268 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



levulose, and is absorbed in this form. It is evident, therefore, that the bulk 

 of our carbohydrate food reaches the liver as dextrose, or as dextrose and 

 levulose, and these forms of sugar must be converted into glycogen in the 

 liver-cells by a process of dehydration such as may be represented in substance 

 by the formula C 6 H 12 O 6 H 2 O = C 6 H 10 O 5 . In the case of levulose there is 

 reason to believe that it is changed first to dextrose in the liver before being 

 converted into glycogen. However that may be, 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 good glycogen-former a fact which has been abundantly demonstrated 

 by direct experiment. Lusk l has shown, however, that if cane-sugar is in- 

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

 amount of glycogen in the liver, since under these conditions it is probably 

 absorbed into the blood without undergoing inversion. Experiments with sub- 

 cutaneous injection of lactose gave similar results, and it is generally believed 

 that the liver-cells cannot convert the double sugars to glycogen, at least not 

 readily ; hence the value of the inversion of these sugars in the alimentary 

 canal before absorption. The relations of lactose to glycogen-formation have 

 not been determined satisfactorily. If it contributes at all to the direct forma- 

 tion of glycogen, it is certainly less efficient than dextrose, levulose, or cane- 

 sugar. When the proportion of lactose in the diet is much increased, it quickly 

 begins to appear in the urine, showing that the limit of its consumption in the 

 body is soon reached. This latter fact is somewhat singular, since in infancy 

 especially milk-sugar forms a constant and important item of our diet, and 

 one would suppose that it is especially adapted to the needs of the body. 



Effect of Proteids on Glycogen-formation. It was pointed out by Bernard, 

 in his first studies upon glycogen-formation, that the liver can produce glycogen 

 from proteid food. This conclusion has since been verified by more exact 

 investigations. When an animal is fed upon a diet of proteid alone, or on 

 proteid and gelatin, the carbohydrates being entirely excluded, glycogen is still 

 formed in the liver, although in smaller amounts than in the case of carbohy- 

 drate foods. This is an important fact to remember in studying the metabo- 

 lism of the proteids in the body, for, as glycogen is a carbohydrate and con- 

 tains no nitrogen, it implies that the proteid molecule is dissociated into a 

 nitrogenous and a non-nitrogenous part, the latter being converted to glycogen 

 by the liver-cells. The possibility of the production of glycogen from proteids 

 accords with a well-known fact in medical practice with reference to the path- 

 ological condition known as diabetes. In this disease sugar is excreted in the 

 urine, sometimes in large quantities. As the sugar of the blood is formed 

 from the carbohydrates in the food, it was thought that by excluding this 

 food-stuff from the diet the excretion of sugar might be prevented. It has 

 been found, however, that in some cases at least sugar continues to be present 

 in the urine even upon a pure proteid diet. If we suppose that some of the 

 proteid goes to form glycogen, the result observed is explained, for the gly- 

 1 Voit : Zeitschrift filr Biologic, 1891, xxviii. p. 285. 



