THE SKELETON AND MUSCLES 197 



In birds, where the centre of gravity lies in front of the hip- 

 joint, movements of the head, neck, wings and tail are called 

 into play to preserve the balance. In beasts, t\vo legs swing 

 while the other two simultaneously support the body (Fig. IO3\ 

 The propulsive force is obtained from the hind-legs, while the 

 fore-legs merely support the hindquarters as they are thrust 

 forward. There is a regular alternation of the pairs of legs, 

 first a hind-leg then a fore-leg in such fashion that while the 

 one hind-leg is swinging the fore-leg of the " pushing off" side 

 swings too. Comparing the distance between the prints of each 

 individual foot the track of quadrupeds is twice as long as that 

 of men, amounting in a horse without a load to if metres, with 

 a load only - 5-'8 metre. 



FIG. 104. Erect gait of gibbon. FIG. 105. Erect gait of orang-utan. 



The erect gait is no monopoly of man, for the anthropoid 

 apes (Figs. 104, 105) and bears can also walk upright. But 

 there is this all-important difference, that apes and bears can 

 only adopt this attitude for a short time as it is too fatigu- 

 ing to be long maintained, while it is man's natural position 

 and his body is admirably adapted to movements in the erect 

 position. The body rests on one leg (the supporting or thrust- 

 ing leg) (Fig. 106), while the other (the hanging leg) (Fig. 107) 

 swings forwards. Each leg is alternately supporting and at 

 rest, and the centre of gravity of the body is constantly moving 

 from one side to the other, rising, lowering or turning. A 

 consideration of the movements in detail shows that there is a 

 phase in which only one foot touches the ground alternating 



