CHAPTEK XIX 



LACTIC ACID. FATE OF BODY-FOREIGN SUBSTANCES 

 IN THE ECONOMY 



LACTIC ACID 



As an addendum to the acetone bodies it seems desirable 

 at this place to examine into the problem of lactic acid, which 

 is of the greatest importance to the comprehension of the 

 metabolic processes. This is one of those questions which 

 are always of special stimulative interest to the writer, not 

 so much because of the wealth of knowledge which comes im- 

 mediately from them (for this for the present can satisfy 

 only very modest demands) but rather because of the feeling 

 that here we are before the entrance to a steep and laborious 

 mountain trail which may lead one, if he be able to surmount 

 its difficulties, to a broad and as yet unknown plateau. 



Quantitative Estimation of Lactic Acid by the Method of 

 Furth and Charnass. The reason that the lactic acid prob- 

 lem has not advanced more rapidly is primarily due to the 

 fact that it has been only recently that a successful method 

 of overcoming the difficulties of exact lactic acid estimation 

 has been attained. The older authors conducted their de- 

 terminations of lactic acid as a rule by extracting it by 

 simply shaking it for a time with ether in a separatory fun- 

 nel, then forming the relatively insoluble zinc or lithium 

 salt of lactic acid and weighing it. As lactic acid is by no 

 means readily taken up by ether from water, and as quan- 

 titative extraction is seemingly possible only by long con- 

 tinued treatment in some such elaborate apparatus as 

 Lindt's rotating extraction apparatus, as, too, the above men- 

 tioned salts are not entirely insoluble in their mother fluids, 

 and impurities can be removed only by repeated crystalliza- 

 tion, it must be evident that the older methods of estimating 

 lactic acid were attended by very serious faults and that 



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