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THE POPULAR SCIENCE MONTHLY 



nounced fatigue ; and the same is true of his individual muscles, which 

 are incapable of performing as many contractions as the muscles of a 

 normal animal. Feeding such an animal with sugar restores his energy 

 and makes his muscles capable of greater labor. This latter experiment 

 has its counterpart in the common practise, by soldiers, guides and ex- 

 plorers, of consuming sweets, such as maple sugar, chocolate and 

 raisins, when on long marches; while for the farmer in the hayfield 

 nothing is more gratifying than a sweetened drink. It is quite possible 

 that future research will discover other substances, besides oxygen and 

 carbohydrate, the loss of which to the tissues is conducive to the pro- 

 duction of fatigue. 



Oxidation and destruction of carbohydrate result in the formation 

 of at least two waste substances, both of an acid character, namely, 



carbon dioxide and lactic acid. 

 Now it is an interesting fact, 

 derived from laboratory in- 

 vestigation, that both of these 

 substances, when in any but 

 small quantity, are inimical 

 to protoplasmic activity, and, 

 furthermore, that a muscle 

 under their influence shows the 

 very same physical symptoms 

 that are shown by a muscle 

 fatigued through work. A 

 fresh muscle to which has been 

 given a moderate or consid- 

 erable quantity of either one of 

 these substances is a muscle 

 already fatigued, although it 

 may have performed no work 

 (Fig. 5). These two metabolic 

 products are thus believed to be important factors in the causation of 

 fatigue, and to them has been given the name, " fatigue substances." 

 Fatigue substances are poisonous, or toxic, to protoplasm; they dimin- 

 ish its irritability, so that a given stimulus calls out a less response than 

 before. Certain other substances, besides carbon dioxide and lactic 

 acid, are thought to belong to the class of fatigue substances, some of 

 which are probably produced normally, while others occur only in 

 diseased conditions. Among these pathological fatigue substances may 

 be mentioned /^-oxybutyria acid, which is present often in large quanti- 

 ties in a body suffering from diabetes — and it is a well-known fact that 

 a person afflicted with diabetes is incapable of any considerable 

 labor without extreme fatigue. /^-oxybutyria acid occurs also in 

 a starving body. The weakness of a person in starvation is associated 



Fig. 5. Series of contractions of the two 

 gastrocnemius muscles of a frog, excised 

 and stimulated at intervals of two seconds ; 

 the one muscle normal, the other under the 

 influence of carbon dioxide. The longer, or, 

 in the later contractions, the lower curves 

 are those of the poisoned muscle. Every 

 fiftieth contraction of the two muscles is 

 recorded from the same base line. The 

 augmenting action of the fatigue substance 

 is visible in the first two hundred contrac- 

 tions : its fatiguing action in the subse- 

 quent ones. 



