542 MUSCLE-NERVE PHYSIOLOGY 



When the right foot has reached the ground the action of the left leg has 

 not ceased. The calf muscles of the latter continue to act, and, by pulling up 

 the heel, throw the body still more forward over the right leg, now bearing 

 nearly the whole weight, until the time when the left leg should again swing 

 forward, and the left foot be planted on the ground to prevent the body 

 from falling prostrate. As at first, while the calf muscles of one leg and foot 

 are preparing, so to speak, to push the body forward and upward from 

 behind by raising the heel, the muscles on the front of the trunk and the 

 same leg (and of the other leg, except when it is swinging forward) are 

 helping the act by pulling the legs and trunk, so as to make them incline 

 forward, the rotation in the inclining occurring mainly at the ankle joint. 

 Two main kinds of leverage are, therefore, employed in the act of walking, 

 and if this idea be firmly grasped, the details will be understood with com- 

 parative ease. One kind of leverage employed in walking is essentially 

 the same with that employed in pulling forward the pole, as in figure 339. 

 And the other, less exactly, is that employed in raising the handles of a 

 wheelbarrow. Now, supposing the lower end of the pole to be placed in 

 the barrow, we should have a very rough and inelegant, but not altogether 

 bad representation of the two main levers employed in the act of walk- 

 ing. The body is pulled forward by the muscles in front, much in the 

 same way that the pole might be by the force applied at p, while the raising 

 of the heel and pushing forward of the trunk by the calf muscles are roughly 

 represented on raising, the handles of the barrow. The manner in which 

 these actions are performed alternately by each leg, so that one after the other 

 is swung forward to support the trunk, which is at the same time pushed 

 and pulled forward by the muscles of the other, may be gathered from the 

 previous description. 



There is one more thing to be especially noticed 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 evident 

 that there must be some provision made for throwing the center 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 walking of different people. Thus it may be done by ah in- 

 stinctive slight rotation of the pelvis on the head of each femur in turn, in 

 such a manner that the center 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 



