7-38 A MANUAL OF PHYSIOLOGY 



perpendicular passing through it, are detected, and equilibrium is 

 maintained by adjustment of the amount of contraction of this or the 

 other muscular group. 



In standing at ' attention,' the heels are close together, the legs 

 and back straightened to the utmost, and the head erect ; the weight 

 falls equally upon both legs, but the advantage is much more than 

 counterbalanced by the considerable muscular exertion required to 

 maintain this more ornamental than useful position. In 'standing 

 at ease,' practically the whole weight is supported by one leg, the 

 perpendicular from the centre of gravity passing through the knee 

 and ankle-joints. The centre of gravity is brought over the support- 

 ing leg by flexure of the body to the corresponding side, and com- 

 paratively little muscular effort is required. The other foot rests 

 lightly on the ground, the weight of the leg itself being almost 

 balanced by the atmospheric pressure acting upon the air-tight and 

 air-free cavity of the hip-joint. The light touch of this foot varies 

 slightly from time to time, so as to maintain equilibrium. 



When the arms or head are moved, or the body swayed, the 

 centre of gravity is correspondingly displaced, and it is by such 

 movements that tight-rope dancers continue to keep the perpen- 

 dicular passing through it always within the narrow base of support. 



In sitting, the base of support is larger than in standing, and the 

 equilibrium therefore more stable. The easiest posture in sitting 

 without support to the back or feet is that in which the perpendicular 

 from the centre of gravity passes through the horizontal line joining 

 the two tubera ischii. 



Locomotion. In walking, the legs are alternately swung forward 

 and rested on the ground. In military marching, it is directed that 

 toe and heel be simultaneously set down. But with most persons 

 the swinging foot first strikes the ground by the heel ; then the sole 

 comes down, the heel rises, the leg is extended, and, with a parting 

 push from the toe, the leg again swings free. By this manoeuvre the 

 body is raised vertically, tilted to the opposite side, and also pushed 

 in advance. 



The forward swing of the leg is only slightly, if at all, due to 

 muscular action ; it is more like the oscillation of a pendulum dis- 

 placed behind its position of equilibrium, and swinging through that 

 position, and in front of it, under the influence of gravity. For this 

 reason the natural pace of a tall man is longer and slower than that 

 of a short man ; but it may be modified by voluntary effort, as when 

 a rank of soldiers of different height keeps step. 



The lateral swing of the body is illustrated by the everyday 

 experience that two persons knock against each other when they 

 try to walk close together without keeping step. In step, both swing 

 their bodies to the same side at the same moment, and there is no 

 jarring. 



Even in the fastest walking there is a short time during which 

 both foet are on the ground together, the one leg not beginning its 

 swing until the other foot has been set down. In running, on the 

 other hand, there is an interval during which the body is completely 

 in the air. 



