134 COMPARATIVE ELECTRO-PHYSIOLOGY 



response of both. In the next figure (fig. 93) is given a photo- 

 graphic record, showing the marked depression of response 

 induced by a strong solution of KOH, and in order to show 

 that under the given experimental conditions, the variation 

 of resistance does not in any way affect the responses, the 

 deflection produced in the galvanometer by the application 

 of an E.M.F. of 'I volt to the circuit is shown at the 

 beginning and end of the record. The equality of these two 

 deflections shows that the resistance in the circuit has 

 remained practically the same throughout the experiment. 



Before After 



FIG. 92. Abolition of Response at both A and B Ends by the Action 

 of NaOH 



Stimuli of 30 vibration were applied at intervals of one minute to A and 

 B alternately. Response was completely abolished twenty-four 

 minutes after application of NaOH. 



Therefore, the change in the amplitude of the E. M. 

 responses recorded may be taken as due entirely to the 

 variation in the excitability of the tissue. 



In the experiments just described, the stimulus was 

 applied directly at the responding point. By the application 

 of a chemical reagent, not only was the responsive excitability 

 of the tissue modified, but its receptivity, or power of 

 receiving stimulus, also underwent a change. It will be 

 shown later that the receptive excitability and the responsive 

 excitability are not necessarily the same. The records which 

 have just been given show what is, strictly speaking, the 



