The appreciation of the fact that in the reactions of muscle or nerve tissue to electrical stimulation the duration of the current is just as important as, if not more than, its intensity has opened up a new field in the investigation of this branch of physiology. Ever since the introduction of this concept by Lapicque, in 1909, there has been a steadily growing interest in the subject and an ever increasing accumulation of experimental data in its theoretical and practical aspects. At present the literature has reached such dimensions that it would be altogether outside the scope of the present communication to attempt even a brief review of it, especially since it has been done in such a thorough fashion by Lapicque,1 Bourguignon,2 Fredericq,3 Stein,4 von Weizsäcker5 and others. In introducing the special phase of the subject that we shall discuss, we shall restrict