CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 723 



living substance. We may fairly suppose that a quantity of sugar 

 hi solution present in a vacuole, for instance, of the hepatic cell- 

 substance can be, by some action of the cell-substance, converted 

 into glycogen in a solid form, filling up the vacuole. < Again, as we 

 have incidentally mentioned, sugar injected into the jugular vein 

 readily gives rise to sugar in the urine ; but a very considerable 

 quantity can be slowly injected into the portal vein without any 

 appearing in the urine. This suggests the idea that the liver, so 

 to speak, catches the sugar as it is passing through the hepatic 

 capillaries and at once dehydrates it into glycogcn. 



Similar considerations may also be applied to the case men- 

 tioned above of the appearance of glycogen in the hepatic cells of 

 winter (fasting) frogs. We have reason to think that sugar makes 

 its appearance as a product of the metabolism of various tissues. 

 The sugar thus arising finding its way into blood may be made 

 use of at once elsewhere, converted speedily for instance into 

 carbonic acid and so got rid of. But we can readily imagine that 

 under certain circumstances, as for instance when the activities of 

 the animal were lessened by a low temperature, it was not so made 

 use of and remained in the blood. If so it would in the course of 

 the circulation be carried to the liver, and might be at once taken 

 up by the hepatic cells and converted into glycogen; and these 

 might be so active that the blood was never at any time allowed 

 to remain loaded with sugar to such an extent as to permit a loss 

 through the urine. 



461. Upon such a view, the carbohydrate taken as food 

 would be converted into glycogen by the agency of the hepatic 

 cell, without at any time becoming an integral part of the living 

 substance of the cell. Such a view may be the true one ; but it 

 is open for us to look at the matter in another light. We may 

 push still further the analogy between the glycogen of the hepatic 

 cell and the material with which a secreting cell is loaded. In 

 dealing with secretion we saw reasons for regarding such a body 

 as mucin to be a product of the metabolism of the cell-substance 

 of the mucous cell ; and we may similarly regard glycogen. or 

 sugar readily convertible into glycogen, or at least some or other 

 carbohydrate material, as a normal product of the metabolism of the 

 hepatic cell. We may thus conceive of the hepatic cells as being 

 continually engaged in giving rise to carbohydrate material in the 

 form either of sugar or of some other body ; and we may suppose 

 that under certain circumstances, as in the absence of adequate 

 food, the carbohydrate material thus formed is at once discharged 

 into the blood of the hepatic vein for the general use of the body, 

 but that under other circumstances, as when an amylaceous meal 

 has been taken, the immediate wants of the economy being 

 covered by the carbohydrates of the meal, the carbohydrate 

 products of the hepatic metabolism are stored up as glycogen. 

 Under such a view the sugar of the meal is used up somewhere 



462 



