INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 207 
1. The non-nitrogenous ingredients of the food or of the tissues 
are the chief source of muscular energy. In by far the greater 
number, if not all, of the experiments upon this subject the amount 
of protein metabolized, as measured by the nitrogen excretion, was 
insufficient to furnish energy equivalent to the work done, the de- 
ficiency being in many cases very great. This statement, it will be 
observed, does not assert that the proteids are not concerned in the 
production of this energy. We may regard it as very probable that 
the non-nitrogendus matter metabolized has first entered into the 
structure of the muscular protoplasm, which, as we know, consists 
largely of proteids, but in a contraction it is largely, if not wholly, 
the non-nitrogenous groups contained in the protoplasm which are 
metabolized rather than the nitrogenous groups. 
2. With insufficient food there may be a considerable increase 
in the proteid metabolism as a result of muscular exertion, espe- 
cially when pushed to exhaustion. 
3. This increase is far from sufficient to supply energy for the 
work actually done, is not usually proportional to it, and seems 
dependent to a considerable degree upon accompanying conditions. 
4. With sufficient food the increase of the total proteid metab- 
olism consequent upon muscular exertion is at the most slight and 
possibly equal to zero. 
5. In some cases a storage of proteids has been observed to 
result from the performance of work. 
FuNcTIONS OF PROTEIDS.+—If the above conclusions are admitted, 
it is possible to suppose that in a muscular contraction under favor- 
able conditions—that is, when there is an abundant supply of non- 
nitrogenous material—there is no increased metabolism of the 
proteids. This view of the subject would regard the question as 
being simply one of the relative supply of nutrients, the energy 
being evolved from non-nitrogenous nutrients when these are in 
abundance, while in default of them the proteids are drawn upon. 
Another view of the subject, however, is possible, and perhaps 
more probable. It would appear that muscular exertion tends to 
produce two opposite effects upon the proteid metabolism: first, 
to break down additional protein, as is shown when work is done 
upon insufficient food; and second, to build up proteid tissue when 
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