PHYSIOLOGY OF MOVEMENT. 747 



seldom still in the horse. It is characterized by the fact that the eleva- 

 tion of the second leg on the same side occurs sooner than in the walk. 



The walk merges into the amble when, the body being supported by 

 the two legs on the same side, the two opposite legs are elevated simul- 

 taneonsty instead of separately, as in the walk (Fig. 308). 



In the walk the fore leg is alwa} T s one-half the extent of its move- 

 ment behind the hind leg on the same side, while in the amble both legs 

 on one side move together, so that, therefore, there is a regular change 

 between the feet on each side of the body. Consequently, in the amble 

 the centre of gravity is first shifted to the one side and then to the 

 other, the length of the step in the amble and walk being the same. But 

 from the fact that the supporting limbs are on the same side of the body, 

 to preserve equilibrium the movements must be more rapidly performed 



FIG. 308. THE AMBLE. (Colin.) 



than in the walk. The gait is, therefore, a faster one, the greater rapidity 

 of the pace being accomplished by the reduction of the time, which cor- 

 responds to the half of the duration of the movement of one leg, since on 

 each side one-fourth of the time is saved. The rate of movement in 

 pacing may approach that of the trot, the velocity often rising to three 

 meters per second. Since the two unilateral propulsive and the two 

 swinging feet always move together, and are always at one time in the 

 same phase of motion, the swinging feet strike the ground together, so 

 that after one pace but two strokes of the feet have been heard. 



In the rack, which is simply a modification of pacing, the uni- 

 lateral feet act together, but the hind leg in propulsion is somewhat 

 later than the fore foot of the same side in leaving the ground. Four 

 strokes of the feet are heard in this gait, two rapidly following sounds 

 when the feet of one side strike the ground, separated by a longer interval 



