DETERMINATION OF DIFFERENTIAL EXCITABILITY 273 



electrodal points to them. These qualities, however, are 

 counterbalanced by many serious drawbacks. For the re- 

 sultant electrical change, induced at a given point by an 

 electrical shock, is the sum of a number of changing factors, 

 which may be broadly classified as excitatory and polarising. 

 Let us suppose the induction shock to enter at a point. We 

 shall there have the physiological polar effects of anode-make 

 and anode-break. There is again, on the cessation of the 

 current, a counter electro-motive force due to polarisation. 

 The physiological reaction, moreover, will depend on the 

 excitability of the point. 



Next, as regards the second or kathodal point, where the 

 shock-current leaves the tissue. We have here the physiological 

 effects of kathode-make and break, the factor of the ex- 

 citability of the point, and the counter electro-motive force 

 due to polarisation, all contributing to the resulting electrical 

 change at that point. That relative electrical difference 

 induced between the points A and B, which determines the 

 observed electrical variation, thus equals the sum of the 

 fluctuating factors acting on A, minus the sum of the 

 fluctuating factors acting on B. These elements of variation 

 are sufficiently complicated ; but still another, as already 

 said, is added to them, in the fact that, with regard to the 

 excitatory polar effects of currents themselves, we have not 

 to deal simply with the law enunciated by Pfliiger, that the 

 kathode excites at make and the anode at break. The com- 

 plete law, as will be shown in a later chapter, is complicated by 

 the fact that the result depends on the intensity of the electro- 

 motive force. With feeble and again with excessive E.M.F., 

 the actual facts are the opposite of conclusions arrived at by 

 Pfliiger. Under these circumstances it would appear at first 

 sight impossible that any reliable results could be obtained by 

 the employment of the electrical form of stimulation. I shall 

 now, however, proceed to show in what manner all these 

 difficulties may be overcome, and the electrical form of 

 stimulus made extremely reliable. This will perhaps be best 

 understood if we take a concrete example. Let us suppose 



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