68 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



soon goes into death-rigor. On the contrary, if the muscle is 

 supplied abundantly with oxygen, no accumulation of lactic acid 

 can be detected. It is evident from these observations that 

 lactic acid is formed in the muscle as a result of the chemical 

 changes underlying contraction, and also of the changes that 

 occur during dying. The interpretation of this fact and also 

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

 oxygen is freely supplied to the muscle is surrounded with diffi- 

 culties owing to our lack of knowledge of the chemical reactions 

 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. The fact 

 that immediately after the shortening of the muscle there occurs an 

 oxidative process, attended by liberation of heat and carbon dioxid, 

 shows that some material is oxidized, either lactic acid or some 

 other product derived from the sugar. The significance of this 

 reaction has been interpreted differently by various authors. The 

 energy liberated may be used in the synthesis of the lactic acid to 

 sugar, or, more probably (Fletcher and Hopkins), assuming that it 

 is the lactic acid itself that is burnt, it may be utilized to restore 

 potential energy to the muscle by re-establishing the physico- 

 chemical condition that existed before the contraction took place. 

 On the chemical side much uncertainty exists in regard to the 

 origin as well as the fate of the lactic acid. In a general or sche- 

 matic way we may adopt the hypothesis that the lactic acid is 

 derived from the sugar, and this, in turn, from the reserve supply of 

 carbohydrate (glycogen), but the steps in the process are not 

 known. There is reason to believe that the lactic acid is not de- 

 rived directly from sugar but from some unknown compound 

 formed from the sugar. One group of workers f designate this 

 unknown precursor of lactic acid as ladacidogen to emphasize its 

 characteristic as a mother substance of the lactic acid. On the 

 other hand, it is supposed that the change to lactic acid takes 

 place not in one step but through intermediate stages. A further 

 discussion in regard to this point will be found on page 916. 



Chemical Changes During Rigor Mortis. The chemical 

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

 summarized here in brief form: 



* Fletcher and Hopkins, Croonian Lecture, "Proc. Royal Soc.," London, 

 B. 89, 444, 1917. 



f Embden et al, "Zeitschrift. f. physiol. Chemie," 93, 1, 1914. 



