THE CHARACTERS OF THE NERVE CURRENT. 519 



normal tissue and the altered portion of the actual cross section, the 

 resting nerve current has been therefore termed the demarcation current. 



Methods of producing so-called resting or demarcation nerve 

 currents. — Cross section. — Every cross section of a nerve brings into 

 prominence the above-mentioned difference. It is, however, somewhat 

 difficult in the case of a small nerve to adjust the contacts, so that one 

 shall be effectively connected with the cross section ; a more certain 

 method for attaining the same result is furnished by " thermal section." 

 This is accomplished by dipping the end of an excised nerve for a brief 

 space into physiological saline at 60° C, so that several mm . of the 

 nerve may be suddenly destroyed ; the boundary zone between such a 

 destroyed portion and the normal tissue is practically the same as that 

 present on a cross section, whilst the destroyed end now serves as an 

 indifferent moist conductor to connect the whole of the cross section 

 with the necessary contact. 



Localised injury. — Instead of the end, any intermediate portion of 

 the nerve may be similarly injured by local heat, compression, etc. ; 

 its boundary zones on each side are demarcation areas between the 

 normal and the injured fibres, and the injured part is thus galvano- 

 metrically negative to either portion of the normal longitudinal surface. 

 Partial localised injuries of this kind involving superficial fibres of a 

 nerve trunk may be readily produced by nerve exposure, with the result 

 that, owing to the formation of demarcation zones, the electromotive 

 changes just referred to are now observed. 



Temperature. — Localised alterations of the temperature of nerve 

 fibres are accompanied by electromotive changes, even when the tem- 

 perature change is such as to cause no persistent injury. Thus, with 

 two symmetrical contacts showing isoelectrical conditions, localised 

 warmth to 30° C, in the immediate neighbourhood of one causes the 

 ' tissue at this point to become galvanometrically positive to that of the 

 cooler region ; localised cooling produces the opposite condition. 



The methods of ascertaining the characters of the nerve 

 current. — The experimental demonstration of resting nerve currents in the 

 tissue under any of the preceding conditions is one in which the recording 

 instrument may be a high resistance galvanometer, a capillary electrometer, 

 or such a physiological galvanoscope as is afforded by a muscle-nerve prepara- 

 tion. The special precautions attending such demonstration cannot be entered 

 upon here, but some features presented by nerve in common with other moist 

 conductors must be briefly referred to. These are related to the use of the 

 compensator, the restriction of all such investigations to the examination of 

 derivation effects, and the electrical resistance of the nerves themselves. 



The compensator is a graduated rheochord of definite resistance, so constructed 

 that it allows the experimental circuit to comprise a known electromotive force, 

 the amount of which can be varied at will. It is thus possible to exactly 

 counterbalance any current in the instrumental circuit due to electromotive 

 changes at the tissue contacts, by means of one of precisely equal amount but 

 opposite sign. The amount of the antagonistic electromotive force which has 

 thus to be employed is a measure of the amount of that present under the 

 tissue contacts. This latter is, in reality, that produced when an electro- 

 motive source is surrounded by a moist conductor, and contacts are placed 

 upon two points of the surface of the latter ; such a condition, for instance, 

 as is present when two small pieces of dissimilar metals are embedded within 

 a cylinder of kaolin soaked in 0-6 per cent. NaCl. It is obvious that the 

 whole of this moist conductor is now traversed by derivation currents, and 



