9M THE CENTRAL NERVOUS SYSTEM 



traction of the muscles of the back and of the legs. By means of the 

 muscular sense, and the tactile sensations set up by the pressure of the 

 soles on the ground, alterations in the position of the centre of gravity, 

 and consequent deviations of the perpendicular passing through it, 

 are detected, and adjustment of the amount of contraction of this or 

 the other muscular group is promptly made. 



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 may be more than counter- 

 balanced by the muscular exertion associated with this more orna- 

 mental 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-joint. The 

 centre of gravity is brought over the supporting leg by flexure of the 

 body to the corresponding side, and comparatively 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, 

 he light touch of this foot varies slightly from time to time, so as to 

 maintain equilibrium. 



When the head or arms 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 perpendicular 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. 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 mus- 

 cular action; it is more like the oscillation of a pendulum displaced 

 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 walk- 

 ing on level ground there is a short time during which both feet touch 

 the ground together, the one leg not beginning its swing until the 

 other foot has begun to be set down. In running, on the other hand, 

 there is an interval during which the body is completely in the air, 

 while in walking uphill or in carrying a load the one foot is not raised 

 until the other has been firmly planted. 



Functions of the Cerebral Cortex. When an animal, like a frog, 

 is deprived of its cerebral hemispheres, the power of automatic 

 voluntary movement appears to be definitively and entirely lost. 



