CHAPTER X. 
THE FOOD AS A SOURCE OF ENERGY—METABOLIZABLE 
' ENERGY: 
Wirx the establishment of the law of the conservation of 
energy in its application to the animal body, and with the 
development ‘of the methods of calorimetric research briefly out- 
lined in Chapter VIII, it has become possible to study success- 
fully the problems of animal nutrition from a new standpoint, re- 
garding the food as primarily a source of energy to the body and 
tracing, to some extent at least, the transformations which that 
energy undergoes in the organism and particularly the extent to 
which the latter utilizes it for various purposes. 
Some data regarding the total energy of foods and their constitu- 
ents have already been givenin Chapter VIII. It was there pointed 
out, however, that the total energy, taken by itself, does not fur- 
nish a measure of the nutritive value of a substance. It is now 
necessary to enter upon the question of the availability of this 
energy to the organism. 
ToTAL AND METABOLIZABLE EneERGy.—The heat of combustion 
of the food represents to us its total store of potential energy. By 
no means all of this potential energy, however, is accessible to the 
organism. A part of what the animal eats is not food at all in a 
physiological sense, but is simply waste matter which passes through 
the digestive tract unacted upon. Furthermore, that part of it 
which is digested and resorbed is not completely oxidized in the 
body, but gives rise to the formation of excretory products which are 
still capable of liberating energy by oxidation. We have, there- 
fore, at the outset, to distinguish between the total, or gross, 
energy of the food eaten, represented by its heat of combustion, 
and the portion of that energy which can be liberated and utilized in 
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