NUTRITION. 469 



sugar in that liquid falls below a certain percentage. That 

 the chief part of the glycogen found in the normal liver has 

 its ultimate source in carbohydrate foods is shown by several 

 facts. (1) 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 carbohydrates, sugar rarely appears in the urine, and 

 then but temporarily; so that the large quantity of it absorbed 

 from the alimentary canal within a brief time under such cir- 

 cumstances, 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 accumu- 

 lation of glycogen in the liver. We may therefore conclude 

 that the sugar absorbed from the alimentary canal is taken 

 by the portal vein to the liver, and there converted into 

 glycogen and stayed for a time ; and later slowly passed on 

 into the hepatic veins during the intervals between meals. 

 Thus in spite of the intervals which elapse between meals the 

 carbohydrate content of the blood is kept pretty constant: 

 during 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 

 fresh liver-cells are capable of changing glycogen into glucose 

 the latter view is the more probable. Analyses of portal and 

 hepatic bloods, made with the view of determining whether 

 more sugar was carried out of the liver during fasting than 

 into it, are conflicting; and considering the great amount of 

 blood which flows through the liver in twenty-four hours, a 

 very slight increase of sugar (falling within the limits of 

 error of the difficult quantitative determination of that sub- 

 stance in the blood) in the hepatic vein would represent a 

 large total amount during the whole day. The main fact, 

 however, remains that somehow this carbohydrate reserve in 

 the liver is steadily carried off to be used elsewhere: and 

 animal glycogen thus answers pretty much to vegetable starch, 

 which, made in the green leaves, is dissolved and carried away 



