328 



COMPARATIVE ELECTRO-PHYSIOLOGY 



this record the intensity of stimulus was reduced by pulling 

 out the secondary further away from the primary. The 

 responses were now found reversed, as seen in the subsequent 

 series. 



We have last to consider the reversal induced by intense 

 stimulation. Such instances must not be confused with the 



effect of fatigue. The two 

 can be distinguished by the 

 fact that the fatigue-reversal 

 takes place after a series of 

 normal responses, whereas 

 the true reversal, due to 

 strong intensity of stimulus, 

 which we are now discuss- 

 ing, is exhibited at the very 

 beginning. Such an effect 

 I have observed in the 

 response of the human 

 lip. The direction of 

 the responsive current was 

 normally, under moderate 

 from the epi- 

 to the epidermal 

 Under very strong 



exhibit reversed response under sub- ,. , , .,. 



minimal stimulation, current from Stimulus, however, this 

 external to internal surface. normal direction was found 



to be reversed. 



But the employment of excessively strong stimulation 

 introduces other complicating factors. The applied stimulus 

 may be supposed to be localised only when it is of moderate 

 intensity. With intense stimulus the subjacent tissues are 

 liable to be involved in giving rise to excitatory response ; 

 and it then becomes a difficult problem to discriminate how 

 much of the observed effect is due to the superficial layer, 

 and how much to others more deeply situated. 



FiG. 199. Photographic Record showing 

 Reversal of Response in Carpel of 

 Dillenia indica, under Sub-minimal 

 Stimulation 



The first series show normal electrical Stimulus, 



responses under moderate stimulus, 



responsive current from internal to 



external surface; the second series surfaces. 



