EFFECTS DURING CURRENT FLOW. 



545 



whilst polarisation spread is also altered ; hence the bipolar effect is 

 so lessened that the idiopolar one comes into prominence as in the 

 unipolar arrangement, and a turning-point can be found by suitable 

 adjustment of envelope and core. 1 In nerve this thin sheathing 

 envelope exists, so that the extrapolar changes are chiefly those of the 

 idiopolar type. The following table shows the direction of the after 

 effects in various cases of polarisation : — 



Polarisation After Effects. 2 



Polarising current . . 



Intrapolar polarisation ^ 

 current . . . . / 



Anodic extrapolar ~\ 



polarisation current J 



Cathodic extrapolar ) 

 polarisation current J 



Model. 

 Thick Envelope. 



> > 



< 



< 



Model. 

 Thin Envelope. 



> 



^ 





Nerve. 



>- > 



<- 



-> 



(b) Effects during current flow. — All the polarisation currents just 

 referred to are observed on the cessation of the polarising current. 

 They are therefore after effects, but the electrolytic changes, which 

 they represent, commence the moment that the polarising current 

 begins to flow. In the intrapolar region this is evidenced by a diminu- 

 tion in the polarising current ; in the extrapolar regions it is shown 

 by the establishment of extrapolar currents during closure. The 

 moment a current begins to flow through any of the models pre- 

 viously referred to, extrapolar currents are observed ; the anodic, 

 directed through the core towards the anodic contact; the cathodic, 

 through the core from the cathodic contact ; such currents pass through 

 the envelope in the reverse direction in each case. It will be thus 

 evident that during the passage of a current or a series of currents 

 through such a polarisable moist conductor as a medullated nerve, 

 similar changes must be set up outside the line of current flow, so that 

 every point nearer the anode becomes galvanometrically positive to one 

 more remote, and every point nearer the cathode negative to one more 

 remote. If the characters of these changes are compared with those 

 of the excitatory electrical response, it will be noted that since in the 

 latter the transmitted excitatory effect is one in which the point nearer 

 the seat of excitation is negative to one more remote, the character 

 of the excitatory change resembles the polarisation effects produced 

 during closure in the cathodic region. Now, since both polarisation 

 effects, the anodic and the cathodic, extend during closure into the 

 extrapolar regions, electromotive changes of this type might be suc- 

 cessively assumed by more remote parts, and possibly be propagated, 

 like those of the excitatory electrical response. Hence the electro- 



1 Hermann, Arch. f. d. (jca. Physiol., Bonn, Bd. xxxiii. S. 103 et seq. ; see also 

 'Biological Memoirs," Oxford, 1887, for translation of Hermann's paper, pp. 312-319. 



2 Arrows indicate direction in core or axis cylinder. 



vol. ii. — 35 



