746 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



direction of the fore extremity becomes changed, so that instead of being' 

 vertical it is directed from above downward and from before backward. 

 The foot must, therefore, be raised from the ground and advanced in the 

 direction in which the trunk is moving, and, striking the ground, again 

 becomes vertical at the moment when the pendulous hind leg strikes the 

 ground. At the moment, then, when the fore leg becomes vertical, the 

 propulsive movement in the opposite hind leg occurs. Then the motion 

 commences in the opposite hind leg and fore leg, the alternation of the 

 limbs being perfectly regular. Thus, suppose the right hind foot to be the 

 propulsive foot, the left fore foot is extended on the ground, the left 

 hind leg is swinging forward, and the right fore foot is just leaving the 

 ground, the body thus being supported on a diagonal pair of feet. Then 

 the right hind foot leaves the ground, the left fore foot is on the ground, 

 the limb having passed the vertical line, the left hind leg is giving the 

 impulse to the body, while the right fore foot is swinging forward. The 

 body is then supported on unilateral feet, the support being of shorter 

 duration than in the first case. Hence, in walking, there is always at 

 an}' one time an anterior and a posterior limb in the air and an anterior 

 and posterior limb acting as a support, the limbs being raised and replaced 

 in such an order that of the two limbs in the air one is always in advance 

 the half of its course over the other, while of the supporting limbs in 

 one the line of the support is vertical when the other first reaches the 

 ground. 



In quadrupeds the length of the step is measured by the distance 

 between the track formed by each separate foot, so that it is, therefore, 

 twice as extensive as in man. When the hind feet reach the foot-prints 

 of the fore feet it is twice as long as the base of support, i.e., the dis- 

 tance between the pairs of feet at rest. As the motion in walking in 

 quadrupeds is produced by the action of the diagonal extremities, the 

 centre of gravity is first moved to one side and then returned to the 

 centre and then cast to the opposite side. The duration of the step is 

 dependent upon the duration of the swinging of the leg. The higher an 

 animal raises its leg, the shorter is the pendulum and the more rapidly it 

 swings. 



The rapidity of motion in the walk in the horse varies from one to 

 two meters in the second. In drawing heavy weights or in a very slow 

 walk the relative movement of the feet is somewhat different from that 

 detailed above. Then the elevation of each foot is delayed until the sup- 

 porting feet are firmly on the ground ; the body is then always supported 

 on three limbs, by which equilibrium is better preserved. The same 

 sequence of movement is, however, preserved. 



(b) The Amble. — The amble is a modification of the walk, and is 

 seen in the dromedary, giraffe, and occasionally in ruminants, and more 



