THE CHEMISTRY OF MUSCLE. 67 



over, if provision is made to irrigate its blood-vessels with a solution 

 of physiological saline (NaCl, 0.7 per cent.) the recovery from fatigue 

 is hastened. These facts seem to indicate clearly that fatigue is 

 not due to a complete consumption of the material in the muscle 

 that supplies the energy for the contractions. In other words, 

 fatigue as it usually presents itself to us in life or under experi- 

 mental conditions is a phenomenon different from exhaustion. 

 Ranke,* who made the first complete study of this subject, was 

 convinced that a muscle when ,tetanized to the point of complete 

 fatigue consumes only a fraction of the oxidizable or energy-yielding 

 material contained in its substance. He believed that there exists 

 in the fatigued muscle a something brought into existence by the 

 contraction itself, which retards or prevents further physiological 

 oxidation. In support of this view he found that if an extract 

 was made from the fatigued muscles of one frog and injected into 

 the circulation of a second frog, the muscles of this latter animal 

 gave evidence of fatigue, that is, they showed diminished power 

 of contraction upon stimulation. A similar experiment made with 

 an extract from resting muscle gave no such effect. Investigation 

 of the separate products formed in a muscle during contraction 

 demonstrate that the sarcolactic acid, acid potassium phosphate, 

 and carbon dioxid are apparently responsible for this effect. | 

 According to these experiments, the accumulation of these products 

 is responsible for the appearance of fatigue; the muscle's own 

 metabolic products, therefore, serve to limit its responsiveness to 

 stimulation, and thus form a protective mechanism that saves it 

 from complete exhaustion. Under normal conditions these prod- 

 ucts are quickly removed by the blood or, in the case of the 

 lactic acid, destroyed by oxidation. It should be added that Lee 

 has published experiments which indicate that the first effect of 

 these so-called fatigue substances is to increase the irritability of 

 the muscle, while the later effect is to diminish the irritability or 

 to suppress it altogether. In this initial favoring influence Lee 

 finds an explanation of the phenomenon of Treppe (see p. 33). 

 This chemical theory of fatigue does not, however, explain all the 

 phenomena, particularly the after-results. As was stated in describ- 

 ing the experiments made with the ergograph, a very short rest 

 suffices to make the muscle again capable of lifting its load, but 

 a very long interval of rest, two hours, may be required before the 

 muscle is restored entirely to its normal condition. Such a long 

 interval is evidently not necessary for the removal of the metabolic 

 products, and we must recognize that a part of the fatigue is due to a 



*Ranke, "Tetanus," Leipzig, 1865. 



f For discussion and experiments, see Lee ; Harvey Lectures, 1905-06, 

 Philadelphia, 1906; also Journal of the American Medical Association, May 

 19, 1906, and American Journal of Physiology, xviii., 267, 1907. 



