140 ELECTRO-PHYSIOLOGY CHAP. 



at the anode or kathode respectively is manifestly insufficient 

 to neutralise the excitation approaching the muscle, from the 

 kathode at make of the ascending current, from the anode at 

 break of a descending current. While, lastly, the single 

 reaction on stimulating with weak ascending or descending 

 currents is readily explained on the assumption that the magnitude 

 of the two impulses excited by current is unequal the fall of 

 the current in particular being the weaker stimulus. With 

 gradually rising current intensity, the more active kathodic 

 stimulus at make is first to take effect, and the weaker anodic 

 opening stimulus only comes into play when the current is still 

 further strengthened. 



III. CHANGES IN EXCITABILITY AND CONDUCTIVITY PRODUCED 



BY THE PASSAGE OF A GALVANIC CURRENT (ELECTROTONUS) 



We have next to consider the facts from which it is con- 

 cluded that an inhibitory impulse arises at the anode on closing, 

 at the kathode on opening, a constant current. It is again to 

 Pfliiger that we are indebted for decisive evidence. While in the 

 muscle, polar inhibition finds double expression in change of form, 

 along with simultaneous depression of excitability, we are in 

 nerve thrown back solely upon the latter, and should therefore 

 (according to the previous observations) expect, with sufficiently 

 strong currents, to find depression of nervous excitability at the 

 anode during closure, at the kathode after breaking the circuit. 

 Here we are at once confronted by a marked dissimilarity from 

 striated muscle. While in the latter, " electrotonic " changes of 

 excitability are essentially local and confined to the physiological 

 poles, in medullated nerve (under similar conditions) not merely 

 the whole intrapolar, but considerable sections of the extrapolar 

 region also, exhibit alterations of excitability during and after 

 the passage of current, these alterations being unlike and opposite 

 in the vicinity of the two poles. Even the earlier electricians 

 observed indications of such a reaction on sending current through 

 an entire limb, but Eckhardt was the first to show by unex- 

 ceptionable experiments that a nerve of which a portion was 

 traversed permanently by a constant current, underwent substantial 

 modifications as expressed by increased or diminished excitability 

 to artificial stimuli at points of the intra- and extra-polar regions. 



