REVERSED POLAR EFFECTS IN LIVING TISSUES 201 



I soon discovered that the value of the acting electromotive 

 force had an important influence on polar excitation. I 

 found that under an increasing E.M.F. the excitation pro- 

 duced at the kathode underwent, first an increase, and then, 

 on reaching a maximum, a decrease, which might even 

 become negative. The changes produced at the same time 

 at the anode were exactly the opposite. There was thus a 

 progressive variation, resulting in an exchange of the excita- 

 tory properties of the anode and kathode. 



My first observation with regard to this question was 

 made in the course of my investigations on the determination 

 of the velocity of transmission of excitation (Chapter XX.), 

 and on the effect of increasing intensity of stimulation 

 en this velocity. I found that, using for instance thermal 

 stimulation, when this was strong, it was transmitted with a 

 greater velocity than when it was feeble. Hence the speed 

 with which the effect of stimulus travels in a given tissue 

 may be held to afford a measure of the effective intensity of 

 the stimulus. In order, however, to apply a stimulus which 

 might be increased by known amounts, I next tried the electric 

 mode of stimulation, expecting to produce an increasingly 

 effective intensity of stimulus by increasing the E.M.F., excita- 

 tion being produced at the kathode at make. In the course 

 of a particular experiment, on a leaf of BiopJiytum, I found 

 that as the E.M.F. was augmented from eight to thirty-two 

 volts, the excitatory value of kathode at make was also 

 increased, as shown by the fact that the velocity of the 

 transmission of excitation was raised from 3*27 mm. per 

 second, in the former case, to 3-83 mm. in the latter, an 

 increase, that is to say, of 17 per cent. 



But on raising the E.M.F. still higher, I found that the 

 velocity of transmission from this point underwent a progres- 

 sive decrease. From this it would appear that the excitatory 

 effect of the kathode at make had an optimum value, in this 

 case of thirty-two volts, beyond which there was a decline. 

 This optimum value would naturally undergo a certain varia- 

 tion with the nature and condition of the tissue. Now, as the 



