THE STRUCTURE AND ACTIVITIES OF MAMMALS 401 



Claws, Nails, Hoofs, and Horns. - - Mammals protect them- 

 selves from their enemies when in actual combat by means of 

 cheir teeth, claws, nails, horns, and hoofs. The claws, nails, 

 and hoofs are all modifications of the horny covering on the 

 upper surface at the end of the digits (Fig. 272). The foot may 

 rest partially or entirely on these structures, as in the case of the 



FIG. 272. Diagrammatic longitudinal sections through the distal ends of 



the digits of mammals. 



A, spiny anteater; B, dog; C, man; D, horse. 



1-3, phalanges; b, torus; N, nail plate; S, sole horn; W, bed of claw or 

 nail. (From Wiedersheim.) 



horse, but, as a rule, it is partly supported on the pads just be- 

 neath them. The horns of the rhinoceros and the horn sheaths 

 of cattle are, like claws and hoofs, formed from the outer layer 

 of the skin, the epidermis, but in many other animals the horns 

 are of bone, and even in cattle the central core of the horn is 

 bone. Some animals, like the deer and prong-horned antelope, 

 shed their horns annually and a new set gradually grows to take 

 their place; others, like cattle and sheep, normally keep one 

 pair of horns throughout life. In many cases only the male 

 individuals of a species possess horns. 



2D 



