756 PHYSIOLOGY OF DIGESTION AND SECRETION. 



admission that fats may contribute directly to sugar production. 

 Whether the synthesis of sugar (or glycogen) from glycerin is, 

 so to speak, a normal process or occurs only under especial condi- 

 tions, can not be decided at present. 



The Function of Glycogen Glycogenic Theory. The mean- 

 ing 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. According to Bernard, glycogen 

 forms a temporary reserve supply of carbohydrate material that is 

 laid up in the liver during digestion and is gradually made use of in 

 the intervals between meals. During digestion the carbohydrate 

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

 dextrose, levulose, and galactose. If these sugars passed through 

 the liver unchanged, the contents 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 normal limit 

 (condition of hyperglycemia), the excess is excreted through the 

 kidney and is lost. But as the blood from the digestive organs 

 passes through the liver the excess of sugar is abstracted 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 to the use of the tissues. The great importance of the 

 formation of glycogen and the consequent conservation of the sugar 

 supply of the tissues is evident when we consider the nutritive value 

 of carbohydrate food. Carbohydrates form the bulk of our usual 

 diet, and the proper regulation of the supply to the tissues is, there- 

 fore, of vital importance in the maintenance 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 dis- 

 appear and a corresponding increase in dextrose occurs. Within a 

 comparatively short tune 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 gly- 

 cogen is changed to dextrose by the liver is a matter not fully ex- 

 plained. According to most authors, the conversion is due to an 

 enzyme produced in the liver. Extracts of liver, as of some other 

 tissues, yield a diastatic enzyme that changes glycogen to dextrose.* 

 *Tebb, "Journal of Physiology," 22, 423, 1897-98. 



