216 PHYSIOLOGY 



It has long been known that the onset of rigor is associated with an evolu- 

 tion of carbonic acid by the muscle. Fletcher has shown that this increased 

 output of carbonic acid by a surviving muscle is due simply to the driving 

 off of carbonic acid from the carbonates in the muscle as a result of the 

 production of lactic acid. There is no evidence of a new formation of carbonic 

 acid in the dying muscle as a result, for instance, of oxidative changes. 



THE CHEMICAL CHANGES WHICH ACCOMPANY ACTIVITY 

 The principle of the conservation of energy teaches us that the energy 

 of the contraction of muscle must be derived from chemical changes, probably 

 processes of decomposition and oxidation, occurring in the muscle itself. 

 In seeking out the nature of these changes three methods are open to us : 



(1) We can examine the changes in the muscle itself, avoiding so far 

 as possible reintegrative changes by working on excised muscles. 



(2) We can investigate the changes in the medium surrounding the 

 muscle. Muscle may be exposed in a vacuum or in a confined space of 

 air, and its gaseous interchanges during rest and activity compared. Or 

 we may lead a current of defibrinated blood through excised muscles, and 

 determine the change in the composition of the blood in passing through the 

 muscle under various conditions. 



(3) A method which, although apparently complex, has rendered the 

 utmost service to the physiology of muscle is to use the changes in the 

 total metabolism of the animal during rest and muscular work as a clue 

 to the muscular metabolism itself. In such a case the respiratory exchanges 

 of the animal are determined (viz. its oxygen intake and its C0 2 output), 

 and the urine and faeces are carefully analysed, in order to judge of the 

 action of muscular work on the carbon and nitrogen metabolism of the body. 



By the third of these methods we may show that muscular exercise 

 increases largely the intake of oxygen and the output of carbon dioxide 

 by the body. No corresponding changes are found in the nitrogenous 

 metabolism, so that ultimately we may regard the energy of the muscular 

 contraction as derived from the oxidation of the food-stuffs and especially 

 the carbohydrates. That it is this class of bodies which is the immediate, 

 or at any rate the most accessible, source of muscular energy, is shown by 

 the rise in the respiratory quotient which occurs during muscular exercise. 

 When the exercise is moderate there is no evidence of the production of any 

 other substance than carbon dioxide as a result of the muscular metabolism, 

 but with violent exercise it can be shown that lactic acid is not only pro- 

 duced in the muscle, but appears in the blood and is excreted in the urine. 



It has been shown by Ryffel that normal urine contains 34 mg. of lactic acid 

 per hour. In one expsriment the urine passed after running one third of a mile with 

 the production of severe breathlessness contained 454 mg. of lactic acid. In another 

 experiment blood obtained before running contained 12-5 mg. per 100 c.c., and that 

 obtained immediately after running one third of a mile contained 70 mg. lactic acid 

 per 100 c.c. On the other hand, the examination of the urines of competitors in a 

 twenty -four hours track walking race showed no increase in the output of lactic acid 

 above the normal 4 mg. per hour. 



