Page:A Text-book of Animal Physiology.djvu/466

434 The Uses of Glycogen.—Whether the blood of the hepatic vein contains more sugar than that of the portal vein has long been a subject of controversy. If the affirmative could be established, it would be pretty clear that glycogen stored in the liver-cells was transformed into sugar, possibly by a process of hydration. But, considering the rapidity of the blood-stream, it is easy to understand that a large amount of sugar might be conveyed

into the general circulation, and yet the blood, whether of the hepatic vein or of other parts, contain but a small quantity at any one time. The blood is kept of a certain fairly constant composition, both by the action of the excreting organs and by the withdrawal from it of supplies for the tissues. Moreover, that correlation of functional work on which we have already insisted, is not to be forgotten. One must not conceive of the liver-cells or any others doing their work independently of the condition of their fellow cell-units in the organic commonwealth. We mean to say that the amount of glycogen transformed to sugar will depend on a great many circumstances outside of the liver itself. Such aspects of the case have been rather overlooked. According to another theory, glycogen is an intermediate product between sugar and fat, but of this there is very little evidence indeed; and, besides, fat formation is otherwise well enough accounted for, though, of course, too much stress must not be laid upon such an argument.

What is the fate of the transformed glycogen? What becomes of the sugar? We can answer, negatively, that it is not