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O the conscientious schoolmaster the contemplation of a dissertation on the nature of fatigue can hardly seem an unmixed joy; for the subject is one with which he is already practically and sadly familiar. I may say at once, however, that I have not come here to remind you too acutely of this aspect of your professional work, your sensations at the end of a busy day in the class room. I, too, have felt those sensations, and I know how dully depressing, to both mind and body, they may be. They must be reckoned with and eliminated in the reform of the school. Much is written on the fatigue of the school child, but the ideal school course will allow the teacher too to bring an un jaded spirit to his successive tasks. In his Utopia the teacher will have his playground, as the child now has his. My present task, however, is not to limit myself to a discussion of the fatigue that is incident to life in the schoolroom, but to present to you a study of a specific topic in physiology, and to try to show that it has broad biological bearings.

In popular usage the term fatigue is employed loosely, for while it signifies, in general, a depression of physiological activity, resulting strictly from previous activity, physiological depression is often called fatigue when it is not at all clear that previous activity is at the bottom of it. It does not, however, appear to me at present necessary to hold always to the strict significance of the word, since in a given case there are still too many unknown causative factors. Moreover, in the marvelously complex weft of the human organism, where the physical and the psychical are inextricably intermingled, illusion is so readily mistaken for reality, especially in the phenomenon now before us, as often to make the detection of a genuine fatigue well-nigh impossible.

Let me proceed at once to an analysis of the phenomenon of fatigue. Every one is familiar with its sensations; but not every one realizes that the sensations are but signs of physical and chemical conditions permeating the whole body. Fatigue is a general physiological phenomenon; not only is the whole body subject to it, but every organ, tissue and cell of which the body is composed. Like other general physiological phenomena, its study may be best approached by considering its manifestations in the parts of the organism. I propose to