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. 



