238 PHYSIOLOGY 



precursor from which the lactic acid is derived. The immediate precursor cannot be 

 glucose or glycogen since the heat evolved in the initial stage of contraction is two or 

 three times as great as could be derived from the mere conversion of either of these 

 substances into lactic acid. We must therefore conclude that the oxidation of lactic 

 acid which goes on during the process of recovery is used to yield the energy necessary 

 for building up the active molecules, which are the precursors of lactic acid and which 

 have a higher potential energy than glucose itself, so that when it rapidly decomposes 

 sufficient energy is set free to account for the observed heat production. Some such 

 utilisation of the energy of oxidation of the lactic acid is indicated by the results of 

 Parnas, who found that the heat evolved during this recovery process corresponded to 

 only about one half the heat which would be evolved by the formation of the carbon 

 dioxide output of the muscle during the same time as a result of the oxidation of lactic 

 acid. 



