CHAPTER XII 



VARIATIONS OF EXCITABILITY DETERMINED BY METHOD 

 OF INTERFERENCE 



Arrangement for interference of excitatory waves Effect of increasing difference 

 of phase Interference effects causing change from positive to negative, 

 through intermediate di-phasic Diametric balance Effect of unilateral 

 application of KHO Effect of unilateral cooling. 



I HAVE explained how the variations of excitability brought 

 about by various agencies may be determined, by recording 

 the corresponding amplitudes of response. I shall now pro- 

 ceed to describe a new and interesting method of making 

 such determinations, by means of which it will be found 

 possible to elucidate certain questions which without it must 

 remain obscure. This method is, moreover, of extreme 

 delicacy, enabling the investigator to detect the slightest 

 variation of excitability, induced by any agent. 



Let two points in the experimental tissue, say A on the 

 right, and B on the left, be suitably connected with the galva- 

 nometer, and let the occurrence of excitation at A on the 

 right be represented by an ' up ' response record, the excita- 

 tory effect at B, on the left, being represented as 'down.' 

 If now the two points, A and B, be excited simultaneously, 

 the resultant electrical response will be due to the algebraical 

 summation of the two excitatory electro-motive effects E A and 

 E B , these standing for the individual electrical effects at the 

 two points A and B. Now if the intensities of the two effects 

 be the same, and if their time-relations be also the same, 

 it is evident that these two excitatory electrical waves, being 

 of equal amplitude and having the same "phase but of 

 opposite signs, will, by their mutual interference, neutralise 



