CHAPTER XXI 



DETERMINATION OF DIFFERENTIAL EXCITABILITY 

 UNDER ELECTRICAL STIMULATION 



Advantage of electrical stimulation, in its flexibility Drawbacks due to 

 fluctuating factors of polar effects, and counter polarisation-current Difficulties 

 overcome by employment of equi-alternating electric shocks Methods of the 

 After-effect and Direct-effect Experiment of Von Fleischl on response ot 

 nerve Complications arising from use of make and break shocks Rotating 

 reverser Motor transformer Response of Musa to equi-alternating shocks 

 Abolition of this response by chloroform Response records of plagiotropic 

 Cucurbita and Eel Differential excitability of variegated leaves, demonstrated 

 by electric response. 



WE have seen, in Chapter IX. that the differential excitability 

 of any two points in a tissue can be detected, by observing 

 the direction of resultant response, when both the points are 

 simultaneously excited by an identical stimulus. It was 

 seen in the same place also that the more excitable point 

 becomes, under diffuse stimulation, relatively galvanometrically 

 negative. I further described the various forms of quanti- 

 tative stimulus which might be employed for this purpose, 

 those, namely, of mechanical vibration and of stimulation by 

 thermal shocks. By employing these non-electrical forms of 

 stimulus the fundamental law of differential response was 

 firmly established, in such a way as to exclude every source 

 of uncertainty. 



The electrical form of stimulation is characterised never- 

 theless by many advantages. Its intensity, for instance, 

 is easily graduated. But its principal superiority lies in its 

 great flexibility of application. Any two points, however 

 remote or difficult of access they may be, may by this means 

 be subjected to a required stimulus, if we can only apply two 



