Currents on Transmission of Excitation. 487 



Part II. — Influence of direction of electric current on conduction of 

 excitation in animal nerve. 



1. The method of experiment. 



2. Variation of velocity of transmission under heterodromous and 

 homodromous currents. 



3. Variation in the intensity of transmitted excitation under the action 

 of heterodromous and homodromous currents. 



4. After-effects of heterodromous and homodromous currents. 



5. Phenomenon of reversal. 



Part I. — Influence of Direction of Current on Transmission of 

 Excitation in Plant. 



1. The Method of Experiment. 



I may here say a few words of the manner in which the period of 

 transmission can be found from the record given by my resonant recorder, 

 fully described in my previous paper. The writer attached to the recording 

 lever of this instrument is maintained by electromagnetic means in a state of 

 vibration to and fro. The record thus consists of a series of dots made by the 

 tapping writer, which is tuned to vibrate at a definite rate, say 10 times per 

 second. This method of intermittent contact not only removes the error due 

 to friction, but also enables time-relations to be deduced from the record. 

 In a particular case whose record is given in Curve 1 (fig. 3) indirect 

 stimulus of electric shock was applied at a distance of 15 mm. from the 

 responding pulvinus. There are 15 intervening dots between the moment of 

 application of stimulus and the beginning of response ; the time-interval is 

 therefore 1*5 seconds. The latent period of the motile pulvinus is obtained 

 from a record of direct stimulation ; the average value of this in summer is 

 O'l second. Hence the true period of transmission is 1*4 seconds for a 

 distance of 15 mm. The velocity determined in this particular case is 

 therefore 10 - 7 mm. per second. 



Disturbance caused by the Leakage of Current. — Employing this method of 

 record, an attempt was made to determine the changes of velocity under the 

 action of heterodromous and homodromous currents. But a serious difficulty 

 encountered at the beginning of the investigation arose from the leakage of 

 the induction current used as the testing stimulus. This will be understood 

 from a concrete example. The record in fig. 3, for example, shows 15 inter- 

 vening dots between the moment of indirect application of stimulus (at a 

 distance of 15 mm.) and the beginning of response. The recorded time-interval 

 for transmission was thus 1-5 seconds. The latent period of the pulvinus 



