20 THE ENGINES OF THE HUMAN BODY 



thigh bone. It is quite evident, however, when we come 

 to watch what really happens as we walk, that the foot of 

 the stationary limb is being all the time adjusted to the 

 ground, the arch of the foot is being supported, the ankle- 

 joint is being steadied, by the ever-alert action of the 

 muscles and tendons which are placed in the leg and foot. 

 They also are in action all the time. The knee-joint too 

 has to be kept under control and supported. Indeed all 

 the muscles of the stationary limb are set in motion and 

 kept at a necessary pitch of work during the period that 

 the other limb is swinging forwards. There is one great 

 muscle-engine in front of the thigh — the quadriceps or 

 extensor of the knee — which is not thrown into action in 

 the stationary limb. One has only to take hold of the 

 knee-cap to make certain of this ; it will be found not to 

 be fixed but to be quite loose. Thus in taking only a single 

 step almost every one of the muscles or engines of the 

 lower limbs — 108 in number — are set going, not all at 

 once but in a definite and wonderfully regulated order. 



