142 ELECTRO-PHYSIOLOGY CHAP. 



The relations of excitability in the extrapolar ("centropolar") 

 tract of nerve above the ascending .or descending polarising current 

 are much more complicated, particularly in the first case. The 

 augmentation of excitability still appears indeed (as pointed out by 

 Pfltiger) with weak currents, but quickly passes into its contrary 

 with stronger excitation, since at a certain strength of polarising 

 current a stimulus of given intensity discharges a weaker contrac- 

 tion than was previously the case, while at last the strongest 

 stimuli (which excited maximal twitches before the closure) fail 

 to give any effect. This, however, is due less to declining 

 excitability above the kathode, i.e., centropolar diminution of ex- 

 citability, than to decreased excitability and conductivity below, 

 at the anode, which becomes more and more prominent with 

 increasing strength of polarising current, and modifies in a greater 

 or less degree the action of every excitation discharged from 

 above upon the muscle. This is the principal reason why 

 Valentin and Eckhardt failed to detect the extrapolar increase 

 of excitability above the kathode. We must accordingly assume 

 a constant increase of excitability, proportionate with strength of 

 current, above the ascending as well as below the descending 

 current : as in the latter, it is the more pronounced the nearer the 

 point tested lies to the polarised tract of nerve ; at a definite 

 distance from the kathode (according to the intensity of the 

 current) it is 0. The extrapolar excitability above the anode of 

 the descending current is again in complete conformity with the 

 anodic alterations of excitability which appear below the ascending 

 current. There remains only the excitability within the intra- 

 polar tract that is actually traversed by the ascending or descend- 

 ing current. Obviously, whatever has been stated above of muscle 

 under the same conditions, both as regards difficulties of investiga- 

 tion and method adopted, must hold good of nerve also. 



Starting with the erroneous presumption that an induction- 

 current excites the entire tract of nerve through which current is 

 passing, Pfliiger attempted in the first place to determine the 

 " total excitability " of the intrapolar tract, as dependent upon the 

 strength of the polarising current, leading in the induction current 

 employed as test-stimulus by the polarising electrodes (according 

 to a method previously employed by Eckhardt), as was described 

 above for muscle. But since polar action has now been determined 

 for induced currents also, it is clear that the results of these 



