#Fatigue.#--It is assumed that in living substances like muscle, fatigue is caused by the break down or dissimilation of tissue by stimulus. And till this waste is repaired by the process of building-up or assimilation, the functional activity of the tissue will remain below par. There may also be an accumulation of the products of dissimilation--'the fatigue stuffs'--and these latter may act as poisons or chemical depressants. In an animal it is supposed that the nutritive blood supply performs the two-fold task of bringing material for assimilation and removing the fatigue products, thus causing the disappearance of fatigue. This explanation, however, is shown to be insufficient by the fact that an excised bloodless muscle recovers from fatigue after a short period of rest. It is obvious that here the fatigue has been removed by means other than that of renewed assimilation and removal of fatigue products by the circulating blood. It may therefore be instructive to study certain phases of fatigue exhibited under simpler conditions in vegetable tissue, where the constructive processes are in abeyance, and there is no active circulation for the removal of fatigue products. It has been said before that the E.M. variation caused by stimulus is the concomitant of a disturbance of the molecules of the responsive tissues from their normal equilibrium, and that the curve of recovery exhibits the restoration of the tissue to equilibrium. #No fatigue when sufficient interval between successive stimuli.#--We may thus gather from a study of the response-curve some indication of the molecular distortion experienced by the excited tissue. Let us first take the case of an experiment whose record is given in fig. 20, _a_. It will be seen from that curve that one minute after the application of stimulus there is a complete recovery of the tissue; the molecular condition is exactly the same at the end of recovery as in the beginning of stimulation. The second and succeeding response-curves therefore are exactly similar to the first, _provided a sufficient interval has been allowed in each case for complete recovery_. There is, in such a case, no diminution in intensity of response, that is to say, no fatigue. We have an exactly parallel case in muscles. _'In muscle with normal circulation and nutrition there is always an interval between each pair of stimuli, in which the height of twitch does not diminish even after protracted excitation, and no fatigue appears.'_[10] [Illustration: FIG. 20.--RECORD SHOWING DIMINUTION OF RESPONSE WHEN SUFFICIENT TIME IS NOT ALLOWED FOR FULL RECOVERY In (_a_) stimuli were applied at intervals of one minute; in (_b_) the intervals were reduced to half a minute; this caused a diminution of response. In (_c_) the original rhythm is restored, and the response is found to be enhanced. (Radish.)]