THE ACTION OF LOCOMOTOR MECHANISMS. 419 



joints, which would incline the body to fall backward. The line of gravity 

 of the head, trunk, and thighs falls slightly behind the axis of the knee- 

 joints, and the line of gravity of the whole body slightly in front of a line 

 connecting the two ankle-joints, so that the weight of the body would tend to 

 flex the knee- and ankle-joints. 



We cannot here consider in detail the mechanical conditions which limit 

 the movements possible to the different joints in the erect position of the body. 

 Although these conditions help to support the body in the upright position, 

 they are not alone sufficient to the maintenance of this posture, as is shown by 

 the fact that the cadaver cannot be balanced upon its feet. That standing 

 requires the action of the muscles is further proved by the fatigue which is 

 experienced when one is forced to stand for a considerable time. The body 

 may be supported in the standing position in various attitudes. Thus, the 

 soldier standing at " attention " places the heels together, turns the toes out, 

 makes the legs straight and parallel, so as to extend the knees to their utmost, 

 tilts back the pelvis, straightens the spine, and looks directly forward. In 

 this position many of the muscles are relieved from action by the locking 

 of the hip- and knee-joints. The tilting backward of the pelvis causes the 

 line of gravity to fall slightly behind the axis of rotation of the hip-joint 

 and puts the strong ilio-femoral ligament on the stretch, which balances 

 the tendency of the weight of the body to extend the hip. The line of 

 gravity would fall slightly behind the axis of rotation of the knee, and tend 

 to cause flexion; but when the joint is extended, the thigh, because of the 

 horizontal curvature of the internal condyle, receives a slight inward rota- 

 tion, and the knee cannot be flexed without a corresponding outward rota- 

 tion. In standing with the feet turned out, this rotation movement is 

 prevented by the same ilio-femoral ligament that locks the hip-joint. The 

 ankle-joint cannot be locked, and the tendency of the body to tall forward is 

 resisted by the strong muscles of the calf of the leg. The creel position of 

 the spine and the balancing of the head have likewise to be maintained by 

 the action of muscles. Although this position gives great stability, it cannot 

 be long maintained with comfort. It is less fatiguing to allow the joints to 

 be a little more flexed, and to keep the balance by the action of the muscles, 

 the position being frequently changed so as to bring fresh muscles into action. 

 Perhaps the most restful standing position is round in letting the weight of 

 the body be supported on one leg, the pelvis being tilted so as to bring the 

 weight of the body over the femur, and the other being used as a prop to pre- 

 serve the balance. Absolute stability in standing is impossible for any length 

 of time; the body is continually swaying, and a pencil resting on a writing 

 surface placed upon the head is found to write a very complicated curve. 

 There is a normal sway for every individual, and this may become markedly 

 exaggerated under pathological conditions. The maintenance of equilibrium 

 requires that afferent impulses shall continually pass to the co-ordinating cen- 

 tres which control the muscles involved in this act, and if any of these normal 

 impulses fail the sway of the body is increased ; for example, it is more diffi- 

 cult to stand steadily when the eyes are closed than when they are open ; the 



