472 MOTION. 



166, 3, the pelvis is instinctively, by various muscles, made to 

 rotate on the head of the left fernur at the acetabulum, to the 

 left side, so that the weight may fall over the line of support 

 formed by the left leg at the time that the right leg is swing- 

 ing forwards, and leaving all the work of support to fall on 

 its fellow. Such a " rocking " movement of the trunk and 

 pelvis, however, is but an awkward manner of doing what can 

 be done more gracefully by combining a slight " rocking" with 

 a movement of the whole trunk and leg over the foot which 

 is being planted on the ground (Fig. 167); the action being 



FIG. 167. 



accompanied with a compensatory outward movement at the 

 hip, more easily appreciated by looking at the figure (167) 

 than described. 



Thus the body in walking is continually rising and swaying 

 alternately from one side to the other, as its centre of gravity 

 has to be brought alternately over one or other leg ; and the 

 curvatures of the spine are altered in correspondence 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 calf- 



