ELECTRICAL BREAK-EXCITATION. 109 



In a fourth experiment, for which a current of two Daniells was used, the first 

 break-contractions occurred with the following values of the derivation-circuit, counting 

 in the same order. With ascending current at 170, 220, 150, 50; with descending 

 current at 190, 280, 160, 60. In order to have the resistance equal in cases I and 2, 

 and in cases 3 and 4, when the metallic electrodes were placed on the nerve the pre- 

 viously used impolarisable electrodes were brought into the nerve- or muscle-circuit, 

 lying end to end. 



Thus when the external polarisation-current is added to the 

 internal the occurrence of break-contractions on opening the main 

 circuit is favoured, as is evident from the experiments and readily 

 intelligible. (Compare cases 3 with cases 4.) If, however, the 

 current is opened in the nerve-circuit, the rule is that the occur- 

 rence of break-contractions is not only not favoured, but even 

 hindered. (See cases I and 2 in the second series, with ascending 

 current.) This would seem to mean that there is far less resistance 

 to an internal polarisation-circuit lying altogether in the nerve, 

 than to a similar external one. To this point I have not further 

 directed my attention. 



Muscle behaves like nerve according as polarisable or impolaris- 

 able electrodes are used arid the main circuit is opened. 



EXPERIMENT. A sartorius muscle of a medium-sized curarised grass-frog is laid on 

 the electrodes in such a way that no muscle-current passes through them (position of 

 indifference). Two Daniells. There was a break-contraction for both ascending and 

 descending currents in case 3 (polarisable electrodes, main current opened) with 

 190 cm. of wire. In all other cases no break-contraction up to 300 cm. of wire. 

 With a current of six carbon-zinc elements a similar result. Break-contraction 

 only in case 3 with 80 cm., in all other cases no break-contraction up to 300 cm. 

 Arrangement similar to that of the previous experiments. 



We now arrive at the consideration of those cases in which the 

 secondary polarisation-current through the tissue to be excited is of 

 insufficient density, or for any other reason fails. 



A' number of both earlier and more recent observers (Cl. Bernard, 

 Schiff, Valentin, Fick, Rumpf) have drawn attention to the fact 

 that break-contractions occur less readily with uninjured nerves in 

 the living animal than with those which have been cut across and 

 prepared. There are clearly two causes for this. On the one hand 

 the continuance of the circulation must oppose polarisation by 

 removing its products, on the other hand when a nerve is not 

 insulated and lying on the electrodes the polarisation-current can 

 pass along other paths, so that the current passing through the 

 nerve is weakened. 



There is a fact which is evidently related in nature, and is easy 

 enough to observe, though, so far as I know, it has not been more 



