VISION. 451 



the choroid coat is highly glistening, and reflects a 

 great quantity of coloured light : the object of this 

 structure, which is termed the Tapetum^ 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 ser- 

 viceable 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 sub- 

 cutaneous muscles, in order to enable the animal 

 to derive any advantage from its eyes. These 

 organs in the Mole are still more remarkably defi- 

 cient in their developement, not being larger than 

 the head of a pin, and consequently not easily dis- 

 covered.* It is therefore probable that this animal 

 trusts chiefly to its sense of hearing, which is re- 

 markably acute, for intimations of the approach of 

 danger, especially as, in its subterranean retreats, 

 the vibrations of the solid earth are readily trans- 

 mitted to its ears. The 3lus ti/p/dus, or blind rat 

 of Linnaeus, (the Zemni of Pallas,) which is an in- 

 habitant of the western parts of Asia, cannot be 

 supposed to possess even the small degree of vision 

 of the mole ; for no external organ of this sense has 

 been detected in any part of that animal. The 

 whole side of the head is covered with a continuous 

 integument of uniform thickness, and equally over- 



* Magendie asserts that the mole has no special 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. This nerve has also been demonstrated by Koch and 

 Keule. (Muller, by Baly.p. 1071.) 



