x ELECTROMOTIVE ACTION IN NERVE 315 



falls to zero, so soon as the led-off current disappears entirely 

 in the negative phase. The variation consequent on excitation 

 is therefore invariably negative to the initial sign of 

 the nerve current." As Bernstein remarks, these results are 

 easily explained on the assumption " that the current led off from 

 the nerve in a state of electrotonus reacts like an ordinary nerve 

 (demarcation) current. The weaker it is, the weaker is its 

 negative variation, and vice versa. The two disappear together, 

 just as the negative variation disappears on leading off from 

 two symmetrical points of an unpolarised nerve, and reversal of 

 the current reverses the sign of its variation also" (I.e. p. 622). 



Bernstein established by further experiments (in which the 

 excitation, whether suprapolar or otherwise, along the continuity 

 of the nerve, was led off from two longitudinal contacts) that the 

 electrotonic incremental currents give a precisely similar reaction 

 on exciting medullated nerve to that of the ordinary demar- 

 cation current. This appears most plainly when the polarising 

 and exciting electrodes are applied to either end of the longest 

 nerve available, the lead-off being from two points of the inter- 

 mediate tract. Since in this case the electrotonic alterations 

 are not obliged to pass the point of excitation, nor the excitation 

 the polarised tract, in order to reach the leading-off circuit, the 

 effect of excitation on electrotonic currents may be investigated 

 in its integrity. While Bernstein explains these observations by 

 a diminution, consequent upon excitation, of the energy or activity 

 of the supposed electromotive molecules, Hermann is led, by his 

 interpretation of the galvanic manifestations in electrotonus, to 

 refer these facts to alterations of intensity in the negative wave 

 of excitation, during its passage through the nerve, when the latter 

 is polarised. " It is indeed more pronounced at any point of 

 the nerve, the more strongly positive and weakly negative the 

 polarisation of the latter, i.e. it increases when it is becoming 

 algebraically more positive, and diminishes when it advances 

 upon more negative points " (Hermann's law of the " polarisation 

 increment " of excitation). 



VII. THEORETICAL 



Although it is hardly possible at the present time to formu- 

 late any theory of electrical excitation that shall cover all its 



