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Prof. C. S. Sherrington. Reciprocal [Nov. 13, 



relaxation of both muscles followed by simultaneous contraction of them. 

 If the preparation be carefully made, the condition of the twin isolated 

 muscles remaining sensibly similar, and if similar right and left afferent 

 nerves, e.g. right and left peroneal, be fitted with electrodes from similar 

 induction coils similarly supplied and interrupted for faradisation and the 

 intensity of the stimulation of the two nerves be kept as far as practicable 

 equal, observations can be obtained as follows : — Suppose, as in fig. 2, right 

 peroneal stimulated ; right vastocrureus relaxes and left contracts ; if then, 

 while right nerve continues to be stimulated, left peroneal be stimulated in 

 addition, right vastocrureus still remains relaxed and indeed may relax 

 further, but left vastocrureus relaxes also. On then withdrawing the 

 stimulation of left nerve left muscle contracts and right remains relaxed still, 

 and finally on withdrawal of stimulation of right nerve right muscle contracts 

 by rebound. 



This shows that each of the afferent nerves employed taken by itself 

 unfolds in response to stimulation an inhibitory effect on the ipsilateral 

 muscle stronger than is its excitatory effect on the twin contralateral muscle. 

 Stimulation of the right and left nerves concurrently if the stimulations be 

 fairly equal in intensity causes therefore concurrent relaxation of both 

 muscles. Thus when the stimulations of the two nerves are repeated 

 synchronously, as in fig. 3, both muscles relax together at each repetition of 

 the stimulation. Further at each discontinuance of the double stimulation 

 both muscles exhibit synchronous rebound contraction. So that in response 

 to the synchronously repeated and synchronously remitted stimulations both 

 muscles relax and contract synchronously. If, however, the intensities of 

 the two stimuli be markedly unequal the muscular reactions, right and left, 

 though synchronous are, of course, reciprocal, not identical. 



The identical form of reaction holds true over a wide range of intensities 

 of stimulation, so long as the two stimulations, right and left, are kept of 

 approximately equal intensity. The preponderance of ipsilateral inhibition 

 over contralateral excitation obtains therefore both with moderate stimuli 

 and with strong, the ratio between the intensities of the reflex inhibition 

 and reflex excitation remaining apparently about the same for a wide range 

 of stimulus intensities. The difference between the effect of synchronous 

 double stimulation of strong intensity and of weak is in the main merely 

 that with weaker stimuli the synchronous relaxations of the two muscles 

 are weaker and are followed by less powerful rebounds than are strong., 

 though the rebounds are still synchronous. 



