TEGUMENTARY ORGANS— SKIN STRUCTURES. 



4S5 



horn. At the base there is an infolded pocket in which 

 growth is more act'ive, so that the claw is pushed for- 

 ward in proportion as it is worn off by use (Fig. 310). 



Hoofs. — These, again, differ from claws only by the 

 size and shape of the terminal joint on which they are 

 molded. This terminal joint is 

 the hoof core or coffin bone. We 

 have here the same pocketlike 

 infolding, and the forward-push- 

 ing growth in proportion to 

 wear (Fig. 311). If wearing is 

 prevented by shoeing they must 

 be trimmed from time to time. 



The horny armature of the 

 terminal joints of the toes is 

 characteristic of land vertebrates 

 — i. e., reptiles, birds, and mam- 

 mals. They are absent even in 

 amphibians on account of their 

 greater alliance to fishes. Furthermore, mammals, by 

 the character of this armature, are divided into ungulates 

 (hoofed) and unguiculates (clawed). 



The original generalized form of this armature was 

 apparently of flattened shape on the dorsal side of the 

 toe somewhat like that of man. This was the case in 

 some of the earliest mammals, as, for example, the 

 Phenacodus, and has been retained by apes and by man. 

 From this generalized form have been differentiated 

 claws on the one hand and hoofs on the other. 



Horns. — These are almost characteristic of rumi- 

 nants. Paired horns on frontal bones are entirely so at 

 present, although not so in early geological times. Again, 

 frontal horns are of two kinds — viz., solid horns, as in 

 the Cervidce (deer family), and hollow horns, as in the 

 Bovidce and Ovidce — e. g., ox, sheep, goats, etc. 



Fig. 311. — The joints of the 

 toe of a horse. The parts 

 are similar to the last two' 

 figures. 



