136 



COMPARATIVE ZOOLOGY. 



and toes. Claws 

 are sharp conical 

 nails, being devel- 

 oped from the sides 

 as well as upper 

 surface; and hoofs 

 are blunt cylin- 

 drical claws. Hol- 

 low horns, as of the 

 Ox, may be likened 

 to claws sheathing 

 a bony core. The 

 horn of the Rhinoc- 

 eros is a solid mass 

 of epidermal fibres. 



" Whalebone, 



FIG. 103. Vertical Section of the Forefoot of the Horse 

 (middle digit): 1, 2, 4, proximal, middle, and distal, 

 or ungual, phalanges ; 3, seeamoid, or nut-bone ; 5, 

 6, 7, tendons ; 9, elastic tissue ; 8, 10, internal and 



external floor of the hoof; 11, 12, internal and exter- 



nal walls. 



rattles of the Rattlesnake, and the 

 beaks of Turtles and Birds, are like- 

 wise epidermal. 



Hairs, the characteristic clothing 

 of Mammals, are elongated horny 

 cones, composed of " pith " and 

 "crust." The latter is an outer 

 layer of minute overlapping scales, 

 which are directed towards the 

 point, so that rubbing a human 

 hair or fibre of wool between the 

 thumb and finger pushes the root- 

 end away. The root is bulbous, 

 and is contained in a minute de- 



pression, or sac, formed by an in- Fm.m-SectionoftheRootand 



folding of the skin. Hairs are usu- 

 ally set obliquely into the skin. 

 Porcupine's quills and Hedgehog's 

 spines make an easy transition to 



part of the Shaft of a Human 

 Hair; it is covered with epi- 

 dermic scales, the inner layer, 

 o, forming the outer covering 

 of the shaft, being imbricated ; 

 the root consists of angular 

 cells loaded with pigment. 



