in ELECTRICAL EXCITATION OF MUSCLE 313 



to contribute largely to our knowledge of the nature of the polar 



working of the current. 



SUMMARY 



Gathering up the results of these detailed investigations"' 

 into the visible effects of electrical excitation in different con- 

 tractile substances, certain points of view present themselves 

 from which it appears possible at least to conjecture the specific 

 action of current. 



In the first place, it must be remarked that the law of 

 excitation which du Bois-Keymond postulated as universal (only, 

 it is true, with regard to the electrical excitation of motor nerves, 

 but which was subsequently applied to the direct excitation of 

 contractile substances also) is not found to be a correct repre- 

 sentation of fact. It cannot therefore be taken as the basis of 

 theoretical conclusions in regard to the specific nature of electrical 

 excitation. The law in its original form was as follows : " The 

 motor nerve (or muscle, contractile protoplasm) responds by the 

 twitch of the muscle belonging to it (or other excitatory symp- 

 tom), not to the absolute value of current density at the moment, 

 but to the alterations of this value from one moment to an- 

 other the stimulus to movement consequent on these changes 

 being greater in proportion to their rapidity at uniform magni- 

 tude, or amount per unit of time." 



Even if we admit that in many cases, particularly in all quickly 

 reacting and quickly conducting contractile substances, the effect of 

 excitation (so far as it is expressed by visible change of form) is 

 especially prominent at the moment when current is made or 

 broken (closure and opening twitch), there cannot on the other 

 hand be the slightest doubt that the electrical current in every 

 case produces during its entire closure that change in the irritable 

 substance which is fundamental on the one hand to excitation, and 

 on the other to antagonistic inhibitory processes. In many cases 

 these continuous effects are the only consequence of electrical 

 excitation (e.g. smooth muscle, many protozoa). Currents of 

 insufficient duration are ineffective, as may be demonstrated on 

 striated muscle by the application of various methods (supra). 

 Under all conditions the current, in order to excite, must have 

 a certain period of duration greater in proportion with the 



