ANIMAL LOCOMOTION. 39 



The right hind foot of the horse (series 631, in the interval between 

 11 and 12) is in the same attitude. (See p. 43.) 



Wherever occurring the position illustrates the tendency for one 

 or two feet of the same, or opposed sides, to support the trunk at 

 some point between the vertical lines of the limbs. It is proposed 

 to call such support central. Central support is opposed to the 

 support of the limbs in the extreme or forward motion of the fore 

 limb and the backward motion of the hind limb, which may be 

 called terminal. 



The Ilovements of Limbs. 



If a limb can be conceived moving in vacuo it can be at once 

 understood that propulsion is impossible. For propulsion can 

 follow only upon the initiation of an impetus, and this in turn 

 only by the resistance of the limb against the medium in which 

 the animal is moving, or, in the case of the terrestrial animal, the 

 surface of the ground. 



The resistance of the air and the water is so much less than 

 that of the earth that the acts of flying and of swimming become 

 radically different from those of walking, of running, or of any 

 allied movement. In flying and swimming the resistance made 

 by the limb against the medium in effecting an impetus does not 

 arrest the movement of the pinion or the foot; whereas in terres- 

 trial movements the instant that the foot strikes the earth the 

 resistance is great and the arrest is complete. 



In the swimming-turtle the first stage of the recov^er drives the 

 foot in spite of the resistance of the water to the point at which 

 the second stage begins. With some slight modifications the same 

 is true of fossorial animals. Tlius in flying, in swimming, and in 

 burrowing the limb describes a continuous movement which unites 

 the path of the stroke to that of the recover. In the animal 

 moving on the surface of the ground, the foot being brought to 

 rest, an absolute break occurs between the beginning of the act of 

 recover and its completion, — the time which would be required to 

 describe the interval and thus to complete the union corresponds 

 to the period that the foot is on the ground. 



The limb rests on the ground until the trunk moves beyond 

 the point at which it can maintain itself. It is lifted at intervals 

 which are dependent upon the momentum of the moving mass. 



