LOCOMOTION 347 



plane), and circumduction ; but the motion more often takes 

 place in a single direction around one determined axis. For 

 example, the legs generally move in the sagittal plane. The result 

 is an adaptation, an automatism, so perfect that the expenditure 

 of energy is a minimum. We should, therefore, always encourage 

 those modes of movement which " come naturally " to a man 

 unless it is clear that such movements are not capable of effecting 

 useful results. 



Muscles sometimes control two or more joints. Thus the 

 extensors which control the fingers are prolonged to the wrist. 

 This arrangement may be unfavourable for work. For example, 

 when the wrist is bent, the extensors are already considerably 

 stretched, and the fingers cannot be strongly flexed. Thus if the 

 hand is clenched on an object the grip can be relaxed if the wrist 

 is forcibly bent down. 



The same member can call into play in its movements a whole 

 group of muscles, a group whose constitution varies according 

 to circumstances. We think of the fore-arm as being necessarily 

 controlled by the muscles of the upper arm ; but the powerful 

 muscles of the shoulder make also their contribution. To speak 

 quite accurately, the elbow joint operates the tractive movement 

 of the hands ; while the muscles of the shoulder generally operate 

 the arms. The very mobile wrist joint ( 75) is hardly subjected 

 to any effort. It is also very delicate and more adapted to the 

 exercise of speed than of force. The supination and pronation 

 of the hand are effected by the radius and not by the bones of the 

 wrist, although the latter have really several degrees of liberty. 



To the action of the muscles which actually produce move- 

 ment we must, as we have already seen (vide 87) add the action 

 of their antagonists. As Duchenne de Boulogne and Braune 

 and Fischer have shown, it is by a combination of muscular efforts 

 that a limb is moved in the direction, and at the speed, required. 

 Hence, if we wish to estimate the muscular work performed in 

 flexing the forearm, we must know, not merely the force exerted 

 by the biceps, and the magnitude of the displacement, but also 

 the work done by the triceps, its antagonist. The muscular system 

 functions under the control of co-ordinated nervous excitations, 

 It is really this co-ordination which governs and regulates human 

 movement in centra-distinction to the crude idea of muscular 

 antagonism. The action of the extensors, for example, is not 

 always antagonistic to that of the flexors. Thus when a man, 

 standing, bends forward his head, the extensor muscles alone, by 

 their gradual relaxation, contract, -and guide the movement of 

 the head at the required speed. 



Thus muscles act in combination and not in opposition. To 

 raise the arm, the abductor, adductor and levator muscles come into 



