IQO 



THE POPULAR SCIENCE MONTHLY 



our output of energy. The stage of our best work is when irri- 

 tability 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.") 



