288 THE DOG-FISH. 



5. The lens is a solid spherical body, suspended in the eye 



immediately behind the pupil. 



The spherical shape of the lens, together with 

 the flatness of the cornea, is very characteristic of 

 aquatic animals. 



6. The falciform process is an ingrowth of the choroid 



projecting into the vitreous humor from the blind 

 spot. 



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 he in planes of which each is at right 

 angles to the other two. Each canal opens at both its ends 

 into the vestibule. 



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 rides lodging the anterior and posterior vertical 

 semicircular canals, the former running backwards and in- 

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

 wards 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 aquae- 

 ductus vestibuli. 



Hold the skull in your 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 

 surface of the head by the aquaeductus vestibuli, 



