Page:Popular Science Monthly Volume 76.djvu/193

Rh 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 individual; 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 resistance. 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 manifestations 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 performance is at first distinctly up-hill work; we find it difficult to concentrate 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