20 PRINCIPLES OF ANIMAL NUTRITION. 2 
stantially two suppositions are open: On the one hand, we may 
consider that the resorbed carbohydrates of the food, after being 
temporarily stored up in the liver, as described below, are given off 
again without radical change and that the sugar-forming power of 
the hepatic cells is limited to the transformation of the proteids and 
perhaps the fats of the food. Or, on the other hand, we may sup- 
pose that the nutrients brought to the liver by the portal blood 
enter into the constitution of the protoplasm of the hepatic cells, 
and that the vital activity of this protoplasm gives rise to the dex- 
trose found in the blood, to the glycogen found in the liver, and to 
other products of whose nature we are largely ignorant. The 
evidence at hand is doubtless insufficient for a final decision between 
these alternatives, but the latter hypothesis would seem more in 
accord with our general knowledge of cell activity. As relates to 
the carbohydrates, it is supported by the fact that while various 
sugars besides dextrose (lzevulose, mannose, galactose, sorbinose, 
and, as Miinch * has shown, certain artificial hexoses) may be con- 
verted into glycogen, the resulting glycogen is always the same and 
the product of its hydration is always dextrose.t In other words, 
the molecular structure of these sugars is altered in a manner sug- 
gesting an assimilation by the hepatic cells rather than anything 
resembling an enzyme action. The subject can be more intelli- 
gently considered, however, in the light of a discussion of the second 
function of the liver. 
Tue Liver as A RESERVOIR OF CARBOHYDRATES.—When the 
food is rich in carbohydrates, the supply of dextrose to the blood 
through the intestinal capillaries is more or less intermittent. As 
a means of regulating this intermittent supply, the hepatic cells 
have the power of arresting the dextrose brought to them by the 
portal vein and converting it into an insoluble carbohydrate called 
“glycogen” or “animal starch” which is stored up in the liver. On 
the other hand, when the supply of carbohydrate food is cut off, 
and especially if all food be withdrawn, the glycogen of the liver 
rapidly diminishes, being apparently reconverted into dextrose. 
This latter phenomenon may be readily observed in the liver of a 
freshly killed animal. If the fresh liver, after removal from the 
*Zeit. physiol Chem . 29, 493. 
+ Compare Neumeister. Physiologische Chemie, p. 326 
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