EXTRAPOLAR POLARISATION CURRENTS. 543 



Fig. 280. If a galvanometer is connected with the surface at or near 

 the polarising contacts, a portion of the surface derivation will pass 

 through the instrument from the anodic to the cathodic contact. 

 Hence the internal polarisation around a central metallic core, lying in 

 a moist envelope, will be evidenced by the fact that on the cessation of 

 the polarising current, the anodic contact is found to be galvanometrically 

 positive to the cathodic. 



Polarisation in Hering's model. — In this model both the 

 envelope and the core are liquid, hence the surface junction is the seat 

 of dissimilar polarisation, and ions are formed of one kind in the 

 envelope and of another in the core. 1 The change in the envelope 

 is precisely similar to that just described as occurring in the metallic 

 core model, i.e. h ions are found under the surface anode (A), where the 

 polarising current leaves the envelope to enter the core, and ions under 

 the surface cathode {(J), where the current leaves the core to enter the 

 envelope. These envelope changes must produce a current, flowing 

 from A to G in the liquid envelope, and from c to a in the core. 



But, in addition to these, the core is itself the seat of ions. Under 

 the surface anode at a, the polarising current enters this new liquid, 

 producing ions ; under the surface cathode at c, it leaves this, pro- 

 ducing H ions. A current thus passes from c to a within the core, 

 which is the seat of these changes. It leaves this core at a, and 

 passes in the liquid envelope from A to G. It thus augments that 

 produced by the envelope polarisation (see Fig. 281). If the polarising 

 circuit is opened, and a galvanometer connected with the surface at or 

 near the polarising contacts, then a derivation of the current in the 

 moist envelope passes through the galvanometer from the anode to the 

 cathode. The former is therefore galvanometrically positive to the 

 latter. 



Polarisation in nerve. — Analogous effects to those just described 

 are observed in nerve. They are far more pronounced in medullated 

 than in non-medullated nerve. It is therefore concluded that the 

 sources are similar to those present in the above models, i.e. a two- 

 fold polarisation occurs within the nerve fibres at the surface junction 

 of the liquids of the fibre sheaths with those of the axis cylinders. 



On the cessation of a current through a nerve, the anode is found to 

 be galvanometrically positive to the cathode, and it may be thus inferred 

 that the portion of the fibres beneath the surface anode contain basic H 

 compounds in the envelope, and acid compounds in the axis cylinder, 

 whilst a reversed condition is presumably present under the surface 

 cathode. 



Extrapolar polarisation currents. — (a) After effects. — The passage 

 of a current through either the model or the nerve, is followed not 

 only by intrapolar polarisation currents, but by extrapolar effects. 

 The character of these is due to the circumstance that the part imme- 

 diately under the surface contact is the seat of maximum electrolytic 

 change. Thus, on the cessation of the polarising current, the portion of 

 envelope under the anode will be galvanometrically positive to all other 

 parts of the whole envelope. A current will therefore flow from this, 

 not only to the intrapolar, but to the extrapolar portions of the envelope. 

 In the anodic extrapolar region, this will enter and return along the 

 core from more remote points to those near the anode. Hence, when 



1 Hering, "Lotos," Prag, 1888, Bd. ix. 



