refractory period of the nerve, which is characterized by its extremely brief duration. They found, depending upon the temperature, a period of nonirritability of .001-.008 seconds after every stimulus. The investigations of Miss _Buchanan_[129] lead us to conclude that there is a refractory period for the cross striated skeletal muscle. Miss _Buchanan_ stimulated the muscle at times through the nerve, at other times directly after elimination of the nervous element, with very frequent electrical stimuli (about 1000 in the second) and found by means of the capillary electrometer a rhythmical reaction of the muscle of about 50–100 excitation shocks per second. Likewise the _Ritter_ tetanus produced by the breaking of an increasing current proved to be a rhythmical reaction of an analogous nature. In a more direct manner _Keith Lucas_[130] has determined the refractory stage for the musculus sartorius of the frog. He allowed two induction shocks to act successively on the muscle at intervals of varied duration and then registered the action currents by means of the capillary electrometer. He then found that the second stimulus was ineffective for about .005 seconds after the application of the first stimulus. If the second stimulus follows somewhat later, it produces a contraction which is weaker and has a longer latent period the nearer the second stimulus approaches the first in point of time. (Figure 30.) _Massart_[131] and _Jennings_[132] likewise observed the existence of a refractory period for the myoids of unicellular organisms brought about by mechanical stimuli. _Massart_ attributes this cessation of reaction to stimuli following each other at certain intervals, to fatigue, an explanation which has been disputed by _Jennings_ as the result of his investigations made on Stentor and Vorticella. _Jennings_ looks upon the behavior of the infusoria rather as an “adaptation” to the stimulus. _Pütter_ was the first to see in this the existence of a refractory period. His experiments on Spirostomun ambiguum in 1900 showed a refractory period in the reaction to rhythmical mechanical stimuli. I wish to state, however, that these observations of _Pütter_ have not as yet been published. Thus the existence of a refractory period has even today been proved for a whole series of very different kinds of substances. [119] _Marey_: “Des excitations artificielles du cœur.” Travaux du lab. de M. _Marey_ II, 1875. The same: “Des mouvements qui produit le cœur lorsqu’il est soumis à des excitations artificielles.” Comptes rendues de l’academie des sciences T. L. XXXII, 1876. [120] _Bowditch_: “Ueber die Eigenthümlichkeiten der Reizbarkeit welche die Muskelfasern des Herzens Zeigen.” Arbeiten aus der physiologischen Anstalt zu Leipzig, 1872.