PHYSIOLOGY OF MOVEMENT. 737 



Although the flexors participate in many joints, as an example of which 

 it is only necessary to mention the coraco-radial, which, as already stated 

 offers resistance to the flexion of the arm on the forearm, this action is 

 only rendered possible when the radius is fixed by the olecranon muscles. 

 The support of the head in standing in the case of quadrupeds is accom- 

 plished mainly by the ligamentum nucae, which, originating in the spinous 

 processes of the dorsal vertebrae, terminates in the occipital protuber- 

 ance, being connected at the same time with all the cervical vertebrae. 



While standing quietly the weight of the horse is supported by both 

 of the fore legs and only one hind leg, the other hind leg being flexed and 

 only the tip of the toe touching the ground. By this means the muscles 

 of the posterior extremity which are concerned in the act of standing- 

 are enabled to rest, the weight being borne alternately at varying inter- 

 vals by the opposite hind legs. From the fact that the centre of gravity 

 in quadrupeds lies nearer to the anterior than the posterior extremities, 

 the fore limbs sustain the greater part of the weight of the bod}'. In 

 riding two-thirds of the weight of the rider are borne by the anterior 

 extremities and one-third by the posterior. 



In every form of animal locomotion the position of the centre of 

 gravity of the body in space is changed, inertia tending to continue the 

 motion inaugurated by the muscular contraction until friction, resistance 

 of the atmosphere, or opposed muscular action arrests it. 



In man, movements of locomotion are much simpler than in quad- 

 rupeds, and will, therefore, be first analyzed. The movements in man 

 consist in walking, running, and jumping. In the act of walking in man, 

 by the alternate action of each leg the centre of gravity is advanced so 

 that at each step there is a moment in which the body rests vertically on 

 the foot of one leg (say the right), while the other (the left) is inclined 

 obliquely behind with the heel raised and the toe resting on the ground. 

 Then the latter, slightly flexed to avoid touching the ground, is swung 

 forward like a pendulum, and the toe of the moving leg (the left) 

 then brought to the ground. On this point, as a fulcrum, the body is 

 moved forward by a propulsive act of the supporting leg (the right), the 

 centre of gravity becoming thus advanced until it lies vertically over the 

 leg (the left) which has last touched the ground. The body then rests 

 vertically on the left foot, the right now being directed obliquely back- 

 ward. The propulsive movement of the active leg, the one concerned in 

 pushing the body off the ground, gives sufficient impetus to the centre 

 of gravity to carry it by inertia beyond the vertical line on the passive 

 or supporting leg, so that this movement from inertia assists in swinging 

 forward the active leg until it advances a step beyond the passive sup- 

 porting leg (Fig. 299). Hence, after the act of walking is once started, 

 inertia is largely instrumental is maintaining the motion, and but slight 



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