Currents on Transmission of Excitation. 505 



rise once more to an arrest of conduction at 2 microamperes and a reversal 

 at 10 microamperes. It is a curious fact that the reversal under hetero- 

 dromous and homodromous currents takes place, generally speaking, at the 

 same intensity, namely, 10 microamperes. 



Before passing under review the characteristic results obtained under 

 varying conditions of the experiment, I shall discuss briefly the question 

 whether it is possible to explain the observed results merely by considering 

 the induced variation of excitability as the sole cause. We shall take, 

 then, the simple case of arrest of conduction by homodromous current ; I 

 find that the arrest takes place just the same, whether the anodic electrode 

 is placed on the spine or on an adjacent point n, on the nerve itself (see 

 fig. 6). Discarding from our consideration the possibility of an induced 

 variation of conductivity, we may assume that the arrest was due to the 

 depression of excitability of the stimulated point of nerve on account of 

 the proximity of the anode. But the point of stimulation was, in general, 

 placed not near the anode, but in the middle or indifferent region. In 

 fact the diminution or arrest of transmitted excitation was observed even 

 in the case where the stimulus was applied at the far end of the nerve, 

 at a distance of about 70 mm. from the anode at one end of the nerve, and 

 only 20 mm. from the muscle at the other end. Against this it might be 

 urged that under the action of strong currents the anodic depression might 

 extend to a considerable distance. 



It has, however, been shown that for causing a depression or arrest of 

 transmitted excitation a strong current was not at all necessary, such a 

 depression sometimes taking place under an intensity of current as feeble 

 as - 3 microampere, the applied E.M.F. being less than one third of a 

 volt. The difficulty of explaining the observed results by an assumption 

 of induced variation of excitability would thus appear to be insurmountable. 

 This difficulty is greatly intensified — indeed borders on the impossible — 

 when we follow the same reasoning as regards the action of increasing 

 intensity of homodromous current beyond the critical point. With stronger 

 current, not only will the indifferent point be pushed towards the kathode, 

 but the depression induced by the anode will be so great as to render the 

 stimulated point of the nerve inexcitable. There being no excitation to 

 be transmitted, the response should then undergo an extinction. Instead 

 of this we find that the response shows an actual enhancement, on account 

 of the reversal of the induced variation of conductivity which has already 

 been described. This shows conclusively that the phenomenon we have 

 studied is due not to a variation of excitability, but to that of conductivity. 



