LACTIC ACID. 357 



of those that have rested, while the reverse holds good for the 

 aqueous portions. 



Lactic acid is, however, produced by the muscle not only when 

 at work, but also while resting. This has been shown by Zillessen 

 and v. Frey. These observers found that on transfusing the muscles 

 of the hindquarters of a dog, during three hours, an increase in the 

 amount of lactic acid resulted, which, calculated for the entire amount 

 of blood, corresponded to as much as 1.48 grammes of zinc lactate. 



The amount of lactic acid that may be isolated from dead muscles 

 while still rigid varies between 0.1 and 1.0 per cent., and it is note- 

 worthy that for definite groups of muscles this amount is constant, 

 no matter whether the formation of the acid is allowed to proceed 

 rapidly or slowly. This, however, holds good only for correspond- 

 ing muscles, and is different in different groups. In rabbits larger 

 amounts can thus always be obtained from the muscles of the trunk 

 than from those of the extremities. 



To the general rule that the acidity of corresponding muscles is 

 always the same, there is one exception, viz., the heart, which is the 

 only muscle of the body, moreover, that normally presents an acid 

 reaction. Larger amounts of lactic acid are here always found in 

 the left than in the right side. 



As regards the origin of lactic acid in muscle-tissue, it was long 

 thought that the glycogen probably represented its principal source. 

 There are -a number of facts indeed which favor such an assumption. 

 I have pointed out already that after death the glycogen gradu- 

 ally disappears, and we have just seen that lactic acid is then found. 

 Glycogen is similarly decomposed during muscular activity in the 

 living animal, where lactic acid is also constantly produced, and, as 

 I have shown, the same also occurs in the muscle while at rest. 

 Then again there is evidence to show that the decomposition of 

 glucose in the muscle-tissue does not occur in the sense of a direct 

 oxidation, but that a primary division of the molecule occurs, and 

 that lactic acid may be one of the resulting products. But, on the 

 other hand, observations exist which go to show that the amount of 

 lactic acid that is .produced during rigor mortis bears no relation to 

 the amount of glycogen which was present at the time, and it has 

 further been noted that lactic acid is still formed in muscles from 

 which all glycogen has previously been removed by starvation. The 

 conclusion hence suggests itself that while a certain amount of 

 lactic acid may be derived from glycogen, this does not represent its 

 only source, and we must admit that to some extent the albumins 

 of muscle-tissue also contribute toward its formation. There is a 

 tendency among physiological chemists at the present time to regard 

 this source indeed as the most important. This view is largely based 

 upon observations, which go to show that increased amounts of lactic 

 acid appear in the urine whenever the formation of urea is impaired 

 in the liver, or when this organ is entirely excluded from the gen- 

 eral circulation. Similar results also have been obtained in birds, 



