THE NATURE OF FATIGUE 189 



central nervous system exactly with a view of determining its relative 

 susceptibility to fatigue are unsatisfactory. The preponderance of 

 evidence at present seems to me to be in favor of a high degree of 

 resistance to fatigue on the part of the brain and spinal cord, as of the 

 nerve fiber itself. In fact, such a condition is what we should expect 

 a priori. The nervous system is the administrative instrument of the in- 

 dividual ; it directs, controls and harmonizes the work of the parts of the 

 organic machine, and gives unity to the whole. It is not the frail, 

 delicate thing, easily put out of gear, that we at times believe it to be. 

 It is capable of enormous demands on its powers and of enormous re- 

 sistance. It is the last system to succumb in many diseases and in 

 such a dire condition as starvation. It would seem to be only highly 

 advantageous to the organism that its nervous system should be able 

 to resist the oncoming of fatigue, with all the direful consequences 

 that might follow its advent. 



After thus analyzing the phenomena of fatigue in their manifesta- 

 tions in the various organs and tissues of a complex body, let us briefly 

 consider fatigue as we feel it in ourselves. When we perform a long- 

 continued and ultimately fatiguing task, either physical or mental, we 

 can recognize, with little difficulty, three successive stages of working 

 power, although these are not sharply separated from one another. 

 During the first stage our working power gradually increases; during 

 the second it remains approximately stationary at a high level; during 

 the third it gradually decreases. During the first stage our per- 

 formance is at first distinctly up-hill work; we find it difficult to con- 

 centrate our attention; we feel already fatigued; we could easily give 

 up and do no more. But, surprisingly enough, if we keep on we find the 

 work getting easier; we can accomplish more and more, seemingly 

 without greater effort; we seem to be breaking through barriers that 

 have hindered us; our sensations are agreeable; we say that we are 

 getting our second wind; we feel new courage and no longer care to 

 give up. Before we realize it we have gotten our second wind and 

 have passed into the second stage ; our working power is at its best, and 

 we continue to labor, heedless of time; if we attempt to philosophize, 

 we are only conscious of the fact that our labor is easy and our burden 

 light. But this stage, though it may be long continued, ultimately gives 

 place to the third stage when we realize that our powers, after all, are 

 limited, that work is hard, that either we must put forth greater efforts 

 or our output diminishes, and that we are really tired. Now these 

 three stages of individual labor are but the three stages which we have 

 already seen epitomized in the isolated muscle — the treppe, the period 

 of maximum contractions, and the fatigue — and I do not doubt that 

 they are associated with the same chemical phenomena. The stage of 

 getting our second wind is when our fatigue substances are in minute 

 quantity, and they gradually augment our physiological irritability and 



