VISION. 495 



great density. These circumstances are shown 

 in the section of the eye of the Perch, Fig. 430.* 

 The flatness of the cornea leaves scarcely any 

 space for aqueous humour, and but little for the 

 motions of the iris. 



The surface of the eye in fishes, being con- 

 tinually washed by the water in which it is 

 immersed, requires no provision 

 430 ^ of a secreted fluid for that pur- 



pose ; and there are consequent- 

 ly neither lacrymal apparatus, 

 nor proper eye-lids ; the integu- 

 ments supplying only a thin 

 transparent membrane, which 

 passes over and protects the cornea, serving the 

 office of a conjunctiva. The eye retains its form 

 by the support it receives from the sclerotic coat, 

 which is of extraordinary thickness and density. 

 In the Shark and the Skate the eye is supported 

 from the bottom of the orbit, by a cartilaginous 

 pedicle, which enables it to turn as on a pivot, 

 or lever. 



Sir David Brewster has recently made an in- 

 teresting analysis of the structure of the crystal- 



* In this figure, as in the others, c is the cornea ; l, the lens ; 

 V, the vitreous humour ; r, the retina : o, the optic nerve ; and 

 s, the sclerotica. There is also found in the eyes of most fishes 

 an organ, lodged in the space k, termed the Choroid gland, 

 which envelopes the optic nerve, is shaped like a horse-shoe, is 

 of a deep red colour, and highly vascular ; its use is quite un- 

 known. 



