760 USES OF GLYCOGEN. [BOOK n. 



conversion into glycogen is a subsidiary act for the purpose of 

 retaining the carbohydrate material in the grasp of the cell. If 

 this be the case, then until it has been shewn that there is some- 

 thing peculiar about the sugar thus produced by the cell itself, by 

 virtue of which it alone can be converted by the cell into glycogen, 

 we may fairly infer that the cell might also convert into glycogen 

 sugar passing into the interstices of the cell-substance from the 

 blood capillaries, whether that sugar come directly from the food or 

 be a product of the metabolism of muscular or some other tissue. 



462. We may now turn to another question, the answer of 

 which is in a measure dependent on the one which we have just 

 discussed. What is the use and purpose of this hepatic glycogen ? 

 What ultimately becomes of the glycogen thus for a while stored 

 up in the liver ? 



One view which has been put forward is as follows. We have 

 evidence, as we shall presently learn, that a great deal of the fat 

 of the body is not taken as such in the food, but is constructed 

 anew in the body out of other substances. Both carbohydrates 

 and proteids, taken in excess or under certain circumstances, lead 

 to an accumulation of fat; and we have reason to believe that 

 carbohydrates on the one hand and the carbon-holding portions of 

 various proteids on the other, may by some process or other be 

 converted into fat. And it has been suggested that the glycogen 

 in the liver is a phase of a constructive fatty metabolism, that it is 

 material on its way to become fat. 



The positive evidence in favour of this view is very scanty ; it 

 is almost limited to the facts that fat, sometimes in very large 

 quantity, is found in the hepatic cells, that while fat itself taken 

 as food leads to no increase in the hepatic glycogen, carbohydrates, 

 which are especially fattening, are most active producers of 

 glycogen, and that the fat present in the hepatic cells seems to 

 be increased by such diets as naturally increase the glycogen in 

 the liver. No evidence has been offered as to the occurrence in 

 the hepatic cell of any of the several steps of the conversion of 

 glycogen into fat, nor indeed has it been suggested what those 

 steps are. The view indeed is almost exclusively based on the 

 supposed proof that the blood of the hepatic vein contains during 

 life no sugar, or at least not more than does the general blood or 

 even the blood of the portal vein. From this it is inferred that the 

 glycogen in the liver is not lost to the liver by becoming converted 

 into sugar and so discharged into the hepatic blood, and therefore 

 must be converted into some other substance, which substance is 

 presumably fat. But this line of argument is one which cannot 

 safely be trusted. On the one hand it has been maintained both 

 by older and more recent observers that the blood of the hepatic 

 vein under normal conditions is richer in sugar than the blood of 

 the portal vein or indeed of any other part of the vascular system ; 

 and this has been regarded as an indication that the liver is 



