280 



Mr. Graham Brown. 



[Apr. 11, 



IV. Rhythmic Responses to Compounded Stimuli. 



Sherrington* has pointed out that weak inhibition is often accompanied 

 by tremor. In this paper he refers to the reflex inhibition of the state of 

 decerebrate tonus by a stimulus which produces a flexion-reflex. Dr. Forbes 

 has informed me that he observed, some time since, a similar tremor on 

 pitting two opposite reflex stimuli one against the other. Sherrington has 

 also shown that the tremor is reciprocal.^ 



In the present experiments two opposite reflex stimuli were also pitted 

 against one another, but the rhythmic response which was evoked cannot be 

 described as " tremor." It is far more akin to the rhythmic movements of 

 the act of progression. 



The results were obtained in the low spinal and in the decerebrate prepara- 

 tions. In both kinds of preparation it was found in some cases that although 

 a simple contralateral stimulus when applied alone gave a response of 

 steadily maintained extensor contraction, and although a simple ipsilateral 

 stimulus similarly applied gave pure maintained flexor contraction, yet 

 when these were applied together the response became a rhythmic alterna- 

 tion of flexion and extension. 



Thus if the contralateral stimulus was first applied it evoked a maintained 

 contraction of the extensor. If then an ipsilateral stimulus was applied 

 the extensor response ceased to be steady and became rhythmic. On then 

 taking the ipsilateral stimulus off again the extensor response again became 

 steady (figs. 1 and 2). 



In a similar manner the steady maintained flexion evoked by the 

 ipsilateral stimulus might become rhythmic on the application of the 

 contralateral. In some cases there was a contraction of both muscles and 

 each of these was rhythmic. The rhythmic movements were reciprocal — 

 that is to say that one rhythmic " beat " was in process of relaxation while the 

 synchronous " beat " in the other muscle was in process of contraction (fig. 3). 



These phenomena were also seen in muscles after the division of their 

 proprioceptive afferent nerves. 



In all these cases the rhythmic phenomenon occurs only when the 

 strengths of stimuli bear a certain fixed relationship to one another. If one 

 stimulus is progressively increased and compounded with the fixed opposite 

 stimulus a rhythm present gradually decreases in distinctness, and finally 

 disappears. 



The rate of the rhythmic movements usually lies between one and two 

 beats per second. 



* ' Quart. Journ. of Exp. Physiol.,' 1908, vol. 1, p. 67. 

 t ' Koy. Soe. Proe.,' 1909, B, vol. 81, p. 249. 



