CHEMISTRY OF DIGESTION AND NUTRITION. 329 



to be formed ordinarily 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 severe eases 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 ob- 

 served is explained, for the glycogen, as will be explained presently, is finally 

 converted to sugar and is given oft' to the blood. An interesting additional 

 fact that points to the same conclusion is that the percentage of sugar in the 

 blood remains practically constant after prolonged starvation, at a time when 

 the animal is living at the expense of the proteids and fats of its own body. 



Effect of Fats and other Substances upon Glyeogen-formatwn. — It has been 

 found that fats take no part in the formation of liver glycogen. Some 

 attempts have been made to prove that fat in the body, and particularly in 

 the liver, may be converted to sugar, but the evidence at present seems to be 

 against this possibility. 1 



The Function of Glycogen : Glycogenic Theory. — The meaning of the 

 formation of glycogen in the liver has been, and still is, the subject of discus- 

 sion. The view advanced first by Bernard is perhaps most generally accepted. 

 According to Bernard, glycogen forms a temporary reserve supply of carbo- 

 hydrate material that is laid up in the liver during digestion and is gradually 

 made use of in the intervals between meals. During digestion the carbohy- 

 drate food is absorbed into the blood of the portal system as dextrose or as 

 dextrose and levulose. If these passed through the liver unchanged, the con- 

 tents of the systemic blood in sugar would be increased perceptibly. It is now 

 known that when the percentage of sugar in the blood rises above a certain 

 low limit, the excess will be excreted through the kidney and will be lost. 

 But as the blood from the digestive organs passes through the liver the ex- 

 cess of sugar is abstracted from the blood by the liver-cells, is dehydrated to 

 make glycogen, and is retained in the cells in this form for a short period. 

 From time to time the glycogen is reconverted into sugar (dextrose) and is 

 given off to the blood. By this means the percentage of sugar in the systemic 

 blood is kept nearly constant (0.1 to 0.2 per cent.) and within limits best 

 adapted for the use of the tissues. The great importance of the formation of 

 glycogen and the consequent conservation of the sugar-supply of the tissues will 

 be more evident when we come to consider the nutritive value of carbohydrate 

 food. Carbohydrates form the bulk of our usual diet, and the proper regula- 

 tion of the supply to the tissues is therefore of vital importance in the main- 

 tenance of a normal healthy condition. The second part of this theory, which 

 holds that the glycogen is reconverted to dextrose, is supported by observations 



Upon livers removed from the body. It has been found that shortly after the 



removal of the liver the supply of glycogen begins t<> disappear and a corre- 

 sponding increase in dextrose occurs. Within a comparatively short time all 

 the glycogen is gone and only dextrose is found. It is for this reason that in 



1 Kumagawa ami Miura: Arehiv fur Anatomie und Physiologie ("Physiol. Abtheilung"), 



1 s'. is, s. 431, contains also reference to the literature of the subject. 



