Currents on Transmission of Excitation. 



491 



of 40 mm. from either the anode or the kathode, and may therefore be 

 regarded as situated in the indifferent region. This is found to be verified in 

 actual experiments. 



2. Effects of Direction of Current on Velocity of Transmission. 



A very convincing method of demonstrating the influence of electric 

 current on conductivity consists in the determination of changes induced in 

 the velocity of transmission by the directive action of the current. For this 

 purpose we have to find out the true time required by the excitation to travel 

 through a given length of the conducting tissue (1) in the absence of the 

 current, (2) against and (3) with the direction of the current. The true time 

 is obtained by subtracting the latent period of the pulvinus from the observed 

 interval between the stimulus and response. Now the latent period may not 

 remain constant, but undergo change under the action of the polarising 

 current. It has been shown that the excitability of the pulvinus does not 

 undergo any change when it is situated in the middle or indifferent region. 

 The following results show that under parallel conditions the latent period 

 also remains unaffected : — 



Table I. — Showing the Effect of Polarising Current on the Latent Period. 



Specimens 



I. 



II. 



,, „ heterodromous current 



,, „ homodromous current 



sec. 



o-io 



0-11 

 0-09 



sec. 

 0-09 



o-io 



09 



The results of experiments with two different specimens given above show 

 that a current applied under the given conditions has practically no effect on 

 the latent period, the slight variation being of the order of one-hundredth 

 part of a second. This is quite negligible when the total period observed 

 for transmission is, as in the following cases, equal to nearly 2 seconds. 



Induced Changes in the Velocity of Transmission. — Having found that the 

 average value of the latent period in summer is 0*1 second, we next proceed 

 to determine the influence of the direction of current on velocity. 



Experiment 1. — As a rule, stimulus of induction shock was applied in this 

 and in the following experiments on the petiole at a distance of 15 mm. from 

 the responding pulvinus. The recording writer was tuned to 10 vibrations 

 per second ; the space between two succeeding dots, therefore, represents a 

 time-interval of - l second. The middle record, N in fig. 3, is the normal. 

 There are 17 spaces between the application of stimulus and the beginning 



