76 HUMAN PHYSIOLOGY 



The blood, as it sweeps through the small vessels of the 

 mass, enriches the lymph with oxygen and the fibers 

 receive the oxygen from the lymph that bathes them. 

 At the same time the blood is relieving the lymph of 

 carbon dioxid which the fibers have unloaded upon it. 

 In a somewhat indirect way the oxygen brought to the 

 scene by the blood disposes of the sarcolactic acid. 1 

 Finally, the blood ministers to the muscle and defers 

 fatigue by bringing new offerings of food, especially 

 sugar. 



It is easy to conceive of a state of affairs such that a 

 balance may exist between consumption and renewal. 

 If, in addition, the removal of waste-products is adequate 

 a certain average degree of activity may be indefinitely 

 continued without inducing fatigue. This is actually 

 realized in the case of the heart which begins to beat 

 very early in embryonic life and goes on, it may be, for 

 seventy years. We must infer that the decomposition 

 entailed by every beat is perfectly compensated before 

 the next contraction is begun. Some of the muscles used 

 in breathing show a similar ability to go on with serial 

 movements, though there is more variation in their 

 performance than is true of the heart. 



In regard to the Treppe an interesting discovery has 

 been made. It is to the effect that the gain in power 

 during the early period of activity is due to minute 

 quantities of the same substances which we call fatigue 

 products. A little carbon dioxid is favorable to the 

 working capacity of a muscle although any considerable 

 increase of the same compound will depress it. The 

 same can be said of sarcolactic acid and probably of still 

 other bodies. The idea that one and the same sub- 

 stance may be classed as a stimulant or as a depressant 

 according to its concentration is one which is of much 

 value as we consider questions of hygiene. 



In the laboratory we can study true muscular fatigue, 



1 The acid is not, normally, oxidized but is forced back into a chem- 

 ical compound like that which lately gave rise to it. 



