374 



COMPARATIVE ZOOLOGY 



(as the elephant). The horse steps on what corre- 

 sponds to the nail of the middle finger ; and its swiftness 

 is conditioned on the solidity of the extremities of the 



FIG. 326. Feet of Hoofed Mammals: A, Elephant; B, Hippopotamus; C, Rhinoceros; 

 Z>, Ox; E, Horse, a, astragalus; cl, calcaneum, or heel; s, naviculare; b t cu- 

 boides; ce, ci, cm, cuneiform bones; the numbers indicate the digits in use. 



limbs. Horses of the greatest speed have the shoulder 

 joints directed at a considerable angle with the arm. 



The order in which the legs of quadrupeds succeed' 

 each other determines the various modes of progression, 

 called the walk, trot, gallop, and leap. Many, as the 

 horse, have all these movements ; while some only leap, 

 as the frog and kangaroo. In leaping animals, the hind 

 limbs are extraordinarily developed. In many mammals, 

 like the squirrel, cat, and dog, the fore legs are used for 

 prehension as well as locomotion. Monkeys use all four, 

 and also the tail, for locomotion and prehension, keeping 

 a horizontal attitude ; while the apes, half erect, as if 

 they were half quadruped, half biped, go shambling 

 along, touching the ground with the knuckles of one 

 hand and then of the other. In descending the scale, 

 from the most anthropoid ape to the true quadruped, we 

 find the center of gravity placed increasingly higher up 



