VISION. 505 



to this rule, their eyes being comparatively 

 small ; but a compensation has been afforded 

 them in the superior acuteness of their other 

 senses. In many quadrupeds a portion of the 

 choroid coat is highly glistening, and reflects 

 a great quantity of coloured light : the object 

 of this structure, which is termed the Tapetuniy 

 is not very apparent. 



Among the lesser quadrupeds which burrow 

 in the ground, we find many whose eyes are 

 extremely minute ; so much so, indeed, as to be 

 scarcely serviceable as visual organs. The eye 

 of the Sorex, or shrew mouse, is very small, and 

 surrounded by thick hair, which completely 

 obstructs vision, and requires to be removed 

 by the action of the subcutaneous muscles, 

 in order to enable the animal to derive any 

 advantage from its eyes. These organs in the 

 Mole are still more remarkably deficient in 

 their developement, not being larger than the 

 head of a pin, and consequently not easily 

 discovered.* It is therefore probable that this 

 animal trusts chiefly to its sense of hearing, 

 which is remarkably acute, for intimations of 

 the approach of danger, especially as, in its 

 subterranean retreats, the vibrations of the solid 



* Magendie asserts that the mole has no optic nerve ; but G. 

 St. Hilaire and Carus recognise the existence of a very slender 

 nervous filament, arising from the brain, and distributed to the 

 eye of that animal. 



