938 CENTRAL NERVOUS SYSTEM 



external rectus muscle of the left eye, an inhibition which is pronounced 

 enough to bring about a movement of the eyeball, and which exactly cor- 

 responds in point of time with the contraction of the external rectus of 

 the right eye. This movement, due to the atonicity of the external rec- 

 tus, does not however succeed in causing the eye to rotate beyond the 

 midline of the field of vision. This is an instance of a willed reciprocal 

 inhibition ; i. e., a reciprocal inhibition brought about by stimuli coming 

 from the motor centers in the cerebrum. The same result may be 

 obtained by electric stimulation of the center for eye movements on the 

 cerebral cortex. 



The most important details concerning the mechanism of reciprocal 

 inhibition have been obtained by studying the flexion reflex in a spinal 

 animal which has completely recovered from shock. In such an animal 

 the tonus of the extensor muscles of the knees is well marked. If we pre- 

 vent the flexors from acting on the knee joint and the leg is held in an ex- 

 tended position, irritation of the skin of the leg will cause the flexion of the 

 disconnected hamstring muscles simultaneously with a visible relaxation 

 of the extensors. If the leg is held properly, this relaxation may be 

 marked enough to cause a slight flexion at the joint under the influence 

 of gravity. This experiment is very striking when performed on a de- 

 cerebrate animal, in which, as we have seen, the extensor muscles of the 

 limb are in a permanent state of hypertonicity. 



Reciprocal inhibition can also be demonstrated by stimulating the 

 central end of suitable afferent nerves that is, certain afferent nerves 

 acting on the same groups of neurons will produce a flexion reflex, others 

 an extension reflex; thus, stimulation of the homolateral peroneal nerve 

 produces a flexion reflex of the hind limb (excitatory for flexors, in- 

 hibitory for extensors), whereas stimulation of the contralateral peroneal 

 nerve produces an extension (inhibitory for flexors, excitatory for ex- 

 tensors). (Fig. 239.) 



Before we can conclude that the two elements in reciprocal inhibition 

 are part and parcel of the same reflex it must be shown that under what- 

 ever conditions the contraction of one group of muscles is brought about, 

 inhibition of their antagonists must also occur and that the responses 

 take place at exactly the same time. If records are made of the move- 

 ments of the flexors and extensors of the knee in an experiment such 

 as we have just described, it is found that the latent period for the re- 

 sponse of agonist and antagonist, whether it be contraction or inhibition 

 exactly coincides (Fig. 226). If the strength of the stimulus is gradually 

 increased the latent period becomes shorter for the inhibitory and active 

 reaction alike and the synchrony of the reciprocal responses is still pre- 

 served. Moreover the receptive field on the skin from which contraction 



