RUNNING 479 



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 cen- 

 ter of gravity of the body shall fall over the foot of this side. Thus when the 

 body is pushed onward and upward by the raising, say, of the right heel, as in 

 figure 340, 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, 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 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 leap- 

 ing 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 

 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 

 number of combined and harmoniously arranged muscular contractions which 

 are necessary for even the simplest acts of locomotion. 



II. THE PRODUCTION OF THE VOICE. 



Before commencing the consideration of the Nervous System and the 

 special Senses it will be convenient to consider first speech, the production of 

 the human voice, and the physiology of the larynx as a muscular apparatus. 



