rrg STRUCTURE OF VERTEBRATES 



two orders. The horses and some cattle (Figs. 378-9* 383-4) show 

 the Umit of reduction by the two methods. Other artiodactyls, such 

 as the hippopotamus, have four toes, other perissodactyls, such as 

 the rhinoceroses, have three, but whatever the number the axis 

 goes down the third digit in the perissodactyls and between 

 the third and fourth in the artiodactyls. In the graviportal or 

 very heavy mammals such as the elephant, the proximal bones 

 of the limbs, humerus and femur, are longer than the others. 



Fig. 435. — The bones of the hind-limb of Man, compared with those of a monkey, 

 dog, sheep, and horse. Homologous parts bear the same letters or numbers. 

 — From Romanes, after Le Conte. 



The advantage of the digitigrade and unguligrade conditions 

 is that they allow the animal to move faster, and so they are 

 found in the cursorial forms, which run either to catch their prey 

 or to avoid being caught. There is a general tendency in such 

 species for the hind-lirnbs to do more than half the work of pro- 

 pulsion, and this leads, as in the rabbit or dog, to an increased 

 relative size of their muscles and the bones to which they are 

 attached. The extreme of this is seen in the saltatorial or jumping 

 animals in which the fore-limbs are not used in locomotion at all. 

 The kangaroos are the best known examples, but a similar con- 

 dition is found in three families of rodents. It is notable that where- 



