THE CHEMISTRY OF MUSCLE. 69 



of the further fact that the lactic acid does not appear when 

 oxygen is freely supplied to the muscle is surrounded with 

 difficulties owing to our lack of knowledge of the chemical reac- 

 tions that take place. The simplest explanation at present is 

 that the lactic acid is an intermediary product formed from 

 the sugar by enzyme action, and that it subsequently, in the 

 presence of oxygen, undergoes oxidation under the influence 

 of other enzymes; but another possible explanation is that the 

 lactic acid, after causing in some way the process of contraction, 

 is again reconstructed by synthesis into the precursor from which 

 it was derived. A synthesis of this kind would require energy, 

 and it has been suggested* that the oxidation that occurs in the 

 muscle at this point yields the energy necessary for this work. 

 Much uncertainty, however, prevails in regard both to the imme- 

 diate origin and the fate of the lactic acid. Some evidence, as 

 yet incomplete, has been furnished to show that there is present 

 in muscle a compound containing a carbohydrate-phosphoric 

 acid group, f When the juice of a muscle is expressed under great 

 pressure and is kept at body temperature, both lactic acid and 

 phosphoric acid are formed in the mixture and in approximately 

 equimolecular amounts. It is suggested, therefore, that the sugar 

 in the muscle may first be synthesized with phosphoric acid (and 

 perhaps other constituents) to form this complex compound, for 

 which the name lactaddogen has been proposed. It is in the 

 breaking down of this substance that the lactic acid is pro- 

 duced from the carbohydrate group. A further discussion of 

 the chemical relations of the lactic acid to sugar will be found on 

 p. 907. 



Chemical Changes During Rigor Mortis. The chemical 

 changes during rigor have been referred to above, but may be 

 summarized here in brief form: 



1. There is a coagulation of the protein material of the muscle 

 plasma, which at present may be explained by supposing that the 

 contained myosin and myogen, spontaneously, or under the action 

 of acid products of metabolism, pass into their insoluble forms, 

 namely, myosin fibrin and myogen fibrin. 



2. There is an increased acidity, due doubtless to a production 

 of lactic acid. 



3. There is a production of CC>2. Much importance was 

 attributed formerly to this product on the belief that it indicated 

 the occurrence of a metabolic reaction, a catabolic or oxidative 

 change similar to that taking place in contraction. Fletcher 



* Hill, loc. tit., and Fletcher and Brown, "Journal of Physiology," 48, 177, 

 1914. 



fEmbden et al, "Zeitscrift f. physiol. Chemie," 93, 1, 1914. 



