VISION. 357 



structure of the eye approaches to that of fishes, the lens be- 

 ing denser and more convex than usual, the cornea thin and 

 yielding, and both the anterior and posterior segments of 

 the sclerotic thick and firm; but the middle circle is very 

 thin and flexible, admitting of the ready separation or ap- 

 proximating of the other portions, so as to elongate or con- 

 tract 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 between 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 mus- 

 cles. In birds which frequently plunge their heads under 

 water the crystalline lens is more convex than in other 

 tribes; and the same is true, also, of aquatic reptiles. 



