THE METABOLISM OF THE BODY. 461 
The amount of work which a man or other animal can do 
on a given diet may be estimated without the same sources of 
fallacy as attend the calculation of the heat expenditure; for, 
when an animal is confined in a calometric chamber, the con- 
ditions of the normal metabolism are not observed. 
Tue Sources of MuscuLaR ENERGY. 
Experimental.—T wo physiologists (Fick and Wislicenus) as- 
cended a mountain, noting the conditions under which their 
metabolism was performed, and drew certain conclusions in re- 
gard to the question now being considered. They lived exclu- 
sively on a non-nitrogenous diet while the work was being done, 
and estimated the amount of urea excreted at the same time. 
Assuming that the urea does represent the proteid metabolism 
(oxidation) which bore, of course, a definite relation to the 
energy available, it was found that in the case of each of them 
this was only about half enough to account for the work done. 
Even making large allowances for error in the estimates, if 
this experiment is to be trusted at all, it is plain that the 
energy of the muscles of the body is not derivable from their 
proteid metabolism ; and there are other facts which point in 
the same direction. 
It is found, when an isolated muscle is studied, that its 
continued contraction does not produce nitrogenous bodies, but 
very different ones, such as carbonic anhydride. The quantity 
of the latter may be augmented many times by work. But it 
is no longer believed that the severest labor appreciably in- 
creases the secretion of urea. 
The division of foods into heat-producers and tissue-builders 
is unjustifiable, as will appear from what has just been stated, 
as well as from such facts as the production of fat from proteid 
food, thus showing that the latter is indirectly a producer of 
carbonic anhydride, assuming that fat is oxidized into that 
substance. 
ANIMAL HEAT. 
Though a large part of the heat generated within the body 
is traceable to oxidations taking place in the tissues, it is better 
to speak of the heat as being the outcome of all the chemical 
processes of the organism; and though heat may be rendered 
latent in certain organs for a time, in the end it must reappear. 
While all the tissues are heat-producers (thermogenic), the ex- 
