490 THE MECHANICAL FUNCTIONS. 



must be turned forwards in its whole length from 

 the heel to the extremities of the toes. On com- 

 paring the positions of the corresponding divisions 

 of the anterior and posterior extremities, we ob- 

 serve that they incline, when bent, in opposite 

 directions ; for in the former we find, in fol- 

 lowing the series of bones from the spine, that 

 the scapula proceeds forwards, the humerus 

 backwards ; the radius and ulna again forwards, 

 and the fore foot backwards, positions which are 

 exactly the reverse of the corresponding bones 

 of the hind limb. (See Fig. 218, page 507.) 



The weight of the body, in consequence of 

 this alternate direction of the angles at the suc- 

 cessive joints, must always tend, while the 

 quadruped is on its legs, to bend each limb : a 

 tendency which is required to be counteracted 

 by the actions of the muscles which are situated 

 on the external side of each of those angles. 

 These muscles are the extensors of the joints ; 

 that is, the muscles which tend to bring their 

 parts into a straight line. It is, in fact, by this 

 muscular action, much more than by simple 

 rigidity, that the limb supports the super- 

 incumbent weight of the body. It is evident 

 that greater muscular force is necessary for this 

 purpose when the joints are bent, than when 

 they are already extended ; and the portions of 

 the fore legs being naturally in this condition, 

 require less power than those of the hinder legs 

 to retain them in their proper relative positions. 



