264 ON POSITIVE VARIATION OF THE NERVE CURRENT 



no trace of positive variation, but an actual weakening of the nerve 

 current amounting to several divisions of the scale. In this respect 

 the positive after-effect behaves in an opposite way to the negative 

 variation. The weakening or negative variation of the nerve 

 current which takes place during stimulation, is constant with 

 moderate stimulations which may last for several minutes, and in- 

 creases rather than diminishes with the duration of the stimulation. 

 With stimulations which lasted for five minutes, I repeatedly 

 observed that there was no falling off in the amount of negative 

 variation, but rather a moderate increase in it. This increase corre- 

 sponded only in isolated cases and then only partially, with a per- 

 sistent weakening of the nerve current, while in other cases the 

 nerve current was quite unchanged at the close of a long stimulation. 

 The law of the dependence of the positive variation on the duration 

 of stimulation could not be accurately deduced without the statistics 

 of numerous experiments ; because with repeated stimulation of a 

 nerve the diminution of the positive after- variation must present 

 itself even when the duration of stimulation remains unaltered. 

 Immediately after a long stimulation, a short stimulation with the 

 same strength of current gives no positive variation. If, however, 

 the nerve has a longer period of rest, a strong positive variation 

 again appears. 



The following tables, together with those already quoted, give an 

 approximate idea of the manner in which the amount of positive 

 variation changes with the duration of stimulus on repeated stimu- 

 lation of the same nerve. The Roman numerals again indicate the 

 order of the individual stimulations, which followed each other at 

 intervals of about 2-3 minutes. 



Warm frog. Du Bois induction-coil. Galvanometer tract 10 mm. Inter- 

 mediate tract about 30 mm. With switch. 



Current of electrodes +17. Nerve current + 1 70. 



I. Duration of stimulation 3 sec. + 2 



II. 

 III. 

 IV. 



V. 

 VI. 



5 



10 



30 

 30 



5 



+ 3 

 + 4-5 

 + 5 

 + 4 

 + 0.5 



Cold frog. Du Bois induction-coil. Galvanometer tract 10 mm. Intermediate 

 tract 35 mm. With switch. Pause between the several stimulations about 2 min. 

 Current of electrodes 59. Nerve current + 8l. 



I. Duration of stimulation I sec. + 1 



II. 2 , ? 



