70 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



and Brown * give proof, however, so far at least as heat rigor and 

 chloroform rigor are concerned, that the C0 2 formed does not arise 

 from a metabolic breakdown of organic material, but is simply an 

 indirect result of the production of acid. This acid as it accumu- 

 lates acts upon the carbonates in the muscle and thus leads to a 

 liberation of the CO2- From this standpoint the CO 2 production 

 in rigor has no physiological significance. 



4. The consumption of glycogen. According to some observers, 

 glycogen disappears during rigor as it does during contraction; 

 but others find that the amount is not changed during this process. 



The Relation of the Chemical Changes during Contraction 

 to Fatigue; Chemical Theory of Fatigue. As we have seen, a 

 muscle kept in continuous contraction soon shows fatigue ; it 

 relaxes more and more until, in spite of constant stimulation, it 

 becomes completely unirritable. We may define fatigue, there- 

 fore, as a more or less complete loss of irritability and contractility 

 brought on by functional activity. But even when the fatigue is 

 complete and the muscle fails to respond at all to maximal 

 stimulation, a very short interval of rest is sufficient to bring about 

 some return of irritability. For a complete restoration to its 

 normal condition a long interval of time may be necessary. If 

 the muscle is isolated from the body and is thus deprived of its 

 circulation and its proper supply of oxygen, fatigue appears 

 more rapidly and is recovered from less completely. Rankest 

 to whom we owe the first thorough investigation of this subject, 

 was led to believe that as a result of the chemical changes occur- 

 ring in the muscle during contraction certain substances are 

 formed which depress or inhibit the power of contraction. In 

 support of this view he found that extracts made from the 

 fatigued muscles of one frog when injected into the circulation 

 of another fresh frog would bring on the appearance of fatigue 

 in the latter. Control experiments made with extracts of 

 unfatigued muscles gave no such result. He designated these 

 inhibitory products as fatigue substances and made experiments 

 to prove that they consist of the known products of muscular 

 metabolism, namely, lactic acid (or the lactates), carbon dioxid, 

 and possibly also acid potassium phosphate (KH 2 PO 4 ). These 

 results have been confirmed by other observers, { and we may ac- 

 cept, therefore, the view that the products of muscular activity, 

 if they are allowed to accumulate in the muscle, serve to diminish 

 or suppress its contractility. We know that when muscular ac- 



* Fletcher and Brown, "Journal of Physiology," 48, 177, 1914. 

 fRanke, "Tetanus," Leipzig, 1865. 



j Lee, "American Journal of Physiology," 1907, 20, 170, and Burridge, 

 "Journal of Physiology," 1911, 41, 285. 



