THE CARBOHYDRATES AND THEIR METABOLISM 263 



exceedingly soluble and therefore offer great technical difficulties in their 

 isolation, purification and identification. Secondly, most of the sub- 

 stances are oxidized with great, ease so that at no time can one find more 

 than traces of them, even though throughout the twenty-four hours large 

 quantities may have been produced. Our information therefore must be 

 pieced together from various and indirect sources. 



It has long been known that in the presence of alkali, glucose undergoes 

 decomposition, giving rise to lactic acid. In the animal body lactic acid ap- 

 pears in the blood and urine- in cases of asphyxiation, severe anemias, and 

 after great muscular exertion. The following experimental proof shows 

 that this lactic acid can have its origin in glucose. Mandel and Lusk 

 (1006) found that after giving phosphorus to a dog lactic acid appeared 

 in the urine in large quantities. When they administered phlorhizin to 

 the same dog the animal of course became diabetic, and the lactic acid 

 disappeared from the urine, indicating that the lactic acid could have been 

 derived only from the catabolized glucose. This work is corroborated by 

 von Fiirth (1914, b) who found that the amount of lactic acid excreted in 

 phosphorus poisoning is increased after administering glucose to the ani- 

 mal. Final and most convincing evidence was brought forward by Levene 

 and Meyer (1913, b) when they showed that leucocytes and kidney tissues 

 possess the power of converting glucose into lactic acid, and by Embden 

 and Krauss (1912) who found that the addition of glucose to blood that is 

 perfused through a surviving liver causes the appearance of considerable 

 amounts of lactic acid. 



Embden, Baldes and Schmitz (1912) also demonstrated that washed 

 blood corpuscles have the power of converting glyceric aldehyde into lactic 

 acid to the same extent that they do glucose, indicating the possibility of 

 glyceric aldehyde being an intermediary stage. They also showed that 

 glyceric aldehyde when perfused through the liver is reduced to glycerol, 

 and S. Oppenheimer (1912) added the information that glycerol when 

 perfused through the liver gives rise to lactic acid. 



Then follow experiments by Mayer (1912) in which he showed that 

 after administering pyruvic acid to animals lactic acid appeared in the 

 urine, and by Embden and Oppenheimer who obtained large amounts of 

 lactic acid after perfusing the liver with pyruvic acid. 



Finally, there is a whole array of experimental proof, showing with 

 what ease various substances which are believed to be products of inter- 

 mediary metabolism are converted back into glucose when fed to diabetic 

 {nimnls; for glyceric aldehyde, Woodyatt (1915) ; for dioxyacetone, Ringer 

 and Frankel (1914(c)) ; for pyruvic aldehyde, Dakin and Dudley (1913) ; 

 for pyruvic acid, Ringer (1913), Dakin and Janney (1913), Cremer 

 ( 1913) ; for lactic acid, Mandel and Lusk (1906). 



In the following chart the various reactions that may take place in 

 the intermediary metabolism of glucose are indicated. 



