EYE -BALL OF FRESH-WATER FISH. 291 



rays of light which pass through the transparent 

 window (cornea) in the front of the eye are imme- 

 diately received, and are as suddenly concentrated 

 on the smooth expanse of the optic nerve, to be 

 transferred by the latter to the vital apparatus. 

 As is usual in optical instruments, there is a thin 

 partition pierced with a circular aperture (the 

 pupil), interposed between the transparent mem- 

 brane of the front of the eye and the lens. This 

 membrane is intended to regulate the admission of 

 the luminous rays. In some fishes there are two 

 apertures, or pupils, in place of one in this parti- 

 tion, a curious modification, while in some others, 

 as in the common skate, a broad curtain with a 

 vandyked border, is suspended immediately behind 

 the pupil at its upper part, and serves to veil the 

 interior of the eye against the vertical rays which 

 would otherwise interfere with vision. The 

 enumeration of all the modifications in the con- 

 struction of the eye of the fish with which the 

 naturalist is acquainted, would form of itself an 

 interesting chapter, but the subject is too compre- 

 hensive to be more than glanced at in this brief 

 sketch. In most of the fresh- water fishes, the eye- 

 ball plays in a bony niche, which is specially des- 

 tined for its use ; while in other fishes in which 

 the bony niche is absent the eye is jointed to a 

 movable bony stalk, and moves on the extremity 

 of that stalk as upon a ball and socket hinge, 

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