356 THE SENSORIAL FUNCTIONS. 



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 serviceable as visual organs. 

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

 surrounded by thick hair, which completely obstructs vi- 

 sion, and requires to be removed by the action of the subcu- 

 taneous 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 development, 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 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 ani- 

 mal. The whole side of the head is covered with a conti- 

 nuous integument of uniform thickness, and equally over- 

 spread with a thick velvetty hair. It is only after removing 

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

 ceedingly small size, and apparently the mere imperfect ru- 

 diment of an eye, and totally 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 accommodating the refractive power of that 

 organ to this variation of circumstances. Accordingly, it is 

 found that in the Seal, and other amphibious tribes, the 



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

 and Cams recognise the existence of a very slender nervous filament, arising 

 from the brain, and distributed to the eye of that animal. 



