CHAPTER XII. 
NET AVAILABLE ENERGY—MAINTENANCE, 
Tus organic matter contained in the body of an animal we have 
learned to regard in the light of a certain capital of stored-up energy, 
at the expense of which the vital activities of the organism are 
carried on. The function of the food is to make good the losses 
thus occasioned. The food is frequently spoken of as “the fuel of 
the body.” In a certain limited sense the comparison is admissible, 
but it may easily be pushed too far, and a closer analogy is that with 
a stream of water supplying a reservoir and serving to replenish the . 
drafts made upon it for water. 
The food in the form in which it is consumed, however, is by no 
means ready to enter directly into the composition of the tissues of 
the body and add to its store of potential energy, but on the con- 
trary, as we have seen, a very considerable amount of energy must 
be expended in the separation of the indigestible matters from the 
digestible and in the conversion of the latter into such forms as are 
suitable for the uses of the living cells of the body. 
When, therefore, we give food to a quiescent fasting animal we 
do two things: we supply it with metabolizable energy, depending 
in amount upon the quantity and nature of the food, to take the 
place of the energy expended in its internal work, but we at the 
same time increase its expenditure of energy by the amount neces- 
sary to separate the metabolizable from the non-metabolizable 
energy of the food. 
The case is analogous to that of a steam-boiler which is fired 
by means of a mechanical stoker driven by steam. from the same 
boiler. Each pound of coal fed into the fire-box is capable of 
evolving a certain amount of heat, representing its metabolizable 
energy in the above sense, and that heat is capable of producing a 
394 
