456 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



pieces are found in the sclerotic of birds, as of 

 reptiles. 



The eyes of Mammals agree in essential points 

 with those of man, 110 mammal above the Prototheria 

 having a bony sclerotic ; as we ascend the scale we 

 observe that the eye becomes more and more com- 

 pletely protected, owing to the formation by the 

 cranial bones of a bony orbit, which is to be seen 

 in the dried skull. The eyes are reduced in moles 

 and in burrowing rodents ; in the mole-rat (Spalax) 

 they are covered by the skin. In those that seek 

 their prey by night or twilight, the cornea is larger 

 and more convex, and the pupil broader than in 

 the rest ; in such, too, the lens is nearly spherical. 

 In aquatic forms, just as in fishes, the cornea is 

 much less convex than in their terrestrial allies ; 

 compared with the bulk of their bodies, the eyes 

 of the Cetacea are exceedingly small. 



While the body of an albino is perfectly white, 

 it is often a matter of astonishment that the eyes 

 are red ; but a little reflection will show that this 

 redness is due to the blood in the vessels of the 

 eye, and that the colour is seen in the eye, though 

 not in other parts of the body, in consequence of 

 the transparency of its tissues. It is not so fre- 

 quently or so easily recognised, that the " colour 

 of the eye " is dependent also on this blood. In 

 light-grey or blue eyes no pigment is deposited in 

 the iris, but there is pigment in the retina, the light 

 reflected from which is, owing to interference, of a 

 blue colour. When pigment is more thickly laid 

 down in the retina, and becomes also deposited in 

 the substance of the iris, we have dark-blue or brown 

 eyes ; and it is because this pigment is ordinarily 

 laid down after birth that we have the somewhat 

 strange phenomenon of the blue eyes of the babe 

 becoming brown eyes in a child. 



