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THE POPULAR SCIENCE MONTHLY 



stage of fatigue proper. Decrease in working power may, in fact, be 

 said to be the universal physical phenomenon of fatigue, whatever form 

 of protoplasm we may be considering. Decrease in working power is 

 accompanied by a decrease in irritability. The stimulus remaining 

 the same, the work is diminished; but if the stimulus be increased in 

 intensity, the protoplasm may again perform more work for a brief 



time. Sooner or later, however, all 

 stimuli cease to be effective, and 

 the living substance is then either 

 exhausted or dead. 



If, in our graphic record of 

 muscular fatigue, we are employing 

 a favorite subject of physiological 

 study, the muscle of the frog, we 

 observe another striking physical 

 change. Early in the series, even 

 before the treppe has reached its 



Fig. 2. Series of contractions of the 

 frog's gastrocnemius muscle, excised and 

 stimulated at intervals of two seconds. 

 Every fiftieth contraction is recorded. 

 The increase in the duration of the proc- 

 ess of relaxation as fatigue proceeds is 

 shown in the progressive lengthening of 

 the descending limb of the curves. 



maximum, the duration of each 



contraction begins to increase, 



mainly by a slowing of the process 



of relaxation (Figs. 2 and 3). 



This may reach great proportions 



before exhaustion sets in. This slowing of relaxation appears to be 



wholly absent in the fatigue of warm-blooded, and presumably of human 



muscle (Fig. 4). 



The fatigue of mnscle tissue is thus characterized by marked phys- 

 ical peculiarities. It is only natural to ask what are the causes of these. 

 Happily it is becoming the fashion in physiology, if only slowly 

 and following long after, it is true, the usage of John Stuart Mill, to 

 speak less of causes than of conditions. The cause of a phenomenon 

 is the sum total of its conditions. All conditions are causes, and it is 

 illogical to select one or two conditions and dignify them by the seem- 

 ingly superior designation. In speaking of the causes of fatigue, as is 

 often done, one usually means its chemical conditions; for within 

 protoplasm, when in activity, there occur certain chemical or metabolic 

 changes, with which the phenomenon of fatigue is closely associated. 

 These chemical changes involve two general processes, namely, the con- 

 sumption of certain existing substances which are essential to the 

 activity of the protoplasm, and the production and accumulation within 

 it of certain waste substances. Here again the muscle has yielded us 

 our chief knowledge. Of the substances that are consumed in proto- 

 plasmic activity, we know most about two, oxygen and carbohydrate. 

 For all aerobic tissues or organisms a continual supply of oxygen is 

 essential to the continuance of working power — in fact, one way of 

 bringing on the main phenomena of fatigue seems to be by eliminating 



