CAUSES AND PHENOMENA OF MOTION. 43 



There is one more thing to be noticed especially in the act of walking. 

 Inasmuch as the body is being constantly supported and balanced on each 

 leg alternately, and therefore on only one at the same moment, it is evi- 

 dent that there must be some provision made for throwing the centre of 

 gravity over the line of support formed by the bones of each leg, as, in its 

 turn, it supports the weight of the body. This may be done in various 

 ways, and the manner in which it is effected is one element in the differ- 

 ences which exist in the walking of different people. Thus it may be 

 done by an instinctive slight rotation of the pelvis on the head of each 

 femur in turn, in such a manner that the centre of gravity of the body 

 shall fall over the foot of this side. Thus when the body is pushed on- 

 ward and upward by the raising, say, of the right heel, as in Fig. 291, 3, 

 the pelvis is instinctively by various muscles, made to rotate on the head 

 of the left femur 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 swinging forward, and leaving all the work of support to 

 fall on its fellow. Such a "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 (Fig. 292)-, the action 



FIG. 292. 



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 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 alter- 

 nately 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-muscles, that the body is jerked 



