OF STANDING. 65 



this account, the albatross, and some other aquatic birds 

 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 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 con- 

 tract firmly, so that it can sleep standing upon the roost 

 without effort, in perfect security. 



Fig. 46. 



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

 with the body bend readily in one direction only, 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 extreme of the body. The same antagonism 

 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 backwards, and the fore-arm forwards. Different 



