286 



Mr. Graham Brown. 



[Apr, 11, 



by McDougall,* and a theory of rhythmic movement founded upon this 

 conception can hardly be a correct one. 



In the second place the intrinsic factor in the production can hardly be a 

 metabolic one. That is to say, that it cannot be intrinsically conditioned by 

 the metabolic activity in the moto-neurones. Were this the case, a rhythmic 

 response would follow each expression of activity of the centres. But this is 

 not the case. The application of a continuous stimulus to a single peripheral 

 afferent nerve usually evokes a response in which the one muscle contracts 

 continuously — that is (as regards such coarse rhythms as those of the scratch- 

 reflex or of progression), arrhythmically. But two such stimuli, which evoke 

 antagonistic activities when compounded together, may give a rhythmic 

 response. The intrinsic condition of the rhythmic phenomenon lies not in 

 the efferent centres themselves, but in some property of their inter- 

 dependence and inter-relationship. 



The third theory to account for rhythmic activity is that which assumes a 

 peripheral self-generated antagonistic stimulus. That this is not a correct 

 one is shown by the occurrence of the phenomenon after the de-afferentation 

 of the muscles.t 



The theory suggested to account for rhythmic activity is the following : 

 The cell-bodies and their processes of the efferent neurones of the antago- 

 nistic muscles form centres which mutually inhibit each other. A stimulus 

 which falls upon one will therefore through it inhibit the other. But if this 

 inhibition reduce the activity of the second centre, that will inhibit the first 

 less, and so the process will proceed until there is a limit set to this 

 " progressive augmentation of excitation." 



But if a stimulus fall more or less equally upon the two antagonistic 

 centres — or if two equal stimuli fall upon them — that which is most 

 activated will have its excitability increased by " progressive augmentation " 

 up to a certain point. The limit may be set by a process of inhibitory 

 fatigue. If this proceeds the balance will turn in the opposite direction, and 

 there will be a progressive augmentation of excitation in the other centre 

 until it too reaches its limit, when the process will set in in the other 

 direction again. In such a scheme there is, however, no explanation of the 

 occurrence of inhibition before excitation in time. It is not difficult to 

 overcome the difficulty by postulating a pair of " interposed centres " 

 between the afferent neurone and the efferent centres, and by supposing 

 that these too mutually inhibit, and that, in addition, they inhibit the 



* ' Brain,' vol. 26, p. 153. 



t 'Roy. Soc. Proc.,' 1911, B, vol. 84, p. 308 ; 'Quart. Journ. of Exp. Physiol.,' 1911, 

 vol. 4, p. 331. 



