100 DYNAMICS OF LIVING MATTER 



ing or a much weaker one occurs when a spark is produced. The same 

 effect can be produced when a Rumkorff induction apparatus is used 

 in place of a Toepler-Holtz machine. 



The explanation of the experiment is as follows: let us assume 

 that at a given moment the electrode a (Fig. 16) be charged positively 



and b negatively; in this case the 



nerve will possess at c a negative, at 



d a positive charge. As soon as the 

 spark passes, the charges in the nerve 

 will disappear also; a current will go 

 lengthwise through the nerve, and a 

 twitching will result. If, however, the 

 nerve is placed at right angles to the 

 spark discharge and symmetrical in 

 regard to the two electrodes (Fig. 17), 

 the current must go crosswise through 



the nerve, and no effect or only a minimal effect ensues. In this experi- 

 ment no electrical charges are withdrawn from the ions, and the only 

 effect of the current exists in a change in the concentration of the ions 

 at various places in the nerve. Since, however, in this case of stimula- 

 tion of the nerve by induction, the effect is the same as in the case of a 

 direct application of the electrodes to the nerve, we must draw the con- 

 clusion that in the latter case also the change of concentration is suffi- 

 cient for the physiological effect, and that the withdrawal of the charge 

 from the ion cannot be the cause. 



Nernst has tested the idea that the electrical current only stimu- 

 lates the nerve by bringing about changes in the concentration of ions 

 at various places in the nerves.* It had been known for some time 

 that the interrupted current is an excellent medium for stimulating 

 nerves or muscles, and it was also generally known or accepted that 

 alternating currents produce only weak effects when the number of 

 alternations becomes very high. Quantitive determinations had also 

 shown that the minimum intensity of an alternating current which is 

 required to bring about tetanus, increases with the number of alterna- 

 tions. These facts Nernst used as a starting point to test the idea that 

 the current acts only by changing the concentration of ions at the place 

 of stimulation. "According to our present knowledge, the galvanic 

 current cannot produce in a tissue, i.e. a purely electrolytic conductor, 

 any other effects than displacements of ions, i.e. changes of concen- 

 tration; we therefore conclude that the latter must be the cause of 

 the physiological effects. In the case of an alternating current, changes 



* Nernst, Nachrichten der Gesellschaft dcr Wissenschaften zu Gottingcn, p. 104, 1899. 



