282 ELECTRO-PHYSIOLOGY CHAP. 



the other end of a horizontally-stretched pair of nerves, there 

 will, after compensation of the demarcation current, be (as 

 described above) a more or less considerable deflection of the 

 magnet at each closure of the exciting circuit, in the direction of 

 a diminution (negative variation) of the nerve current, the 

 magnitude of which depends essentially upon the direction of 

 the polarising current. It is invariably greater when the current 

 is flowing towards the leading-off end (3). We shall term this 

 the descending, and its contrary the ascending direction of 

 current. 



The fact that the deflection in the two cases is homodromous, 

 but unequal in magnitude, as well as its independence of distance 

 between the led-off and polarised tracts, leaves no doubt that 

 these are not pure electrotonic manifestations, but are compli- 

 cated by the sequelae of excitation of the nerve by current. The 

 time-relations of the negative variation are remarkably different, 

 according as the exciting current is ascending or descending. In 

 the latter case it declines slowly during closure, in the former 

 with great rapidity. If the polarised part of the nerve is at 

 maximal distance from the leading-off tract, the negative varia- 

 tion at closure is, as a rule, the sole effect of excitation with 

 either direction of current. If the intermediate tract is reduced, 

 a positive deflection regularly succeeds the initial negative varia- 

 tion with ascending direction of current, its development and 

 character being essentially conditioned by the ratio of its magni- 

 tude to that of the previous negative variation. Where this 

 is large, the entrance of the positive after-effect is delayed after 

 closure, and it increases more slowly during the passage of the 

 current. An increasing acceleration of the deflection, or distinct 

 swelling of the positive effect to its maximum, may often be 

 observed, after which the magnet rests at its new position of equi- 

 librium as long as the current is passing. The maximum of the 

 positive variation invariably corresponds with a higher intensity 

 of current than is required by the initial negative variation. 

 Regarding the latter as an effect of closing excitation, it is .signi- 

 ficant that it becomes weaker in proportion as the intensity of 

 the current increases, and finally disappears altogether (according 

 to the third stage of the law of contraction). Under these con- 

 ditions closure either produces no visible effect, where the 

 distance between the exciting tract and the end led off is too 



