754 PHYSIOLOGY OF DIGESTION AND SECRETION. 



glycogen is formed directly from them or from the products into 

 which they are converted during digestion. The bulk of our car- 

 bohydrate food reaches the liver as dextrose, or as dextrose and levu- 

 lose, and these forms of sugar may be converted into glycogen in the 

 liver cells by a simple process of dehydration, such as may be repre- 

 sented in substance by the formula C 6 H 12 O 6 H 2 = C 6 H 10 5 . 

 There is no doubt that both dextrose and levulose increase markedly 

 the amount of glycogen in the liver; and, since cane-sugar is inverted 

 in the intestine before absorption, it also must be a true glycogen- 

 former, a fact that has been abundantly demonstrated by direct 

 experiment. Lusk* has shown, however, that, if cane-sugar is in- 

 jected under the skin, it has a very feeble effect in the way of increas- 

 ing the amount of glycogen in the liver, since under these conditions 

 it is probably absorbed into the blood without undergoing inversion. 

 Experiments with subcutaneous injection of lactose gave similar 

 results, and it is generally believed that the liver cells can not convert 

 the double sugars to glycogen, at least not readily; hence the value 

 of the hydrolysis of these sugars in the alimentary canal before 

 absorption. We may assume, therefore, that dextrose, levulose, and 

 galactose are the true glycogen-formers that occur normally in the 

 blood, and that the disaccharids (cane-sugar, milk-sugar, etc.) and 

 the polysaccharids (starches) are true glycogen-formers to the ex- 

 tent that they are converted into dextrose, levulose, or galactose. 



Effect of Protein on Glycogen Formation. In his first studies 

 upon glycogen Bernard asserted that it may be formed from protein 

 material. Since that time there have been much discussion and 

 experimentation upon this point. The usual view is that protein 

 must be counted among the true glycogen-formers in the sense that 

 some of the material of the protein molecule is directly converted to 

 glycogen. The protein in digestion undergoes, it will be remem- 

 bered, a splitting process, the limits of which are not definitely settled. 

 It is assumed, however, that the nitrogenous split products are 

 acted upon in the liver, the nitrogen being converted first to an 

 ammonia compound and then to urea, while the non-nitrogenous 

 residue is converted to sugar by a synthetic process. Among 

 the split products of protein that have been especially investigated 

 in this relation the results with leucin and glucosamin have been 

 chiefly negative. f Experimentally observers find for the warm- 

 blooded animals, at least, that feeding with proteins, even in the 

 case of those proteins, such as casein, that contain no carbohydrate 

 grouping, causes an increased production of glycogen. J The con- 

 clusion to be drawn from these experiments is strengthened by 



*Voit, "Zeitschrift f. Biologie," 28, 285, 1891. 



f Halsey, "American Journal of Physiology," 10, 229, 1904. 



j See Stookey, "American Journal of Physiology," 9, 138, 1903. 



