484 



Prof. J. C. Bose. Influence of Electric 



causing response of the muscle. Conversely, the application of anode at P 

 causes a depression of excitability of S. Stimulus previously effective now 

 becomes ineffective. In this manner the transmission of excitation may be 

 indirectly modified by the polar variation of excitability of the stimulated 

 point (fig. la). 



In the above experiment it will be noted that for inducing a variation of 

 excitability, the tract of nerve SM, along which excitation is transmitted, need 

 have no current passing through it. The presence of a given pole is enough 

 to induce a definite variation of excitability in its neighbourhood. For 

 convenience I shall call this the Inductive action of a pole. 



The characteristic variations of excitability induced by polar action are :— 



(1) The enhancement of excitability at or near the kathode ; 



(2) The depression of excitability near the anode. 



The boundary between the two polar extensions is reached at a point 

 between the anode and kathode ; this point at which the excitability is 

 unaffected is known as the indifference point. 



The question whether the inductive action of electric poles affects the rate 

 of conduction has been investigated by von Bezold* and by Rutherford. | 

 Von Bezold found that both descending and ascending currents at A (fig. lb) 

 increased the propagation-time between B and C above the normal. 

 Rutherford found on the other hand that the descending current diminished 

 it. The results obtained are thus seen to be indefinite as regards the 

 inductive effect of extrapolar current on conduction. 



Turning from the inductive effect of neighbouring poles, we have the 

 definite object of enquiry : Does the direction of an electric current, as such, 

 cause any selective variation in the propagation of excitation ? In other 

 words, will a homodromous current, i.e. one which flows in the direction of 

 propagation, help or retard transmission of excitation ? Will a heterodromous 

 current on the other hand give rise to an opposite effect ? The object of this 

 particular enquiry is to determine the pure effect of. direction of current on 

 conduction of excitation in a tissue through which a current is flowing. We 

 shall call this the Dynamic effect of a current on conductivity and distinguish 

 it from the Inductive effect. 



The experimental difficulties in isolating the pure effect of current on the 

 intensity and rate of propagation of excitation are very great. In the 

 experiment where " the whole polar region is interposed between the exciting 

 electrode and the muscle, the conditions are (very) complex. I have been 

 unable to find evidence of any marked alteration in propagation rate, unless 



* von Bezold, 'Elektr. Erreg. Nerven u. Muskeln,' Leipzig, 1861. 



t Kutherford, ' Journ. Anat. and Physiol.,' London, vol. 2, p. 87 (1868). 



