INTERNAL POLARISATION. 541 



dye will change colour in accordance with the reaction. Methylene blue, 

 such as is used for intra vital injection, which blanches on reduction and 

 becomes of a more pronounced tinge on oxidation, is a very suitable dye 

 for the purpose; it is observed to blanch at the cathode and become more 

 intense in colour at the anode. A far more delicate proof is that afforded 

 by the fact that the products are themselves electromotive. Thus, if a 

 " polarising " current is passed through a moist conductor by means of metallic 

 contacts, and these are disconnected from the battery and connected with a 

 galvanometer, the anode is now found to be strongly positive to the cathode 

 (Fig. 279). The electromotive changes thus caused are such that a current 

 flows along the moist conductor from the cathode of the battery circuit to 

 the anode; it is thus opposed to the "polarising" current, and is termed the 

 polarisation current. It is sometimes further denned as the "negative" 

 polarisation current in consequence of its direction; the term is a useless 

 refinement, since in all cases the anodic ions of polarisation are galvanometrically 

 positive to the cathodic, and therefore electro-negative to those at the cathode 

 of the moist conductor. The anodic ions being acid are for simplicity denoted 

 in the figures thus, ; the cathodic being basic thus, H + . 



Internal polarisation. Since a nerve is a moist conductor, the 

 passage of an electrical current through it, by means of metallic contacts 

 on its surface, must be 

 accompanied by similar 

 changes. The main seat 

 of these, under such con- 

 ditions, is at the surface 



junctions of the metals -o o<- < H H H H H - H "> 



with the liquid of the - gg..:5i < L - H M -> 



nerve, hence the polarisa- ! ' 



tion (ie.^ the electrolytic ^ 279- _ The polarising current (P) has been led 



Change) IS said to be ex- through the moist conductor in the direction of the 



ternal. black arrow by means of metallic contacts from 



When non r>olariqahlp A to G - On P enin g tne circuit and connecting 



it with the galvanometer(), polarisation currents 



electrodes are used, SUCh are found directed as shown by the dotted arrows, 



external polarisation is 



reduced to a minimum, but nerve fibres are still polarisable. This is 

 especially the case in medullated nerves, in which there is galvanometric 

 proof of "negative" polarisation, following the passage of a galvanic 

 current. The nerve substance itself is thus polarisable. 



Effects very similar in character to those of nerve can be obtained 

 when a current is passed through any schematic arrangement, comprising 

 a good conducting core, surrounded by a moist envelope. 



Hermann's model consists of a glass tube, with side openings. The 

 tube contains a thin core of platinum wire, forming its central axis. 

 Both the central and the side tubes are filled with saturated solution 

 of ZnS0 4 , and into the latter dip amalgamated zinc rods, which serve 

 for the polarising and the galvanometric contacts. 



An effectual and simple model consists of a fine copper wire, well 

 covered with cotton. The cotton is soaked in 0'6 per cent. NaCl, and 

 the whole plastered with kaolin similarly moistened. It is used in a 

 moist chamber to diminish evaporation, and the contacts are made by 

 non-polarisable electrodes of the usual type. 



Hering's model is without a metal core. It consists of a tubular 

 stem of hollow grass, which is soaked in water, and is filled with strong 

 solution of NaCl just before use. The concentrated NaCl in the 



