254 THE DOG-FISH 



transparent, and serves to bring the rays of light to 

 a focus on the retina. 



The spherical shape of the lens, together with 

 the flatness of the cornea, are very characteristic of 

 aquatic animals. 



C. The Ear. 



The ear consists of a membranous sac, or vestibule, filled 

 with fluid, and produced into three curved tubes, the semi- 

 circular canals, which are placed in planes at right angles to 

 one another, and open into the vestibule at both ends. 



The whole sac is lodged within the cartilaginous auditory 

 capsule, which is firmly united with the side of the hinder 

 end of the skull. 



Scrape clean the hinder end of the skull so as to expose the 

 left auditory capsule fully. Note, on the dorsal surface, the 

 prominent ridges formed by the anterior and posterior vertical 

 semicircular canals, the former running backwards and in- 

 wards from the hinder margin of the orbit, and the latter 

 backwards and outwards to the posterior end of the skull. 

 Note also at the inner side of the point of meeting of the two 

 canals, a hole leading downwards and outwards into a canal, 

 the aqueductus vestibuli. 



Hold the skull in~the left hand and carefully pare away 

 the cartilage with a sharp scalpel until the several parts of 

 the auditory organ are fully exposed. Owing to the trans- 

 parency of the cartilage, the parts can be seen before they are 

 met with, and the dissection, though requiring care, is not 

 difficult. 



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

 lower and inner part of the auditory capsule. It is 

 partially divided into two ; contains milky 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. 



