230 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



intestinal epithelium they are evidently broken down into simple 

 sugars or monosaccharides, but the modifications induced by the 

 intestinal epithelium go even further, for the hydrolysis of cane-sugar 

 yields Levulose as well as glucose, and the hydrolysis of lactose yields 

 Galactose as well as glucose. We are compelled to assume that the 

 levulose and galactose fractions of these molecules are converted, 

 either in the intestinal mucosa or else in the liver epithelium, into 

 glucose. In the case of levulose this presents little theoretical difficulty, 

 for the partial conversion of levulose into glucose can be brought 

 about in vitro by the prolonged action of dilute alkali. We are ac- 

 quainted with no mechanisms, however, which will accomplish the 

 direct transformation of d-galactose into d-glucose, much less with 

 any enzyme which will bring it about. That the converse process, 

 the transformation of glucose into galactose may be brought about in 

 living tissues is shown by the Glycosuria which immediately succeeds 

 extirpation of the mammary glands in milch-cows and goats. The 

 sugar that appears in the urine is glucose, and glucose only, although 

 the lactose for the manufacture of which the excess of glucose had 

 previously been utilized, is a compound of glucose and galactose. The 

 remarkable feature of this transformation is that it involves disruption 

 of the oxide-ring of glucose and its reformation upon the opposite side 

 of the molecule: 



HCOH HOCH 



HCOH 



[OCH / 



\ / HCOH 



\ 

 HOCH / \ HOCH 



HC/ 



HCOH HCOH 



CH 2 OH CH 2 OH 



ot-d-Glucoae. d-Galactose. 



The normal circulating form of hexose is therefore d-Glucose and d- 

 glucose only. Whatever form of hexose or polysaccharide derived 

 from a hexose may be ingested, if it is absorbed at all, it appears under 

 normal circumstances as Glycogen in the liver, having either reached 

 the liver-cells in the form of glucose, or else been transformed by them 

 into glucose as a preliminary step in the formation of this colloidal 

 reserve-carbohydrate. From the liver the carbohydrate material is 

 redistributed over the body as the need arises, being broken down to 

 glucose again before it makes its appearance in the circulation. The 

 determining factor which regulates the discharge of this carbohydrate 

 reservoir is probably the concentration of glucose in the blood. This 



