SENSE ORGANS 261 



4. The retina is a soft pulpy membrane lying on the inner 



surface of the choroid, and readily separable from it 

 except at the point of entrance of the optic nerve, to 

 which it adheres firmly; and with which it is con- 

 tinuous. It is the only part of the eye sensitive to 

 Hght. 



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



immediately behind the pupil. During life it is trans- 

 parent, 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, is 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 lie 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 su/rf ace, the 

 prominent ridges lodging the anterior and posterior vertical 

 semicircular canals, the former running backwards and in- 

 wa/rds 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 outwa/rds into a canal, 

 the aqneductus vestibuli. 



Hold the skull in the left hand and carefully pare away 

 the cartilage with a sharp scalpel until the several pa/rts 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 

 ^fficult. 



