Vlll PREFACE. 



tion, the action of the current is directly exerted on certain 

 hypothetical carriers (Trager) of electromotive forces which are 

 distributed throughout the polarisable structure, and which the 

 current is capable of ' directing ' (see concluding paragraph of 

 No. 6). 



Hering rejects this theory on the ground that the changes of 

 which * positive polarisation ' is the expression, have their seat 

 exclusively at the anode, and explains the experimental facts 

 used by du Bois in proof of ' internal polarisation ' (that is of 

 a polarisation affecting all traversed structures, and consisting in 

 the coming into existence of electromotive forces all along the 

 line of flow, and not merely at anodes or cathodes), on the sup- 

 position that he employs structures in which, in consequence of 

 their heterogeneity, such sudden transitions from less to greater 

 density occur in the course of the current-paths, that the effects 

 of anodes are produced. On the assumption that positive polari- 

 sation has its seat at the anode exclusively, he has no difficulty 

 in referring what is observed to the well-recognised facts 

 (i) that the effect of opening a current led through a muscle 

 or nerve is to produce excitation at the anode ; and (2) that the 

 characteristic expression of the excitatory state is negativity; 

 and accordingly, while recognising the essential relation of the 

 change in question to the functional activity of the organ in 

 which it manifests itself, he prefers to speak of it as c anodic after- 

 effect ' rather than as ' positive polarisation.' 



Hermann objects to the term 'positive polarisation' even more 

 decidedly than Hering, on the ground that the phenomenon 

 is not analogous to physical polarisation. He, like Hering, 

 believes it to be excitatory, and holds that it is due to the 

 setting up of a persistent change of this nature in the neigh- 

 bourhood of the spot at which the current has entered. Among 

 the various experiments which lead to this conclusion, those are 

 of most importance which show ( i ) that in muscle it is confined 

 to the anodic region ; (2) that it is annulled whenever the 

 current is led into the muscle or nerve through a dead part ; 

 and (3) that the anode becomes relatively negative not only 



