288 Rhythmic Activity of the Nervous System. 



spinal cord, and the rhythmic movements which occur in narcosis — are closely 

 similar, and seem to be instances of the rhythmic phenomenon of progression. 



10. It occasionally happens that a rhythmic response which more resembles 

 the scratch-reflex is obtained on simple peripheral stimulation. There both 

 antagonistic muscles contract at the same time, and there occur reciprocally 

 alternate " beats " as depressions from the plateaus of maintained contraction. 



11. The explanation of the occurrence of rhythmic action seems best to 

 be given on the assumption that the two antagonistic centres mutually inhibit 

 each other, and that they are very nearly equally activated by the evoking 

 stimuli. In such a case there will be balanced against each other in each 

 centre antagonistic forces of excitation and inhibition. And in this balance 

 and a phenomenon of fatigue and recuperation of the mutual inhibition is 

 probably to be found the explanation of rhythmic activity. 



12. The occurrence in the simple reflex of " rebound contraction after 

 excitation," described previously, and of the allied " rebound relaxation 

 after inhibition," here described for the first time, seems to shew that 

 even there there may be a double excitation of the two antagonistic centres. 



13. Evidence given here and in other cases seems to cast doubt upon 

 certain hypotheses of the nature of rhythmic activity : — 



(a) As the phenomenon of relaxation not infrequently occurs in time 

 before the accompanying phenomenon of contraction in the antagonist it 

 seems hardly possible that the " drainage " theory of inhibition is an 

 adequate one, or that it can serve as a basis for a theory of rhythmic 

 activity. 



(b) That the phenomenon is not conditioned intrinsically by the con- 

 ditions of metabolic activity in the moto-neurones is shewn by the 

 experiments here given, in which two stimuli, when given separately, 

 condition arrhythmic responses, but when compounded in appropriate 

 strengths condition rhythmic responses. This can only mean that the 

 centres normally discharge arrhythmically — as regards such coarse rhythms 

 as that of progression— and therefore that the rhythm is conditioned by 

 some property of their interdependence and inter-relationship. 



(c) On the other hand, the present experiments demonstrate — as the 

 author has done previously — that the rhythm is conditioned centrally and 

 not by an " automatic " peripheral proprioceptive interference. In addition 

 to this the present experiments show that the rhythm on compounding of 

 opposite stimuli may be obtained in the low spinal preparation. This 

 shows that the rhythm is conditioned at the lowest levels, and is not 

 produced, for instance, by the evocation of the activity of a rhythmically 

 discharging centre in the higher parts of the nervous system — a possi- 



