an 

 oval ampulla, which is subdivided internally by partitions, 

 and receives nerve-branches. 



The ampuUse are collected together into a large mass at * 

 the anterior end of the snout, in front of the cranial cavity ; 

 and two smaller masses at the sides of the head, between the 

 nose and the orbit. From these masses the tubes diverge to 

 their external openings, which are arranged in rows on the 



