114 
ON THE STRUCTURE OF THE EYE 
and consequently their cornea is flat. This flatness of the 
cornea, however, could not be preserved, if the eyes of 
fishes were formed of soft membranes, like the eyes of 
quadrupeds, as the lens would then be pressed forward by 
the muscles surrounding the eyeball against the iris, and 
completely prevent the pupil from changing its dimensions. 
Hence we find a firm cartilaginous or osseous framework 
in the eyes of fishes, to preserve the flatness of the cornea^ 
analogous to what is employed in birds to preserve a great 
convexity of that part. The great thickness of the sclero- 
tic coat, the presence of the choroid gland and adipose 
matter between the sclerotic and choroid coats behind, and 
the flatness of the cornea before, shorten very much the 
axis of the eye in fishes. Hence a complete spherical form, 
and greater hardness of the crystalline lens, is necessary in 
these animals, to bring the rays of light more quickly to a 
focus. Two small portions of a sphere would have been 
sufficient, if the texture of the lens were homogeneous like 
glass ; but this organ possesses a highly complicated struc- 
ture, consisting of fibrous layers, beautifully disposed to 
check the aberration of light, and increasing in density from 
the surface to the centre. The distance between the cornea 
and the retina in the swordfish is about one inch and a half, 
while the transverse diameter of the eye is three inches. The 
shortness of the axis, and the enormous size and spherical 
form of the lens in the eyes of fishes, reduce greatly the pro- 
portion of their vitreous humour. In the common herring, 
the axis of the vitreous humour is only a fifth part of that of 
the lens ; whereas in man, it is nearly four times greater than 
the axis of the lens. In the large eye of the swordfish, the 
axis of the vitreous humour is not one-half of that of the 
lens. The ciliary processes and foramen centrale are not 
found in fishes. The choroid gland, the solid sclerotic coat, 
the fatty matter within it, and many other peculiarities in 
