SENSE-OEGANS 255 



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 ampuUa 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 inferior end, 

 on the under surface of the skuU. 



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 ampulla 

 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 ampullae are collected together in a large mass at the 

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

 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 along the 

 dorsal and ventral surfaces of the head. The tubes are filled 

 with a transparent gelatinous matter, which can be readily 

 squeezed out from their openings on pressure. 



Their function is not known with certainty, but they are 

 generally believed to be sense organs of some kind. The nerves 

 supplying them are the ophthalmic branches of the fifth and 

 seventh nerves, and the maxillary division of the fifth. 



