260 ELECTEO-PHYSIOLOGY CHAP. 



the longitudinal lead-off) after a single stimulus, within a certain 

 limited period, to its maximum, and slowly disappears again. 



According to Bernstein's method (in which that arrangement 

 of the slider is determined on the rheotome, which gives the 

 beginning and ending of the variation, the time occupied by the 

 galvanometer closure being then subtracted from the total period 

 of difference between these two positions) the duration of the 

 negative variation does not exceed 0'0007 (j^Vo) sec - * n mec ^ u l" 

 lated frog nerve. This very low, and theoretically improbable, 

 figure is as a matter of fact incorrect. Hermann (27) claimed a 

 much longer period for the negative variation ( = 0'0056 sec.), 

 while Head (10), by means of Bering's specially-constructed 

 rheotome, estimated it at 0'024 sec. i.e. a value more than 

 thirty times as great as that of Bernstein. 



The duration of the negative phase varies with the state of 

 the frog from - 0079 to 0'0239 sec. Hermann at first referred 

 this extension to his system of cooling the nerve, as retarding the 

 transmitted excitation, and corresponding negativity, of each 

 element of the nerve. Further experiments, however, showed 

 that even at normal temperature the variation takes longer than 

 was stated by Bernstein. Hermann then concluded that his use 

 of a highly-sensitive galvanometer, and packet of six nerves, were 

 responsible for the more complete expression of the last part of 

 the declining variation. The far larger figure given by Head 

 implies that his rheotome followed the descending portion of the 

 curve in each single negative phase, beyond that employed by 

 Bernstein : the longer closure of the galvanometer circuit magni- 

 fies the effect of the variational, or action, current upon the 

 magnets, and multiplies it much more with increased stimulation- 

 frequencies than was possible to Bernstein and Hermann (Fuchs, 4). 



Head's experiments show that the magnitude of the negative 

 variation is chiefly conditioned by the magnitude of the nerve 

 current. On the other hand, it is to a remarkable degree 

 independent of fatigue in the nerve (according to Fuchs, there 

 is some relation between the two in non-rnedullated nerve), in 

 which respect it behaves quite differently from the positive after- 

 variation. Lastly, the duration of the single negative phase is 

 shown to be affected in a marked degree by the condition of the 

 frog. In winter frogs there is a comparatively prolonged single 

 negative phase, although the total negative variation is, relatively 



