434 THK MECHANICAL FUNCTIONS. 



with the joint of the hip, and even extends farther 

 backwards, the thigh bone must necessarily be 

 brought forwards ; then the tibia and fibula, which 

 compose the bones of the leg, must be carried 

 backwards to their junction with the bones of the 

 foot ; and again, the foot must be turned forwards 

 in its whole length from the heel to the extremi- 

 ties of the toes. On comparing the positions of 

 the corresponding divisions of the anterior and 

 posterior extremities, we observe that they incline, 

 when bent, in opposite directions ; for in the 

 former we find, in following 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 449.) 

 The weight of the body, in consequence of this 

 alternate direction of the angles at the successive 

 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 ex- 

 tensors 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 su- 

 perincumbent 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 



