98 INTRODUCTION. 



the current may be led in to the nerve by closing the exciting 

 current in the main or battery circuit, that is to say by placing a 

 mercury key between element and compensator. But it is also 

 possible to close the nerve-circuit after the main circuit by em- 

 ploying a key placed between electrodes and compensator. 



Du Bois-Reymond 1 experimented with regard to this point with 

 polarisable electrodes, and found marked differences in the effects 

 of stimulation according as he employed the one or the other 

 method. ' When I opened and closed the main circuit,' he says, 

 'contraction resulted with a length 2 of even a few cm., while often 

 the whole length of the derivation- circuit wire (of about a m. long) 

 was insufficient to evoke contraction on opening and closing the 

 nerve-circuit. Immediate excitation of muscle showed the same 

 difference, but between limits whose absolute values were higher.' 



Du Bois-Reymond's explanation is as follows : ' When the main 

 circuit is closed after the nerve-circuit, the one division of the 

 current enters the nerve-circuit with an intensity conditioned by 

 the relationship of the resistances. When the main circuit is 

 opened, the current which is abolished in the nerve-circuit has 

 been previously reduced to a minimum by polarisation. But the 

 latter current, since a path is left open through the derivation- 

 circuit, is immediately followed by the polarisation-current in the 

 opposite direction, whose strength is perhaps equal at first to that 

 of the primary current, and which is therefore well suited to elicit 

 contractions. The charges are for the most part got rid of in- 

 stantaneously in the polarisation-current, so that, when the key in 

 the main circuit is again closed, the same series of events is re- 

 peated. If, however, the nerve-circuit is closed after the main 

 circuit, the current strength will be the same at the first closure as 

 at closing the main circuit after the nerve-circuit. Since, how- 

 ever, opening the nerve-circuit prevents the charges which im- 

 mediately develop from discharging themselves, opening of this 

 circuit is comparatively inoperative, and, secondly, the current 

 strength at closure after a not too long interval will not be so 

 great as before.' 



In the more or less complete leading off of the external polarisa- 

 tion-current arising between electrodes -and nerve-tissue, du Bois- 

 Reymond thus finds the explanation of this striking difference. 



1 Ges. Abhandlungen, i. p. 196, 1875. 



2 That is, the wire of the compensator or rheochord used. See diagram on a subse- 

 quent page. 



