440 THE HUMAN BODY. 



being constantly used up, and its maintenance in normal, 

 quantity depends on food. 



The Source and Destination of Liver Glycogen. AIL 

 foods are not equally efficacious in keeping up the stock of 

 glycogen in the liver; fats by themselves are useless; pro- 

 teids by themselves give a little; but by far the most is 

 formed on a diet rich in starch and sugar; so it would seem 

 that glycogen is mainly formed from carbohydrate materials- 

 absorbed from the alimentary canal and carried to the- 

 hepatic cells by the portal vein. These materials ara 

 mainly glucose, since the starch eaten is changed into that 

 substance before absorption. This view* of the matter is- 

 supported by several facts. (1) Grape sugar if it exist, 

 in the blood in above a certain small percentage passes out, 

 by the kidneys and appears in the urine, constituting the- 

 characteristic symptom of the disease called diabetes. In 

 health, however, even after a meal very rich in carbohy- 

 drates, no sugar appears in the urine; so that the large 

 quantity of it absorbed from the alimentary canal within a* 

 brief time under such circumstances, must be stopped 

 somewhere before it reaches the general blood current. (2) 

 Glucose injected into one of the general veins of an animal,, 

 if in any quantity, soon appears in the urine; but the same- 

 amount injected into the portal vein, or one of its radicles,, 

 causes no diabetes, but an accumulation of glycogen in 

 the liver. We may therefore conclude that the grape sugar 

 absorbed from the alimentary canal is taken by the portal 

 vein to the liver; there stayed and converted into glycogen;, 

 which is then more slowly passed on into the hepatic 

 veins during the intervals between meals. Thus in spite 

 of the intervals which elapse between meals the carbo- 

 hydrate content of the blood is kept pretty constant: dur- 

 ing digestion it is not suffered to rise very high, nor dur- 

 ing ordinary periods of fasting to fall very much below the 

 average. 



In what form glycogen leaves the liver is not certain; 

 it might be dissolved out and carried off as such, or 

 previously turned again into glucose and sent on in that 

 form; since the blood and the liver both seem to contain. 



