THE CHEMICAL CHANGES IN MUSCLE 243 



of energy. It has been suggested therefore that, in the absence of 

 oxygen, the energy for contraction is derived from a process of dis- 

 integration, each molecule of grape sugar breaking down into two 

 molecules of lactic acid, thus : 



C 6 H 12 6 = 2C 3 H 6 3 . 

 sugar lactic acid 



On the other hand, in the presence of sufficient oxygen the sugar would 

 be entirely oxidised with the formation of C0 2 and water, thus : 



C 6 H 12 6 + 60 2 = 6C0 2 + 6H 2 0. 

 sugar 



The change from sugar to lactic acid involves, however, practically 

 no evolution of energy so that in the absence of oxygen the energy 

 of contraction must be derived from some other source. 



It seems more probable that we are dealing here with two stages of 

 one process, and that in the muscle under normal conditions (i.e. richly 

 supplied with oxygen) the first chemical change is one of disintegra- 

 tion, leading to the formation of lactic acid (and probably other 

 substances), and that this is followed by a process of oxidation, in 

 which all the products of the first stage are converted into C0 2 , which 

 can be rapidly eliminated from the muscle. If the supply of oxygen 

 is deficient, the products of the first stage remain in the muscle, giving 

 rise to the phenomena of fatigue, and finally inducing the coagulation 

 of the muscle-proteins which determines rigor mortis. 



That lactic acid is a normal metabolite, and not simply the result 

 of an alternative chemical change occurring only under abnormal 

 conditions, namely, want of oxygen, is indicated by the fact demon- 

 strated by Hopkins and Fletcher, viz. that the muscle possesses in 

 itself a chemical mechanism for the removal of lactic acid when once 

 formed. These observers have shown that if a fatigued muscle be 

 exposed to pure oxygen, 30 per cent, of the lactic acid produced by 

 the fatigued muscle may disappear within two hours, and 50 per cent, 

 within ten hours. Thus even apart from the circulation, which of 

 course would remove large quantities of any lactic acid which might 

 be produced in the muscles, the muscles themselves can deal with this 

 metabolite locally. 



Since the main products of muscular activity are C0 2 and lactic 

 acid, and no change has been found to occur in the creatine or other 

 nitrogenous extractives of the muscle during contraction, it has been 

 thought that the sole source of muscular energy is the combustion of 

 carbohydrate or fatty material, the proteins of the body taking no 

 part in the process. In dealing with the general metabolism of the 



