METABOLISM. 35 
Computations of the proportion of the energy of the added fat 
which was recovered in the total gain of flesh and fat (compare 
Chapter XIII, § 1) showed, according to the method of computa- 
tion employed, a loss of from 31 to 48 per cent. 
The comparatively small losses observed in Rubner’s and in 
Pettenkofer & Voit’s experiments may well be ascribed to a con- 
sumption of energy in the work of digestion (compare Chapter 
XI), but it hardly seems possible to account in this way for 
the large losses observed by Kellner. Apparently the peanut 
oil in these experiments, after its digestion and resorption, must 
have been subjected to extensive molecular changes involving a 
considerable expenditure of potential energy, and if this be true, 
the suggestion of an assimilation by the fat cells and a construction 
of animal fat from the oil is obvious. 
Constancy oF ComposITIon or Fats.—The relatively constant 
and characteristic composition of the fat of the same species of 
animal, notwithstanding differences in the food, has been urged in 
favor of the view that the fat of the animal is a product of the 
protoplasmic activity of the fat cells. “The fat of a man differs 
from the fat of a dog, even if both feed on the same food, fatty or 
otherwise” (M. Foster). The steer produces beef fat and the sheep 
mutton fat on identical rations. Unless, however, we are prepared 
to discredit the experimental results above cited, it would appear 
that this general and approximate uniformity of composition is 
largely due to a general uniformity of food, and that marked changes 
in the nature of the latter may result in altering the former. To this 
must be added, as already insisted upon, the fact that much of the 
fat found in the body, especially in the herbivora, is undoubtedly 
produced in the organism. We may fairly presume that this fat 
will be the characteristic fat of the species. If we may suppose 
further that a considerable share of the food fat is oxidized directly, 
and if we take into consideration the general uniformity of diet of 
our domestic animals and the relatively small total amount of fat 
which it often contains, we have at least a plausible explanation of 
the observed facts and one which does not preclude a direct deposi- 
tion of food fat in the body and a consequent effect upon the com- 
position of the body fat. ° 
The Katabolism of Fat.—The proportion of the food fat which 
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