LOCOMOTION 509 



"rocking" movement of the trunk and pelvis, however, is accompanied 

 by a movement of the whole trunk and leg over the foot which is being 

 planted on the ground, figure 341, the action being accompanied with a 

 compensatory outward movement at the hip, more easily appreciated by 

 looking at the figure (in which this movement is shown exaggerated) than 

 from the description. 



Thus the body in walking is continually rising and swaying alternately 

 from one side to the other, as its center of gravity has to be brought alternately 

 over one or the other leg; and the curvatures of the spine are altered in cor- 



FIG. 341. 



respondence with the varying position of the weight which it has to support. 

 The extent to which the body is raised or swayed differs much in different 

 people. 



In walking, one foot or the other is always on the ground. The act of 

 leaping or jumping consists in so sudden a raising of the heels by the sharp 

 and strong contraction of the gastrocnemius muscles that the body is 

 jerked off the ground. At the same time the effect is much increased by 

 first bending the thighs on the pelvis, and the legs on the thighs, and then 

 suddenly straightening out the angles thus formed. The share which this 

 action has in producing the effect may be easily known by attempting to 

 leap in the upright posture, with the legs quite straight. 



Running. Running is performed by a series of rapid low jumps pro- 

 duced by each leg alternately; so that, during each complete muscular act 

 concerned, there is a moment when both feet are off the ground. 



In all these cases, however, the description of the manner in which any 

 given effect is produced, can give but a very imperfect idea of the infinite 



