THE CENTRAL NEVROUS SYSTEM 839 



that is, within the area enclosed by the outer borders of the feet 

 and lines joining the toes and heels respectively. The centre of 

 gravity alters its position with the position of the body, which tends 

 to fall whenever the perpendicular cuts the ground beyond the base 

 of support. 



In the comfortable and natural erect position the centre of gravity 

 of the head is a little in front of the vertical plane passing through the 

 occipital condyles, and as much as 4 centimetres in front of the 

 vertical plane passing through the ankle-joints. A certain degree of 

 contraction of the muscles of the nape of the neck is required to 

 balance it. When these muscles are relaxed, as in sleep, the head 

 must fall forward, and this is the reason why Homer or any lesser 

 individual nods. In animals which go upon all-fours none of the 

 weight of the head bears directly upon the occipito-atloid articula- 

 tion ; its support by muscular action alone would be an intolerable 

 fatigue, and the ligamentum nuchae is specially strengthened to hold 

 it up. 



The vertebral column is kept erect by the ligaments and muscles 

 of the back. The centre of gravity of the trunk lies almost vertically 

 over the horizontal line joining the two acetabula, but the centre of 

 gravity of the whole body is about the level of the third sacral 

 vertebra, and a little more than 4 centimetres in front of the vertical 



Elane passing through the ankle-joints. Equilibrium is maintained 

 y contraction 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 

 counterbalanced by the muscular exertion associated with 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 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. The light touch of this foot varies slightly from time to timr, 

 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 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 



