818 



THE CENTRAL NERVOUS SYSTEM 



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). By taking advantage of these facts further proof may be 

 supplied that inhibition and contraction occur simultaneously, as shown 

 in Fig. 214. 



It is impossible to demonstrate any trace of inhibition of the skeletal 



Fig. 215. Sherringtoa's diagram illustrating the mechanism of reciprocal innervation. The 

 afferent fibers (5) from the skin of the leg and (5') from the flexor muscles of the knee (in 

 hamstring nerve) pass to the spinal cord, where each gives off a branch which divides into two 

 others, of which one in each case goes to a motor neuron of the extensor muscles (E) and the 

 other to a motor neuron () of the flexor muscles (F). Branches also pass across the median 

 line to similar motor neurons on the opposite side of the cord. As indicated by the plus and 

 minus signs, the afferent stimuli either stimulate or inhibit the activities of the motor neurons, 

 the determination of the exact effect being a function of the synapsis. (From Sherrington.) 



muscles by stimulation of their motor nerves, thus indicating that in- 

 hibition is dependent upon the nerve center. Furthermore, since inhibition 

 occurs along with flexion of the antagonistic muscle, we must assume 

 that the afferent impulse on entering the spinal cord divides into 

 two branches, one going to one motor neuron so as to excite it, the other 

 to another neuron so as to inhibit the tonic stimuli which it is con- 

 stantly sending to the muscles (Fig. 215). 



Since the seat of the inhibition is in the nerve center, it is to be ex- 

 pected that impulses transmitted from other parts of the nervous system 



