50 ANATOMY AND PHYSIOLOGY 



pursuing their prey, at unfathomable depths, for 

 aught we know, the hydrostatic pressure would 

 crush in the whole globe were it of mem- 

 branous tunics, like the land-seeing eye ; but 

 being of bone, it resists the pressure, and thus 

 saves the organ. Shoal water fishes, possessing 

 the membranous eye, if forced down to a certain 

 depth, would be made blind, and come to the sur- 

 face with the entire loss of the internal humors. A 

 pretty accurate judgment may be formed of the 

 depth to which any species of fish swim, by an 

 examination of the coats of the eye, as in one case 

 there are ribs of bone, as in bird's and turtle's eyes, 

 and in others, flat pieces are inserted, merely to 

 offer firmer resistance to the pressure of the water, 

 which is greater there, than on the body. 



There are no eye-lids, because it is necessary to 

 see, even while they sleep. Fishes undoubtedly 

 sleep, but necessarily with open and vigilant 

 eyes. 



There are no eyelids, because they would have 

 been entirely useless, it being necessary for fishes 

 to perceive, even while they sleep. It has been 

 said to us that the vitreous, and other humors resist 

 the external pressure, and therefore our hypothe- 

 sis in regard to the object of an osseous scleroti- 

 ca, is quite objectionable. If it were true, that 

 the vitreous humor, and crystalline lens, were 



