570 STORAGE OF GLYCOGEN. [BOOK n. 



substance, wliich in its changes is continually giving out energy, 

 has the power of acting on molecules of starch or of sugar in 

 contact with, or even only near to itself, and so of hydrating 

 starch into the sugar or of dehydrating sugar into starch. The 

 latter process may be a more difficult one than the former, but 

 not one beyond the power of the living substance. We may 

 fairly suppose that a quantity of sugar in 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 inciden- 

 tally 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 glycogen. 



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 in- 

 stance 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. 



365. 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 liv- 

 ing 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 re- 

 garding such a body as mucin to be a product of the metabo- 

 lism 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 en- 

 gaged in giving rise to carbohydrate material in the form either 

 of sugar or of some other body ; and we may suppose that under 



