228 



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



occurring concomitantly in the symmetrical muscles is not without interest. 

 T. Graham Brown* has recently pointed out that in antagonistic muscles in 

 many cases the terminal relaxation following an excitatory reflex may be 

 regarded as of the nature of an inhibitory rebound, the converse of rebound 

 contraction. It would seem, therefore, that the terminal rebound following 

 a reflex of reciprocal effect on a muscle-pair is often itself of reciprocal 

 character in the two muscles. In the symmetrical muscles dealt with in 

 this paper the terminal effects when the reflex itself has been of reciprocal 

 influence on the two muscles are quite usually of reciprocal character in the 

 two muscles (see fig. 8). When, however, the character of the reflex itself has 

 been changed, by the procedure described, from reciprocal into identical 

 the terminal rebound also is changed from reciprocal into identical (fig. 3). 



VI. Factors Outside Algebraic Summation Involved in the Change. 



The above seems to me what the experiments clearly indicate as the 

 main principle involved in the change from reciprocal innervation of 

 symmetrical muscles to identical innervation of them when the stimuli are 

 appropriately duplicated. This principle rests on the inequality of the 

 excitation-potency and inhibition-potency respectively inherent in the 

 components of the summed duplicate reflex. But the experiments have 

 shown certain further features outside this principle. In the observations 

 on the vastocrureus muscles, when the reflex inhibition due to the ipsilateral 

 nerve is in progress, and stimulation of the contralateral nerve is then added, 

 the effect of the latter is very occasionally not a mitigation but a distinct 

 increase of the inhibitory relaxation (fig.«2, abscissae 4, 5). A similar result is 

 sometimes met with in the flexor, tensor fasciae femoris, though there conversely 

 in regard to the nerves used. A phenomenon comparable with these, although 

 in the opposite direction, was reported previously by Miss Sowton and 

 myself working with the knee-flexor, semitendinosus.f We noted that if 

 the stimulation of contralateral nerve is relatively weak in comparison 

 with that of the ipsilateral nerve, the former, if added when the latter is in 

 progress, may, instead of lessening the reflex contraction due to the latter, 

 actually increase it. In the observations of this paper a strong reflex 

 inhibition of the extensor centre already in progress seems to convert a 

 weak excitatory influence into an inhibitory one. In the previous obser- 

 vations a strong reflex excitation of the flexor centre seemed to convert a 

 weak inhibitory influence into an excitatory one. And it has been the case 

 in some of the present observations on the hip-flexors that the addition of a 

 * Ibid. 



t 'Koy. Soc. Proc.,' 1911, B, vol. 84, p. 204. 



