156 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



SECTION VI. 

 Comparative Physiology and Morphology of the Eye. 



VERTEBRATES. 



Mammals. — The structure of the eye and the physi- 

 ology of vision in all mammals and, indeed, in all verte- 

 brates is substantially the same as that already given 

 for man, yet there are some points of difference worthy 

 of note. 



Color. — The iris is, we have seen, a continuation of 

 the choroid coat. Normally and most usually, there- 

 fore, it has the dark, chocolate-brown color characteris- 

 tic of the pigment of that coat. Doubtless this is the 

 original and normal color of the human eye. The blue 

 and gray are the result of peculiar structure, together 

 with a deficiency in pigment. Nearly all mammals have 

 the normal brown color. In the cat tribe, however, as 

 is well known, it is brilliant yellow. 



Pupil. — The form of the pupil is usually round, as in 

 man, but in the two most highly specialized and differ- 

 entiated orders — the cat tribe on the one hand, and the 

 grazing animals on the other — the pupil is greatly elon- 

 gated, vertically in the former and horizontally in the 

 latter. The vertical elongation is probably connected 

 with the habit of springing on its prey ; the horizontal 

 elongation, certainly with wide horizontal view, neces- 

 sary in grazing. This shape of the pupil, combined with 

 the prominence of the eyes and their position on the 

 margin of a broad front, makes the view of these ani- 

 mals sweep the whole horizon without turning the head 

 or even the eyes. 



Tapetum. — In many mammals, especially those of noc- 

 turnal habits, such as the cat tribe and ruminants, there 

 is found at the bottom of the retinal concave a large 



