220 PRINCIPLES OF ANIMAL NUTRITION. 
Many eminent physiologists, however, notably Zuntz and his 
pupils, go further and regard both the fat and the carbohydrates of 
food or body tissue as immediate sources of muscular energy and as 
of value for this purpose in proportion to their content of potential 
energy—that is, to their heats of combustion. In other words, they 
hold that either fat or carbohydrates may be in effect directly 
metabolized by the muscular tissue and that each under like condi- 
tions yields substantially the same proportion of its potential energy 
in the form of mechanical work. 
On the other hand, Chauveau * and Seegen f{ and their followers, 
as has already been indicated, regard the carbohydrates as the im- 
mediate source of energy for all the vital activities and hold that fat 
(or proteids) must first be converted into dextrose by the liver before 
it can be utilized. It is particularly with regard to muscular exer- 
tion that this theory has been elaborated, the conclusions as to other 
forms of vital activity being to a considerable extent based upon 
analogy with the former. 
Functions of the Liver.—According to this theory the material 
which is actually metabolized in a muscular contraction is a carbo- 
hydrate, viz., either the dextrose carried to the muscle by the blood 
or the glycogen which is stored up in it. Muscular activity is thus 
brought into intimate relations with the sugar-forming function of 
the liver, and a chief office of that organ is considered to be the 
preparation of the necessary carbohydrate material from the various 
ingredients of the food. The main facts which have been estab- 
lished may be summarized as follows (compare Chapter II, §§1 
and 2): 
1. Dextrose is being constaritly formed by the liver, which not 
only modifies the carbohydrates of the food but likewise appears to 
produce dextrose from proteids and particularly, according to this 
school of physiologists, from fat. 
2. Dextrose is as constantly being abstracted from the blood by 
the tissues, particularly the muscular tissues, as is shown by the 
constancy of the proportion of dextrose in thé blood. 
3. The dextrose content of the blood is, according to Chauveau, 
* La Vie et. ]’Energie chez l’ Animale. 
t Die Zuckerbildung im Thierkérper. 
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