INADEQUACY OF PFLUGER'S LAW 



585 



E.M.F. of -5 and of 2 volts respectively. The lower voltage 

 now gave rise to intense excitatory pain. On the cessation 

 of the current the normal smarting sensation, due to salt, 

 was restored, and on now applying 2 volts, this slight 

 irritation was superseded by a sensation of soothing. This 

 result was found to be repeated many different times. 



The kathodic effect was next put to the test, and found 

 to induce responsive sensations exactly the reverse. The 

 application of *5 volt caused a soothing sensation, due to 

 depression of excitability. An E.M.F. of 2 volts, on the 

 other hand, induced an increase of excitability, with con- 

 sequent pain. 



These results are shown in the following table : 



METHOD OF SUCCESSIVE CONTRASTS TO SHOW REVERSAL OF SENSATION 

 UNDER POLAR CURRENTS 



From these experiments, then, it will be seen that during 

 the passage of the current, and when the E.M.F. is low, it 

 is the anode which increases the excitability, and the 

 kathode which depresses. Pfliiger's Law is thus seen not 

 to be universally applicable, but to be true only within 

 certain limits, the very reverse of this law holding good, in 

 the case of excessively high, and in that of low E.M.F. 

 The demonstration which has just been given of the latter 

 of these two facts, is independently borne out by the results 

 of electrotonic variations of excitability in nerves, described 

 in the last chapter, where we saw that, with moderately 

 feeble E.M.F., excitability was enhanced by the anode and 

 depressed by the kathode. 



It will thus be seen that polar variations ot excitability 

 are not always the same, but differ in character, according as 

 the intensity of the acting E.M.F. is moderate or low. The 

 great significance of this fact is apparent, with regard to the 



