PROGRESSION OF ANIMALS, xiii.-xiv. 



outwards, as in D. No biped or quadruped bends its 

 limbs as in figure A or B, but quadrupeds bend them 

 as in C. The bendings illustrated by figure D occur 

 in none of the quadrupeds except the elephant, and 

 in the movement of the arms and legs by man, for 

 he bends his arms concavely and his legs convexly. 



In man the bendings of the limbs always take 

 place alternately in opposite directions ; for example, 

 the elbow bends concavely but the wrist convexly, 

 and the shoulder again convexly. Similarly in the 

 legs, the thigh bends concavely, the knee convexly, 

 and the foot, on the other hand, concavely. And 

 obviously the lower limbs bend in opposite directions 

 to the upper ; for the origin of movement bends in 

 opposite directions, the shoulder convexly and the 

 thigh concavely ; therefore also the foot bends con- 

 cavely and the \\Tist convexly. 



XIV. The bendings, then, of the legs take place 

 in this manner and for the reasons stated. But the 

 back legs move diagonally in relation to the front 

 legs ; for after the right fore leg animals move the 

 left hind leg, then the left fore leg, and after it the 

 right hind leg. The reason is that, if they moved the 

 fore legs at the same time and first, their progression 

 would be interrupted or they would even stumble 

 forward, with their hind legs as it were trailing behind. 

 Further, such movement would not be walking but 

 jumping ; and it is difficult to keep up a continuous 

 movement from place to place by jumping. An illus- 

 tration of this is that, in actual fact, horses that move 

 in this manner,^ for example in religious processions, 

 soon become tired. For this reason, then, animals do 



" i.e. prancing instead of walking. 



529 



