506 THE SENSORIAL FUNCTIONS. 



earth are readily transmitted to its ears. The 

 Mus typhlus^ or blind rat of Linnaeus, (the 

 Zemni of Pallas,) which is an inhabitant 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 overspread with a thick velvety 

 hair. It is only after removing the skin that 

 a black spot is discovered on each side, of ex- 

 ceeding small size, apparently the mere imper- 

 fect rudiment of an eye, and, as far as we can 

 perceive, incapable of exercising any of the 

 functions of vision. 



Those mammalia whose habits are aquatic, 

 having the eye frequently immersed in a dense 

 medium, require a special provision for accom- 

 modating the refractive power of that organ to 

 this variation of circumstances. Accordingly it 

 is found that in the Seal, and other amphibious 

 tribes, the structure of the eye approaches to 

 that of fishes ; the lens being denser and more 

 convex than usual, the cornea thin and yield- 

 ing, and both the anterior and posterior seg- 

 ments of the sclerotic thick and firm ; but 

 the middle circle is very thin and flexible, 

 admitting of the ready separation or approxi- 

 mating of the other portions, so as to elongate 



