218 PHYSIOLOGY 



given no contractions at all. Fletcher and Hopkins interpreted this result 

 as showing that under the influence of oxygen, lactic acid is put back into 

 the precursor from which it arose, and would assume that part of the lactic 

 acid is completely oxidised to carbonic acid and water, the energy so evolved 

 being employed in the building up of the precursor from the rest of the lactic 

 acid. On the other hand it is possible that the lactic acid produced in the 

 initial stage of contraction may be under normal circumstances completely 

 removed by oxidation, and that the energy or part of the energy so made 

 available is used to build up some precursor substance, not out of the lactic 

 acid, but out of the glycogen already present in the muscle (Parnas). It 

 is certain that prolonged activity of muscle, especially in the presence of 

 oxygen, may be associated with a diminution in the glycogen store of the 

 muscle. We cannot, however, discuss this question further without refer- 

 ence to the total energy changes in muscle contracting with or without 

 oxygen, and the clue to these changes is given by a study of the heat pro- 

 duction in muscle. 





