THE BIOCHEMISTRY OF RESPIRATION 257 



of its decomposition. Thus he showed ' that muscles kept in 

 nitrogen give out very little extra C0 2 on tetanisation for twenty 

 minutes, the very small increase of output observed being due, 

 in all probability, to the liberated lactic acid turning it out from 

 the carbonates present in the muscle. Similarly the CO2 dis- 

 charged from muscle when caused to pass into rigor by heating 

 to 40° C. is pre-existent C0 2 expelled from the carbonates by the 

 lactic acid formed during the onset of the rigor, and if, previous 

 to the induction of the rigor, the muscle is kept for several 

 hours in nitrogen, whereby its C0 2 is expelled by the steadily 

 accumulating lactic acid, there is practically no C0 3 given off. 2 In 

 fact Fletcher considers that the increased CO2 output which 

 accompanies the contraction of muscles supplied with oxygen is 

 not due to the combustion of the lactic acid at all. Other suit- 

 able material is burnt, and its oxidation supplies energy for the 

 building up of the lactic acid into its precursor, which is then 

 able, when required, to break down again with the evolution of 

 heat and mechanical energy, and the re-formation of lactic acid. 

 Thus Hill has shown that muscle, when kept in oxygen, gives 

 out about twice as much heat on contraction as when kept in 

 nitrogen, but the extra amount of heat is not evolved at the time 

 of contraction. It is produced slowly during several minutes 

 subsequent to it, and it is accompanied by the extra C0 2 output. 

 Similarly Verzar 3 found that the oxygen consumption of an 

 active mammalian muscle may occur largely after the contraction 

 is over, and Barcroft and Piper 4 showed that the oxygen con- 

 sumption of the active submaxillary gland is considerably 

 delayed. 



Fletcher and Hill point out that their results lend no support 

 to the hypothesis of intramolecular oxygen. In fact the inter- 

 pretation they put on their experiments is diametrically opposed 

 to it. The continued evolution of C0 2 by frogs, snails, worms, 

 and other animals when kept in nitrogen was held by Pfliiger, 

 and has since been held by most other physiologists except 

 Detmer and Winterstein, to be formed at the expense of intra- 

 molecular oxygen ; but as I pointed out in my previous article, 



1 Fletcher, ibid. 23, p. io, 1898 ; 28, p. 474, 1902 ; Fletcher and Hopkins, ibid. 

 35, p. 247, 1907. 



2 Fletcher and Brown, ibid. 48, p. 177, 1914. 



3 Verzar, ibid. 44, p. 252, 191 2. 



4 Barcroft and Piper, ibid. 44, p. 372, 191 2. 



