744 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



oxygen. There is no doubt that the production of lactic acid in 

 functional activity and its transformation into other substances 

 are processes that go on also in the muscles of the intact body. 

 The formation of the acid in the excised muscle, far from being a 

 sign of death, is an index of the ' survival ' of a process by which 

 it is normally formed, as the accumulation of it is an index of the 

 crippling, in the absence of oxygen, of a mechanism by which it is 

 normally transformed. 



The lactic acid which accumulates in the excised muscle in rigor 

 and activity does not remain free, since blue litmoid paper is not 

 reddened as it would be by free lactic acid. It causes a repartition 

 of the bases at the expense of the sodium carbonate and disodium 

 phosphate, the latter being changed into monophosphate, which, 

 in part at least, accounts for the acid reaction to turmeric (Roh- 

 mann). It is of great interest that this oxidative transformation 

 of lactic acid only occurs in muscle whose structure is so far pre- 

 served that its irritability is not lost. In minced or triturated 

 muscle it does not take place. 



The relations between the heat production, the formation of 

 carbon dioxide, and the production of lactic acid indicate that 

 liberation of lactic acid from some precursor is an essential stage 

 in the sudden, ' explosive ' reaction or series of reactions which 

 precedes and induces the mechanical response to stimulation. This 

 stage takes place whether oxygen be present or absent, and it seems 

 to be accompanied by a considerable liberation of energy, at the 

 expense of which alone the anaerobically contracting muscle works. 

 It is most probable that the liberation of lactic acid follows the same 

 course in the muscle abundantly supplied with oxygen, although it 

 has not been shown that oxidative processes, resulting in the forma- 

 tion of carbon dioxide, do not contribute also at this stage to the 

 energy which is transformed into the mechanical effect. But while 

 in the absence of oxygen the reaction stops at the formation of 

 lactic acid, when oxygen is available the cycle is completed by 

 restitution processes which lead to the disappearance of the lactic 

 acid either by restoration to its original position in the precursor 

 from which it is derived, or perhaps in the case of a portion of the 

 lactic acid to its combustion to carbon dioxide and water. For 

 these restitution processes oxygen is essential, and it is to be 

 supposed that the energy required for the rebuilding of the lactic 

 acid precursor, or, to speak more generally, for the restoration of 

 the muscle to its original state in readiness for a fresh contraction, 

 is derived largely from oxidations in which carbon dioxide makes 

 its appearance. 



The Precursor of Lactic Acid. What material is the lactic acid 

 formed from ? There are reasons for thinking that lactic acid is an 

 intermediate substance which in metabolism serves as a link between 



