OF STANDING. 89 



which have the feet placed very far back, cannot use them 

 for walking. 



183. The more numerous and the more widely separated 

 are the points of support, the firmer an animal stands. On 

 this account, quadrupeds are less liable to lose their balance 

 than birds. If an animal has four legs, it is not necessary 

 that they should have a broad base. Thus we see that 

 most quadrupeds have slender legs, touching the earth by 

 only a small surface. Broad feet would interfere with each 

 other, and only increase the weight of the limbs, without 

 adding to their stability. Birds are furnished with long toes, 

 which, as they spread out, subserve the purpose of tripods. 

 Moreover, the muscles of the toes are so disposed that the 

 weight of the bird causes them to grasp firmly ; hence it 

 is enabled to sleep standing in perfect security upon the roost, 

 without effort. 



Fig. 46. 



184. In quadrupeds, the joints at the juncfion of the limbs 

 with the body bend freely in only one direction, that is, to- 

 wards the centre of gravity ; so that if one limb yields, the 

 tendency to fall is counteracted by the resistance of the 

 limbs at the other extremity of the body. The same antag- 

 onism is observed in the joints of the separate limbs, which 

 are flexed alternately in opposite directions. Thus the thigh 

 bends forwards, and the leg backwards ; while the arm bends 



8* 



