946 THE CENTRAL NERVOUS SYSTEM 



non-muscular structures, the bones and ligaments. But muscles also 

 play an essential part, for it is only peculiarly-gifted individuals, like 

 some of the fishermen of the North Sea, who can go to sleep on their 

 feet, and a dead body cannot be made to stand erect. The condition 

 of equilibrium is that the perpendicular dropped from the centre of 

 gravity to the ground should fall within the base of support 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 contrac- 

 tion 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 articulation; 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 plane 

 passing through the ankle-joints. Equilibrium i.-, maintained by con- 

 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. 

 The 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 



