114 THE NATURE OF THE 



latter, and not the disappearance of the former, that excites. In 

 accordance with this inference, I should express as follows the 

 law according to which currents excite a nerve or muscle : 



1. A nerve or a muscle is excited by the coming into existence of a 

 current through it. The height of current-strength and the rapidity 

 with which this height is attained are of account, in that the excita- 

 tion increases with them up to a certain limit. 



2. A muscle or a nerve is also excited, though to a less extent, by a 

 current flowing through it at a constant strength *. For muscle this is 

 known ; for sensory nerves and the dilating vascular nerves of the 

 skin I have demonstrated it 2 ; that it is also true for motor and 

 secretory nerves follows if we assume that all nerves are similar, 

 or at least not very different, even though we may not be able 

 to demonstrate excitatory effects in the end-organs of these nerves : 

 for the reaction of the end-organs depends on conditions inherent 

 in themselves, and on other factors not affected by excitation 

 of a nerve. Moreover, Pfliiger 3 and von Bezold 4 long ago asserted 

 that under certain circumstances motor nerves react to a current of 

 constant strength. 



3. On the other hand, a nerve or muscle is not excited by the dis- 

 appearance of a current from it (at any rate this has not been 

 proved). The excitations observable on opening exciting cur- 

 rents correspond with the coming into existence of other currents 

 (muscle- or nerve-currents or polarisation after-currents), and it is 

 to this that the excitatory action is due. 



4. The direction of the exciting current, provided only that it be 

 parallel to the axis of currentless nerves or muscles, does not appreciably 

 affect the strength of the excitation of these organs themselves. In the 

 case of a nerve, however, the effect on its end-organ is of course 

 modified if at make of a strong ascending current its anode, or if at 

 break of a descending current the anode of the negative polarisation- 

 current lies in the way and obstructs the passage of the excitation. 

 I agree with Tigerstedt in distinguishing two strengths of stimulus 

 here, namely, weak currents, at whose closure the excitation at the 



1 Strictly speaking, no currents at least, no currents of short duration, such as are 

 commonly employed for stimulation flow through a nerve or muscle at a constant 

 strength. For since both organs are polarisable there must occur in the first moments 

 after closure an increase of current strength from positive, and immediately afterwards 

 a decrease from negative polarisation. 



3 Pfluger's Archiv, xvii. p. 215. 



3 Elektrotonus, p. 453. 



4 Untersuchungen iiber die elektrische Erregung, Leipzig, 1861, p. 325. 



