THE CHEMISTRY OF MUSCLE. 65 



distinctly increased. We must believe, therefore, that lactic acid 

 is a constant product of the chemical changes of nutrition going 

 on in the muscle, and that its production is increased by the greater 

 chemical activity which occurs during visible contraction. Normally 

 we must suppose that this lactic acid, as stated above, undergoes 

 oxidation with the production of heat, and this oxidation takes 

 place in the muscle itself. When the production is rapid or ex- 

 cessive some may be carried off by the blood and be oxidized else- 

 where or even be excreted in the urine as a lactate. The increased 

 acidity of the muscl^ during activity, especially when the circulation 

 is interrupted, is referable, in the long run, to this greater production 

 of lactic acid; but as the acid after its formation probably reacts 

 with the alkaline salts present it is frequently stated that the 

 actual acidity shown to litmus or other indicator is due to acid 

 salts produced by reaction with lactic acid, presumably the acid 

 phosphate of potassium (KH 2 PO 4 ). 



Much interest has been shown in the question of the origin of 

 the lactic acid. According to some observers, it arises from the 

 carbohydrates in the muscle, the glycogen or the sugar. In support 

 of this view it has been claimed that in contraction and especially 

 in rigor mortis the glycogen disappears as the lactic acid increases. 

 This relationship, however, is denied, as far as rigor mortis is con- 

 cerned, by competent observers*. Chemical studies, however, 

 upon the action of the enzymes contained in muscle tend strongly 

 to support the view that normally the glycogen after conversion 

 to sugar is split first into lactic acid before undergoing oxidation 

 (consult section on Nutrition, p. 824). Another suggestion is 

 that the lactic acid arises from the phosphocarnic acid described 

 above. This compound, when split by hydrolysis, yields lactic 

 acid; so that if we could obtain convincing proof that such 

 a compound exists in living muscle it would serve very well 

 to explain the production of lactic acid. From experiments 

 made upon general nutrition it has been shown that in birds 

 the uric acid in the urine is replaced largely by lactic acid 

 (ammomium lactate) when the liver is excised. Under these 

 conditions the quantity of lactic acid secreted varies with the 

 albumin destroyed in the body, and some physiologists are of the 

 opinion that the lactic acid produced in the muscle or in other 

 tissues is derived from the breaking down of the living protein 

 material. A decisive answer to this problem is not possible at 

 present, but it may be said perhaps that the trend of modern work 

 tends to support the view that lactic acid constitutes what is called 

 an intermediary product in the metabolism of the sugar (glycogen) 

 of the muscle. 



*B6hm, "Pfliiger's Archiv f. d. gesammte Physiologic," 23, 44, 1880. 

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