34 GENERAL PHYSIOLOGY 



in more recent years by Hertz and Rontgen, and those emitted by uranium and 

 radium. 



(/) Electrical stimuli, produced by the exposure of the organism to the electri- 

 cal current. Magnetic stimuli are no longer recognized, because it seems that 

 living substance cannot be influenced by magnets. 



Besides the quality of the stimulus, we must also take into account 

 its ''strength, " this term being employed at this time in a quantitative 

 way to characterize the sum total of its intensity, duration and fre- 

 quency. Every organism is constantly under the influence of stimuli 

 of all sorts which, as long as they retain a normal intensity, give rise 

 to normal reactions. The conditions prevailing at this time, may 

 be said to be optimum in their character. Living substance reacts 

 toward these in the best possible manner. But, stimuli may also be- 

 come excessive, and force the living substance to react maximally. It 

 is only natural to assume that a continued maximal activity must finally 

 produce injurious effects. Lastly, stimuli may possess so slight an 

 intensity that they fail absolutely in producing an effect. Minimal 

 stimuli, and especially subminimal stimuli, must eventually prove as 

 dangerous to life as maximal ones. 



STlli 7?!' 



Trt STR 



T 



A b K 



Fig. 5. — Intensity of Stimulation. 

 L, life; D, death; SMi, subminimal; Mi, minimal; 0, optimum; M, maximal; SM, 

 supramaximal stimuli; T, threshold. 



It appears, therefore, that Hfe is possible only between these 

 two extremes and that death must result whenever this realm is 

 exceeded in either direction. To begin with, therefore, living matter 

 may be subjected to the subminimal stimuli toward which it does 

 not react at all, at least not visibly. Eventually, however, a strength 

 of stimulus will be reached toward which it reacts just barely. At 

 the point where these minimal reactions just begin to appear lies 

 the threshold of stimulation. If the strength of the stimulus is now 

 increased still further, a point will be reached at which the reactions 

 become maximal and finally, a point at which they show a supra- 

 maximal character. 



It should be emphasized, however, that the preceding outline can- 

 not be applied rigidly in all cases, because living substance exhibits 

 certain differences in its behavior which are dependent upon differ- 

 ences in its chemical and physical constitution. Thus, optimum 

 conditions are not always found midway between the minimal and 

 maximal extremes, and neither does a certain kind of maximal stimulus 

 invariably cause fatigue or death. It is a matter of common observa- 

 tion that the energy contained in a stimulus is always very much 

 smaller than the subsequent production of kinetic energy. To some 

 extent living substance also possesses the power of adapting itself 



