THE METABOLISM OF THE BODY. 433 
of groups of animals including some invertebrates. 2. Among 
mammals it is most abundant when the animal feeds largely 
on carbohydrates. 3. It is found in the liver of the carnivora, 
and in those of omnivora, when feeding exclusively on flesh. 
4, When an animal starves (does not feed), the glycogen grad- 
ually disappears. 5. A fat-diet does not give rise to glycogen. 
6. During early foetal life glycogen is found in all the tissues, 
but later it is restricted more and more to the liver, though 
even in adults it is to be found in various tissues, especially the 
muscles, from which it is almost never absent. 
From the facts the inference is plainthat glycogen is formed 
from carbohydrate materials; or, to be rather more cautious, 
that the formation of this substance is dependent on the pres- 
ence of such material in the food. Inasmuch as glycogen oc- 
curs in muscle, it does not follow, from the fact of its presence 
in the liver of carnivorous animals, that it is manufactured 
from proteid substances, though this is not more difficult to 
understand chemically than the formation of fat from this 
source which is well established. 
Starch, it is well known, occurs abundantly in plants, and 
there is no doubt that the sugar often present in abundance has 
starch as its antecedent, and vice versa. Analogy, then, points 
to such a relation between carbohydrate food and glycogen for- 
mation on the one hand, and reconversion of glycogen into 
sugar on the other. And recent investigations tend to show 
that plant metabolism bears a greater resemblance to that of 
animals than was till recently supposed, thus giving greater 
force to the argument from analogy, though this is recognized 
as generally a dangerous one. 
Assuming this relation between food-stuffs and glycogen to 
hold, the question arises, How is the substance formed by the 
liver? There are three conceivable methods: 1. The liver-cells 
may, we know not how, simply dehydrate the sugar of diges- 
tion as carried to them in the portal blood. 2. The cells may 
manufacture glycogen from their own protoplasm, in which 
process the portal sugar is in some way used. 3. The liver-cells 
may always be engaged in the construction of glycogen as the 
gastric cells of pepsinogen, but the accumulation or removal of 
the substance depends on the character of the food especially ; 
thus, if the latter abounds in carbohydrates, the blood will be 
well supplied with sugar, so that the glycogen need not undergo 
its usual conversion into that substance. None of these views 
has been definitely proved to be the correct one. 
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