ELECTROTONUS 571 



decrement. But we shall presently find that the direction of 

 the responsive current is the only constant factor here, de- 

 termined as this is by the relative excitabilities of the two 

 electrodes. An identical variation of excitability may, as 

 I shall show, appear under different circumstances, either as 

 a polarisation-increment or as a decrement. 



I have obtained results precisely like the foregoing, with 

 the sciatic nerve of frog. It should be mentioned here that 

 such effects are obtained without much difficulty in the first 

 stages of polarisation. But, if this be prolonged, there is 

 a certain liability to reversal. 



From the experiments which have been described on 

 variations of excitability by the polar action of currents, we 

 arrive at the following law : 



A feeble E.M.F, induces modifications in the excitability of a 

 tissue : the anode enhances and kathode depresses excitability, 



This result is startling, contravening, as it does, PfHiger's 

 Law. A factor that had not been taken into account 

 was the range of E.M.F., within which this law might 

 be applicable. In the present case, the acting E.M.F. is 

 relatively feeble, and we shall see later that Pfliiger's Law 

 does not apply above or below a certain medium range. 



Having thus obtained the isolated effects of electric 

 currents on conductivity and excitability respectively, I 

 next took up those more complex cases in which both 

 effects were present in various combinations. This problem 

 was attacked by means of the Conductivity Balance. In 

 this experiment, carried out on the petiole of fern, the led-off 

 points E and E' were at a distance of 6 cm. from each other. 

 The distance of each of the polarising electrodes A and K 

 outside the led-off circuit E' and E was 2 cm. (figs. 351 and 

 352). The thermal stimulator S was so adjusted before 

 the passage of the current that the excitations at E' and E 

 were exactly equal, as seen in the balanced horizontal record 

 n fig- 353 a - It should be mentioned here that the galvano- 

 meter connections were so arranged that an increased 

 excitability of the left-hand contact E', would be shown by 



