250 ZOOLOGY. 



arm as well as in the leg. Generally, these leg and arm bones 

 are short and strong, or long and slender, according to the 

 habits of the animal. The mole and chamois may serve as 

 the examples of this modification : strength characterizes the 

 actions of the mole, and swiftness those of the chamois. 

 When the hand no longer serves for prehension, but merely 

 for support, the radius loses its power of rotation on the ulna, 

 and ends by uniting with it so intimately as to be no longer 

 distinguished from it. In the solidungulous animals, the same 

 happens with the fibula and tibia. 



The foot and hand vary in mammals according as they are 

 used for prehension, swimming, flight, or walking on ground 

 more or less firm. These circumstances have been already 

 alluded to. The fingers or toes are never more than five, and 

 their number diminishes according as the extremity is more 

 exclusively used for simple progression. 



398. Organs of the Senses. The perfection of the 

 fingers and toes as instruments of touch seems to depend on 

 the fineness of the integuments, the form of the nail, and the 

 flexibility of the instrument. In this respect man stands pre- 

 eminent, in accordance, no doubt, with his higher intelligence 

 (Fig. 98). 



399. The organs of the other senses are also analogous 

 in all the mammals to those of man. In those remarkable for 

 the fineness of their powers of smell, the dog:, for example, 

 the nasal fossse and frontal sinuses are much enlarged, and 

 the turbinated bones of the nostrils are greatly developed, 

 thus extending the surface of the pituitary membrane, on 

 which the nerves of smell are distributed. 



400. In nocturnal animals, the eyes are generally larger 

 than in the diurnal ; and in the former the pupil of the eye, 

 when contracting, assumes the form of a fissure, losing its 

 circular character. In moles, whose residence is underground, 

 the eyes become extremely small, and seem in some to be 

 mere vestiges or rudimentary organs ; in the aquatic mam- 

 mals, the lens is, for an obvious reason, spherical. In many 

 mammals there exists a coloured part of the choroid tunic of the 

 eye called tapetum, the uses of which are unknown.* Many 

 have a third eyelid placed vertically at the inner angle of the 

 horizontal eyelids. Finally, the direction of the eyes varies 

 much : in man they are directed forwards ; but, as we descend 



* The glaring of the eyes in the dark depends on the presence of this 

 tapetum; its colour varies in different animals. R. K. 



