THE NEUROMUSCULAR MECHANISM 95 



move rather than to cause a movement. When a limb 

 which was previously held firm by such means is suddenly 

 bent, it can be shown that two changes have occurred. 

 Not only have the flexors shortened, but their opponents, 

 the extensors, have been inhibited. They do not actually 

 push, but they cease, in a perfectly definite way, to resist 

 the movement which is to be brought about. Sherrington 

 showed this in the most convincing manner by an experi- 

 ment in which the rotation of the eyeball was under ob- 

 servation. He first determined conditions under which 

 he could unfailingly secure the turning of the eye in a 

 certain direction, we will say, toward the nose. The 

 responsibility would seem to lie with the internal rectus 

 muscle. By cutting the nerve of this muscle he paralyzed 

 it. Then, repeating the stimulation which had caused the 

 reaction before, he still saw the eyeball swinging in the 

 old direction, though not so far as before. 



What happened, that the eye should continue to turn 

 as though drawn by a muscle now disconnected from the 

 nervous system? The movement could be explained only 

 upon the supposition that the antagonistic muscle did not 

 fully relax until the action of its governing neurons was 

 suspended at a moment when those normally presiding 

 over its opponent were stimulated. A diagram will be 

 helpful. In the leg of an animal the same type of inhibi- 

 tion has been demonstrated. If a reflex extension has been 

 secured so frequently as to be predictable and a flexor 

 muscle is then detached from the joint, but left in connec- 

 tion with its nerve, this muscle will be found to lengthen 

 whenever the reflex is executed. It is the "law of recipro- 

 cal innervation" that when the centers excite a set of 

 muscles the antagonists are inhibited. By inhibition 

 in this case is meant the cessation of a mild continuous 

 stimulation which is normally brought to bear upon all 

 muscles excepting at such times (Fig. 18). 



It will now be evident that a coordinated movement 

 requires the excitation of some muscles, each in an ap- 

 propriate degree, and the abolition of the habitual residue 



