our output of energy. The stage of our best work is when irritability is at its highest, we have a store of oxidizable fuel, and toxic products have not yet begun to exert their deleterious action. The stage of fatigue is when our fuel is becoming exhausted, its waste products are clogging the furnaces, and physiological irritability is low.
Fatigue, as we feel it after excessive work, is often spoken of as a sensation. Really it is a great complex of sensations. These sensations differ in some degree according to the character of the work, whether it is mental or physical, and if physical, according to the particular groups of muscles employed. But in extreme fatigue such differences are comparatively slight. There may be a "tired" feeling in the head of obscure origin; pain and soreness in the muscles, resulting from an excessive accumulation of blood or lymph, or perhaps from an actual rupture of muscle fibers; stiffness in the joints, resulting from lymph accumulation; swelling of hands and feet, from the same cause; sleepiness, which is accompanied by cerebral anemia; even a feverish temperature because of derangement of the temperature regulating mechanism; and many other sensations, but, most general of all, a disinclination to perform either mental or physical labor, which may be due in part to general depression of the nervous system, in part to the presence of the unusual sensations, and in part to the mental recognition of the fact that the irritability of our tissues has become diminished and a greater stimulus than before is now required to induce a given action. It is not often possible for the individual to make a satisfactory analysis of the excessively complicated compound of sensations, which he may possess when his body is in a fatigued state. But it has come now to be generally accepted that the sensations
Fig. 6. Series of contractions of the flexor muscles of a human finger. The muscle was stimulated electrically every two seconds, and the resulting contractions were therefore involuntary. Record 1 was made when the muscle was fresh: record 2 immediately after three and one half hours had been spent in the oral examination of students; record 3 two hours after the completion of the examination. (From Mosso's "Fatigue.")
