264 



ORGANS OF THE SENSES. 



organs. From the imperfect development of the nervous 

 system of fishes and the obscurity of the element through 

 which they move, their organs of vision are of great size, and 

 from the density of the watery medium around them, they 

 have little necessity for aqueous humour in the eye, and their 

 cornea is flat. To preserve the eye of fishes flat in front, 

 the sclerotic is thickened and consolidated, as in the cetacea, 

 and it is also to prevent its assuming the spherical form in 

 birds, by the equal pressure of the contained fluids, that the 

 sclerotic is there strengthened with osseous plates, which pre- 

 serve the tubular form of the eye and the great convexity of 

 the cornea in that class. This thickness of the sclerotic coat 

 and the presence of the choroid gland and adipose substance 

 between the layers of the choroid coat behind, and the flat- 

 ness of the cornea in front shorten very much the visual 

 axis of the eye and diminish the space for containing the 

 vitreous humour ; hence a completely spherical form and 

 great size and density of the crystalline lens are here required 

 to bring the rays of light more quickly to a focus as seen in 

 the eye of the perch (Fig. 104. A. e). The crystalline lens is 



composed, as in other classes, of minute transparent fibres, 

 which are disposed in concentric layers and variously united 

 by their serrated edges, the layers encreasing in density from 

 the surface to the centre of the lens. The diameter of the 

 lens (104. A. e) is often greater than that of the aqueous and 

 the vitreous humours, and concentrates the rays of light to a 

 focus before the retina, so as to form an inverted image on 

 that membrane. The conjunctiva is now more easily sepa- 

 rated from the cornea than in the cephalopods, and the 

 choroid and retina are here remarkable for the distinctness of 



