48 ANATOMY AND PHYSIOLOGY 



The eye, though widely different from the land- 

 seeing organ, is readily explained, in all its rela- 

 tions, by the familiar principles of optics. 



In the first place, the fish's eye is nearly glo- 

 bular and the cornea flat ; in terrestrial animals, 

 on the other hand, the cornea is a segment of a 

 small sphere applied to the side of a larger one, 

 which gives the possessor the manifest advantage 

 of having the eye extended further into the field of 

 vision ; — it is also kept convex by the aqueous 

 humor. Now in the fish, it requires no aqueous 

 fluid within the eye, because the element in which 

 it swims is an equivalent. We are supposing 

 that the reader is perfectly acquainted with the 

 laws of refraction and reflection of light, but 

 if he is not, he cannot understand why the lens 

 is placed so far forward in the fish, towards the 

 pupil, nor why the cornea is necessarily nearly flat. 



quarrels between anatomists on the subject, that all the skates 

 in the ocean would not pay for the paper, which has been 

 wasted about a certain little hole in their head, therefore we 

 shall be careful about getting into, the ring. Fishes have just 

 so much acoustic apparatus as constitutes the central por- 

 tion of the ear in man, viz : the vestibule and semi-circular 

 canals, but the whole is boxed up in the solid bones of the 

 skull, so that sound, propagated through the water, gives a vi- 

 bratory motion or tremor to the whole body, and which, agi- 

 tating the auditory nerve, produces the sensation of hearing. 

 Fishes can easily be taught to feed, in ponds, by the ringing 

 of a bell. 



