26 



ELECTRO-PHYSIOLOGY 



CHAP. 



surface of the ventricle. The galvanometer, of course, shows the 

 same thing with the rheotome method. 



In order rightly to interpret this " diphasic action current," 

 it is essential to determine the rate at which the excitation 

 (? the effect of stimulus) spreads in the parenchyma of the 

 leaf. 



Burd on- Sanderson used a pendulum rheotome for this purpose, 

 with which it is easy to determine the time between the moment 

 of excitation and commencement of the consequent electrical 

 variation of the leaf-current. This current was, as before, led off 

 from the middle of the opposite surfaces of a lamina. In a 

 preliminary series of experiments, the exciting electrodes were 

 placed on either side of a leading-off electrode on the upper 



Fio. 148. 



surface of a leaf, so that a straight line connecting the two passed 

 through the leading-off contact. The first perceptible trace of phase 

 I. of the excitatory variation generally appeared 0*041 sec. after 

 the moment of excitation. If time is required for the spread 

 of the excitatory activity, it is evident that the " latent period " 

 must be very much longer (with unaltered position of the leading- 

 off electrodes) when the stimulus is applied to the lobe that is 

 not led off. That this is so appears from Burdon - Sanderson's 

 experiments, where the interval between excitation and beginning 

 of the variation as a rule exceeds 0*073 sec. Accordingly, if we 

 estimate the distance between the two points excited in succession 

 at 6 mm., the transmission of excitation must occur at about 200 

 mm. per sec. (at a temperature of 30-32 C. in air saturated with 

 aqueous vapour). An even greater disparity between the latent 

 periods in the two cases might have been expected, supposing 

 that, as in muscle, there were no perceptible latent period in the 



