RESPIRATION 173 



in acidity precede the mechanical shortening of muscle.* 



(2) Blood analyses indicate that the carbon dioxide output 

 lags behind the oxygen intake, therefore escape of carbon 

 dioxide cannot be the cause of the increased rate of oxidation. 

 We must remember, however, that the escape of carbon 

 dioxide from the cells and diffusion through the lymph to the 

 blood stream takes an appreciable interval of time. Lillie,f 

 from studies on isolated cells, believes that an increase in 

 permeability allows carbon dioxide to escape, thus permitting 

 the oxidation to proceed more quickly because of removal of 

 the products of ^oxidation. The difficulty here is to be sure 

 that the escape of carbon [dioxide is not the result of a rise in 

 carbon dioxide pressure due to activity. 



(3) Oxidation is brought about by oxidising enzymes, and 

 it is possible that more enzyme is set free during activity. 

 Change in hydrogen ion concentration always affects the rate 

 of reaction brought about by enzymes, and as mentioned above 

 increase in acidity precedes muscular contraction ; thus the 

 action of an oxidising enzyme may be accelerated without an 

 actual increase in amount of enzyme. 



In plants various toxic substances start reactions, possibly 

 due to the liberation of an enzyme. { 



At present the only evidence that we have about the early 

 stage of activity is that there is a chemical change preceding 

 activity. This change is shown by an increase in acidity 

 probably by the formation of lactic acid. 



The process of muscular contraction is possibly analogous 

 to other kinds of activity, but in any activity lasting a longer 

 time the products of reaction appear and cloud the picture. 



Heat production in muscle takes place in two stages : (i) an 

 early stage corresponding to the mechanical shortening ; (2) 

 a later stage of repair, during which the tissue is restored to its 

 resting state. 



The amount of heat in each of these stages is the same, and 

 this is what one would expect from the mechanics of the 

 reaction. A. V. Hill agrees with Macdonald in ascribing 

 stage i to the liberation of some chemical reaction without 

 oxidation, and stage 2 to an oxidation to restore the system 

 to its former state. A reversible isothermal reaction would 

 require the same amount of energy to restore the products to 

 their original condition as to produce the reaction. 



* H. E. Roaf, Journ. Physiol., 1914, vol. 48, p. 380. 

 f R. S. Lillie, Amer. Journ. Physiol., 1909, vol. 24, p. 14. 

 j M. L. Guignard, Compt. Rend., 1909, vol. 149, p. 91. 

 J . S. Macdonald, Quart. Journ. Exper. Physiol., 1909, vol 2, p. 15, 

 and A. V. Hill, Journ. Physiol., 1913, vol. 46, p. 28. 



