156 RESPIRATION 



but in their observations they omitted to take account of the 

 increase in the number of yeast cells consequent upon the 

 stimulation of reproduction by the oxygen. So for years the 

 matter rested ; brewers and distillers, aware of the beneficial 

 action of oxygen, continued to practise a certain degree of 

 aeration of the fermenting liquor, a process which has the 

 additional advantage of stirring the liquid and removing 

 excess of carbon dioxide. 



The difficulty encountered by Pasteur in his theory of 

 " vie sans air " was that a plentiful supply of air should sup- 

 press alcoholic fermentation rather than stimulate it. An 

 explanation of this apparent contradiction was furnished from 

 an entirely different source, namely from animal physiology. 



Fletcher and Hopkins * in a study of the relationship 

 between muscular contraction and lactic acid production, 

 showed that lactic acid is formed by resting muscle, and this 

 in increased quantity in an atmosphere of nitrogen ; on 

 exposing the muscle to air, the lactic acid disappears with the 

 formation of carbon dioxide and water. The lactic acid is 

 derived from a precursor, or lactacidogen, which is probably 

 a hexosephosphate, the hexose constituent of which is formed 

 from the store of glycogen contained in the muscle. A. V. 

 Hill f by accurate measurements found that when a muscle 

 is stimulated, heat is liberated and continues to be liberated 

 so long as contraction is maintained and also while the muscle 

 relaxes. This constitutes the initial process in muscular 

 contraction, and it is noteworthy that the production of 

 heat occurs in the absence of oxygen, the process being 

 anaerobic. Recovery next follows : the muscle after re- 

 laxing ceases to give off heat, but after a short interval 

 heat production begins again, provided oxygen is present, 

 reaches a maximum and then falls away to zero. This second 

 evolution of heat, which is about 1*5 times as great as that 

 occurring in the initial stage, is accompanied by an absorption 

 of oxygen and a disappearance of lactic acid ; the amount of 

 heat evolved is, however, only sufficient to account for the 



* Fletcher and Hopkins : " J. Physiol.," 1907, 35, 247. 

 f A. V. Hill : id., 1914. 48, 10. 



