152 THE HUMAN BODY. 



knee being bent so as to raise the foot, the left leg swings for 

 wards at the hip-joint to make a fresh step. 



During each step the whole Body sways up and down 

 and also from side to side. It is highest at the mo- 

 ment when the advancing trunk is vertically over the 

 foot supporting it, and then sinks until the moment 

 when the advancing foot touches the ground, when it is 

 lowest. From this moment it rises as it swings forward 

 on this foot, until it is vertically over it, and then sinks 

 again until the other touches the ground; and so on. At 

 the same time, as its weight is alternately transferred from 

 the right to the left foot and vice versa, there is a slight 

 lateral sway, commonly more marked in women than in men, 

 arid which when excessive produces an ugly "waddling" 

 gait. 



The length of each step is primarily dependent on the 

 length of the legs; but can be controlled within wide limits 

 by special muscular effort. In easy walking little muscular 

 work is employed to carry the rear leg forwards after it has 

 given its push. When its foot is raised from the ground it 

 swings on, like a pendulum; but in fast walking the muscles, 

 passing in front of the hip- joint, from the pelvis to the limb, 

 by their contraction forcibly carry the leg forwards. The 

 easiest step, that in which there is most economy of labor, is 

 that in which the limb is let swing freely, and since a short 

 pendulum swings faster than a longer, the natural step of 

 short-legged people is quicker than that of long-legged ones. 



In fast walking the advanced or supporting leg also aids in 

 propulsion; the muscles passing in front of the ankle-joint 

 contracting so as to pull the Body forwards over that foot 

 and aid the push from the rear foot. Hence the fatigue and 

 pain in front of the shin which is felt in prolonged, very fast 

 walking. From the fact that each foot reaches the ground 

 heel first, but leaves it toe last, the length of each stride is 

 increased by the length of the foot. 



Running. In this mode of progression there is a moment 

 in each step when both feet are off the ground, the Body 

 being unsupported in the air. The toes alone come in con- 

 tact with the ground at each step, and the knee-joint is not 

 straight when the foot reaches the ground. When the rear 

 foot is to leave the support, the knee is suddenly straight- 

 ened, and at the same time the ankle-joint is extended so as 



