CHEMISTRY OF DIGESTION AND NUTRITION. 269 



cogen, as will be explained presently, is finally converted to sugar and is given 

 off to the blood. 



Effect of Fats and other Substances upon Glycogen-formation. It has been 

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

 increases the amount of glycogen in the liver, but the evidence goes to show 

 that it is not a direct or an indirect glycogen-former. Glycerin seems to 

 prevent the reconversion of glycogen to sugar by the liver-cells, and thus 

 leads to an increased percentage of this substance in the liver. 



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

 formation of glycogen in the liver has been, and still is, the subject of discussion. 

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

 cording to Bernard, glycogen forms a temporary reserve supply of carbohydrate 

 material which is laid up in the liver during digestion and which 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 

 glycogeu 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 to 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 

 the estimation of glycogen in the liver it is necessary to mince the organ and to 

 throw it into boiling water as quickly as possible, since by this means the liver- 

 cells are killed and the conversion of the glycogen is stopped. How the 

 glycogen is changed to dextrose by the liver is a matter not fully explained. 

 According to some, the conversion is due to an enzyme produced in the liver. 

 Extracts of liver, as of many other organs, do contain a certain amount of an 

 amylolytic enzyme, but this enzyme changes glycogen to maltose, whereas in the 

 liver the glycogen is normally changed to dextrose. It is probable, therefore, 

 that the conversion of glycogen to dextrose is dependent directly upon the 



