Responses to Rhythmical Stimulation in the Frog. 



339 



reaction is not necessarily obtained by stimulating more afferent nerve fibres. 

 The comparison of the maximal reflex reaction produced by exciting either a 

 very slender afferent nerve or the skin of toes with that evoked by faradising 

 a nerve trunk has an important bearing on the question of the physiological 

 unity of the spinal reflex centre. 



The ramus cutaneus medius was isolated with a tiny piece of the skin 

 which it innervates. The threshold value for this nerve is usually higher 

 than that of the peroneal trunk, a fact which may be related to the ease with 

 which it is injured. Nevertheless, sufficiently strong faradisation of the nerve 

 when it is freshly prepared and has a comparatively low threshold value often 

 produces as powerful a reflex contraction as does that of the peroneal. The 

 extent of the reflex reaction evoked by a small nerve appears to depend more 

 upon the intensity than upon the rate of stimulation. The extent of reflex 

 reaction can be graded with graded strength of excitation. Sometimes the 

 skin of toes was excited by applying a pair of electrodes two or three milli- 

 metres apart. The reflex contraction thus produced often attains the intensity 

 of that evoked by stimulating a nerve trunk (fig. 5). The course of the reflex 

 response elicited by a stimulation of a small afferent nerve or a skin-spot is 

 irregular. 



The fact that the reflex contraction produced by a stimulation of a small 



Fig. 5. — Semitendinosus reflex preparation. Eesponses at same frequouc/ ot sumulation 

 (45 per second) with varying intensity. P = peroneal nerve ; D = skin (2nd toe) 

 stimulation. Numbers = distance between coils. Time in sec. 



afferent nerve or of a circumscribed skin point is of the same extent as that 

 elicited from a nerve trunk and sometimes nearly equal to the maximal direct 

 tetanus suggests strongly the physiological unity of the reflex centre, any 

 afferent nerve, even a small skin nerve, being connected with the whole motor 

 reflex centre. 



Conclusiom. — (1) There is an optimal intensity of stimulation at rapid 



