452 THE SENSORIAL FUNCTIONS. 



spread with a thick velvety hair. It is only after 

 removing the skin that a black spot is discovered 

 on each side, of exceedingly small size, apparently 

 the mere imperfect rudiment of an eye, and, as far 

 as we can perceive, incapable of exercising any of 

 the functions of vision. 



Those mammalia whose habits are aquatic, having 

 the eye frequently immersed in a dense medium, 

 require a special provision for accommodating the 

 refractive power of that organ to this variation of 

 circumstances. Accordingly it is found that in the 

 Seal, and other amphibious tribes, the structure of 

 the eye approaches to that of fishes ; the lens being 

 denser and more convex than usual, the cornea 

 thin and yielding, and both the anterior and pos- 

 terior segments of the sclerotic thick and firm ; but 

 the middle circle is very thin and flexible, ad- 

 mitting of the ready separation or approximating of 

 the other portions, so as to elongate or contract the 

 axis of the eye; just as a telescope can be drawn 

 out or shortened, in order to adapt it to the distance 

 of the object to be viewed. The whole eye-ball is 

 surrounded by strong muscles which are capable of 

 effecting these requisite changes of distance be- 

 tween the cornea and the retina. The Dolphin, 

 which lives more constantly in the water, has an 

 eye still more nearly approaching in its structure 

 to that of fishes ; the crystalline lens being nearly 

 spherical, and the globe of the eye furnished with 

 strong and numerous muscles. In birds which fre- 

 quently plunge their heads under water the crys- 

 talline lens is more convex than in other tribes; 

 and the same is true also of aquatic reptiles. 



