THE ACTION OF LOCOMOTOR MECHANISMS. 859 



movements it is necessary that many fibres shall be placed side by side, as in 

 the case of the gluteus : " Many hands, light work." 



Standing-. In spite of the ease with which the many joints of the body 

 move, the erect position is maintained with comparatively little muscular 

 exertion. It is an act of balancing in which the centre of gravity of the 

 body is kept directly over the base of support. In the natural erect position 

 of the body the centre of gravity of the head is slightly in front of the oc- 

 cipito-atlantoid articulation, so that there is a tendency for the head to rock 

 forward, as is seen from the nodding of the head of one falling asleep. The 

 centre of gravity of the head and trunk together is such that the line of 

 gravity falls slightly behind a line drawn between the centres of the hip- 

 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, for the complete 

 extension of the knee, by bringing the line of gravity slightly in front of the 

 axis of rotation and tending to produce further extension, puts the ligaments 

 on the stretch and so locks the joint. Similarly, in the case of the hip-joint 

 the tilting backward of the pelvis causes the line of gravity to fall slightly 

 behind the joint and puts the strong ilio-femoral ligament on the stretch. The 

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

 resisted by the strong muscles of the calf of the leg. The erect 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 found 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 



