208 PRINCIPLES OF ANIMAL NUTRITION. 
the food is sufficient, as is illustrated in the experiments of Caspari 
and Bornstein. 
As a basis for a tentative hypothesis, it seems allowable to sup- 
pose that both these processes—that of anabolism and katabolism 
of proteids—are continually taking place in the muscle and that 
both are exaggerated by exercise. In other words, we may imagine 
that the performance of work by a normally developed muscle 
requires an increased proteid katabolism, which is balanced, at least 
in the course of the twenty-four hours, by a corresponding increase 
in the proteid anabolism. With a liberal supply of food proteids, 
then, a part of the latter would, during rest, simply undergo nitro- 
gen cleavage and be used virtually as “fuel,” but when work was 
done they (or part of them) would be used to replace the proteids 
katabolized in the muscles. Upon this hypothesis, the proteids 
might play a not unimportant part in the production of muscular 
work without any evidence of it appearing in an increased nitrogen 
excretion. It is to be remarked, however, that even on this suppo- 
sition the proteids could not be regarded as furnishing all, or even, 
in many cases, a large share, of the energy liberated. On insuffi- 
cient food, the hypothesis would assume that the energy supply is 
deficient and that proteids which would otherwise be used for 
muscular anabolism are diverted to use as “fuel,” probably under- 
going a preliminary nitrogen cleavage and furnishing their non- 
nitrogenous residue to the muscles as a source of energy. 
The above tentative hypothesis implies that if work were per- 
formed upon a ration containing only the minimum amount of 
proteids required during rest, it would cause an increase of the 
proteid metabolism, no matter how much non-nitrogenous mate- 
rial was supplied, because there would be no proteids available 
which could be diverted to repair the waste assumed to be occa- 
sioned by muscular activity. Up to the present time, however, 
we possess no experimental investigation of this phase of the ques- 
tion. 
However this may be, we know that the performance of work 
requires a well-developed muscular system. To produce and de- 
velop such a system, a liberal supply of protein is essential, while 
we may reasonably suppese that to maintain it involves a larger 
proteid supply in the food than is required to maintain the proteid 
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