314 ELECTRO-PHYSIOLOGY CHAP. 



physiological alterations of the nervous substance. Something 

 may be learned from the reaction of electrotonic currents on 

 exciting the nerve, i.e. the current of action in electrotonised nerve. 

 The first of these points was investigated by Bernstein (50). 



He began by examining into the alterations of the negative 

 variation of the demarcation current, when a tract of nerve is 

 simultaneously traversed above or below the exciting tract by a 

 constant current. If the latter is, in the first place, very weak, 

 the polarising electrodes being also so remote from the trans- 

 verse lead-off that any perceptible interference of electrotonic 

 currents must be excluded, there will, when the exciting elec- 

 trodes in connection with the secondary coil of an induction 

 apparatus are placed between the polarised and led-off tracts of 

 the nerve (are, i.e., " infrapolar "), regularly be an augmentation of 

 the negative variation with the descending, a diminution of it 

 with the ascending, direction. The contrary effect occurs on 

 exciting above the polarised region of the nerve. 



These results obviously agree in the main with the electro- 

 tonic alterations of excitability as determined by Pfliiger, since 

 the galvanometer merely takes the place of the normal index 

 of muscular excitation. But if the polarising electrodes are 

 brought nearer to the led-off transverse section, so that the 

 electrotonic differences of potential are at first weak, and sub- 

 sequently augmented, the demarcation current either diminish- 

 ing (in the negative phase) or growing stronger (positive phase), 

 according to the direction of the polarising current, there will 

 then, with infrapolar tetanising excitation, be a distinct decrement 

 of the negative variation in the negative phase of electrotonus, 

 produced by descending current ; an increment, on the other hand, 

 in the positive phase, with ascending current. In the first case 

 the negative variation may, if the strength of the polarising 

 current exceeds a certain limit, be reduced to 0, or even reversed 

 in sign. The former invariably occurs when the demarcation 

 current vanishes altogether in the negative phase. If, on the 

 other hand, the current is reversed, the P.D. increases during 

 excitation in the same direction. There is thus " a distinct de- 

 pendence of the negative variation upon the strength and direction 

 of the entering electrotonic phase. If the latter augments the 

 nerve current, the' negative variation increases also ; when it 

 diminishes it, the negative variation is also diminished, and 



