PHYSIOLOGY OF MOVEMENT. 745 



but as the impulse of the supporting feet is not simultaneously pro- 

 duced, four strokes of the hoof are heard with each step, but at unequal 

 intervals ; for when the support comes from the two feet on the same 

 side the preservation of equilibrium compels a more rapid shifting of 

 the body weight to the opposite side than when the diagonal limbs are 

 in action. 



If we commence the consideration of the act of walking in quadru- 

 peds at the moment in which the one hind leg after completing its pen- 

 dulum motion comes again to the ground, then it occupies a position in 

 which the axis of the limb is directed from behind forward and from 

 above downward. The centre of gravity in consequence of the propul- 

 sive movement advances, the leg and trunk describe an arc of a circle 

 around the foot as a centre, so that the axis of the leg gradually 

 becomes vertical and then advances, and the axis now tends to become 



FIG. 307. THE WALK. (Colin.) 



directed from before backward and from above downward. At this 

 moment active contraction of the extensor muscles occurs and the leg 

 which was the supporting member now becomes an active propulsive 

 member. It leaves the ground, becomes flexed, and swings forward, 

 the femoral articulation now being the centre of movement and the foot 

 describing an arc of a circle until it becomes advanced in front of 

 its point of support. It then strikes the ground and the movements 

 are repeated (Fig. 307). At the moment when the hind leg is thus 

 swinging forward the fore leg of the opposite side is likewise advanced, 

 the movement commencing as soon as the propulsive hind leg is in a 

 vertical line. Through the forward movement of the trunk the line of 



