526 HANDBOOK OF PHYSIOLO&Y. 



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

 asmuch as the body is being constantly supported and balanced on each leg 

 alternately, and therefore on only one at the same moment, it is evident 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 differences which exist in the walk- 

 ing 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 

 r< =s pushed onward and upward by the raising, say, of the right heel, as in fig. 

 337, 3, the pelvis is instinctively by various muscles made to rotate on the 

 uead 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 



Fig. 338. 



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. 338) : 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 

 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 corre- 

 spondence 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 off the ground. At the 

 same time the effect is much increased by first bending the thighs on the pel- 



