276 THE EYE IN EVOLUTION 



of the fish-eye is therefore elhpsoidal with the shortest diameter the 

 visual axis (Figs. 292 and 365) ; only in sluggish forms such as the 

 bow-fin, Amia, does the globe become spherical. The maintenance of 

 a non-spherical shape in the face of changes in pressure which may be 

 considerable necessitates a sturdy outer coat ; the sclera therefore 

 tends to be thick and is typically reinforced with a supporting layer 

 of cartilage sometimes supplemented by bone. 



The flattening of the anterior segment implies a flat cornea ; but 

 in a watery medium this structure is in any case useless as a refracting 



Fig. 292. — The Eye of the Trout. 

 Note the flat shape with the short antero-iDosterior axis. 



agent. Perfection in its optical properties is thus neglected ; it is 

 therefore often irregular and even ridged, and in the interests of strength 

 is frequently thin centrally and thick in the jDeriphery giving it the 

 construction of a sturdy arch. It follows that the entire responsibility 

 for refraction falls upon the lens. The lens of fishes is consequently 

 enormously large and almost spherical with a highly refractive nucleus 

 and higher total refractive index (1-649 to 1-653) than in any other 

 Vertebrate, making a maximal difference between it and the refractive 

 indices of the other media. With the elimination of the cornea from 

 the dioptric system and the dependence on the lens for refraction, it is 

 necessary that a constant proportion should exist between the size of 

 the leii ■ and its distance from the retina ; Matthiessen (1886), indeed, 

 showed Miat this is so, that the eyes of fishes, no matter what their 

 size ail liape, are standardized in their configuration, the distance 

 from th'. ntre of the lens to the retina being constant (radius of lens 



