340 Refiex Responses to Rhythmical Stimulation in the Frog. 



frequency-rates, i.e., more than 60 per see., for the reflex power produced in 

 the semitendinosus muscle of the spinal frog, if an ipsilateral afferent nerve 

 trunk is stimulated. At moderate frequency-rates (50 per second on average) 

 the reflex result depends but little upon the intensity of excitation. At low 

 rates (under 30 per second) the reflex response increases with the strength of 

 stimulation, at first quickly and then slowly. The maximum, when attained, 

 is maintained for further increments of stimulation ; no optimum is usually 

 seen. 



(2) Similarly there is an optimal rate of stimulation for any given intensity 

 which is usually about 50 per second. At the optimal intensity of stimula- 

 tion for a rapid frequency the reflex power at this frequency may often be 

 equal to or even somewhat greater than that produced at the optimal rate. 



(3) The reflex myogram obtained by the resonance method exhibits 

 mechanical vibrations synchronous w^ith the frequency-rate of stimulation 

 when this is under 30 per second. The spinal reflex centre therefore follows 

 at the same rate the rhythm of successive volleys of centripetal impulses. 



(4) The reflex tetanus evoked by faradising at sufficient intensity an 

 afferent nerve at rates less than 30 per second, is more abrupt and more 

 powerful in the first period of contraction than the direct tetanus produced 

 under the same condition of excitation, though the latter always surpasses 

 the former when the stimulation is prolonged. Therefore some of the 

 component shocks used as stimuli produce a repetitive series of motor 

 impulses from the reflex centre. 



(5) The reflex contraction evoked at the optimal rate of excitation is often 

 nearly as powerful as the direct one obtained by similar stimulation, 

 especially in the first period of contraction. 



(6) The faradisation of a very small afferent nerve-branch (ramus cutaneus 

 medius) or of a circumscribed skin spot often produces as powerful a reflex 

 contraction as that obtained by stimulation of a nerve trunk at the same 

 frequency. The results obtained in (5) and (6) suggest that the motor reflex 

 centre is a physiological unity ; any afferent nerve, even a small one, being 

 connected with all the motor neurons of a reflex arc. 



I am greatly indebted to Prof. C. S. Sherrington for continuous suggestions 

 and advice during the progress of these experiments, and for permission to 

 utilise his instruments. I take also this opportunity in thanking Dr. C. H. 

 Kellaway for kind assistance in expressing the results in the text. 



