SYMMETRY AND HARMONY OF MUSCULAR MOVEMENTS. 831 



running, or leaping. The centre of gravity of the whole body lies, accord- 

 ing to Weber, in the promontory of the sacrum ; but, according to Meyer, 

 in the canal of the second sacral vertebra. As long as a perpendicular line 

 drawn from this point to the earth drops within the basis formed by the feet, 

 the erect posture can be maintained with comparative ease; if it drop out- 

 side that basis, the subject must inevitably fall. The chief muscles called 

 into play in the standing position are those which keep the head vertical on 

 the spinal column, as the recti, obliqui, sterno-mastoids, splenii, complexus, 

 etc., the extensors of the spine, as the erectores spinse and their prolonga- 

 tions, and the extensors of the thigh and leg ; but the opposite muscles are, 

 as a sailor would express it, "taut" and ready to contract instantaneously 

 and unconsciously on the slightest indication of loss of equilibrium. 



681. Walking. In this movement the body, supported in a nearly vertical 

 position, alternately rests on the right and left leg; the limb upon which it 

 does not rest swinging forward, like a pendulum, in a remarkably regular 

 manner, just sufficiently bent to avoid contact with the ground. This is not 

 effected, however, quite apart from muscular action, since, as M. Duchenne 

 has shown, certain muscular paralyses (of flexor muscles) prevent the oscil- 

 lation from occurring. 1 If we examine the successive movements which 

 occur in making two steps, it will be found that if the subject be standing 

 firmly with the left leg in advance, the first act consists in raising the heel 

 of the right foot, which is accomplished by the gastrocnemius and soleus 

 muscles ; and the weight of the whole body thus raised is by the simul- 

 taneous contraction of the psoas and iliacus muscles of the left leg thrown 

 over to the left side, resting for an instant upon the left foot. At this period 

 all the extensors of the left leg, the glutsei, the quadriceps extensor femoris, 

 the tibialis anticus, etc., are powerfully contracted. The right leg, the toe 

 of which is the last part which touches the ground, now swings forward, the 

 knee and hip joints being just sufficiently bent to enable the foot to clear acci- 

 dental irregularities of the surface. Lastly, the right heel is planted on the 

 ground in front, and is immediately followed by the whole surface of the foot ; 

 but coincidentally with this occur the elevation of the left heel and the throw- 

 ing over of the weight of the trunk to the right side, which is followed by the 

 swinging forward and implantation of the left foot, and so on alternately. 

 The weight of the legs in the act of swinging forward has been shown by 

 Weber to be materially diminished by the pressure exerted by the atmos- 

 phere in retaining the head of the femur in the acetabulum. The ordinary 

 rate of movement in walking for each person is to an important extent 

 regulated by the length of the leg, and the consequent duration of its pendu- 

 lum-like oscillation. Various compensatory movements in walking occur in 

 order to maintain the equipoise of the body ; amongst the most important of 

 these are the movements of the arms, for whilst the right leg is swinging for- 

 wards, the twisting of the trunk which occurs at this period would bring the 

 right shoulder forwards, but the right arm at this moment swings backwards 

 and the left forwards, and by thus generating a force in the opposite direc- 

 tion neutralizes this tendency. A corresponding compensation takes place 

 when the left leg swings forward, and this is effected by a good walker with- 

 out any sensible lateral twisting of the trunk. 2 The whole body rises and 

 falls to a small amount (li inch) in walking, etc. The trajectory of any 

 fixed point of the body, as the pubis, may, according to the formula of M. 

 Carlet, be inscribed in a hollow half cylinder with its concave portion up- 

 wards, at the base of which lie the minima, and on the sides of which the 



1 Duchenne (do Boulogne), Physiologie des Mouvements, Paris, 1867, p. 386. 



2 See Bishop in Cyclop, of Anat. and Phys., vol. iii, p. 460. 



