| NET AVAILABLE ENERGY—MAINTENANCE. 395 
certain quantity of steam. A definite fraction of the latter, how- 
ever, is required to introduce the next pound of coal into the furnace 
and therefore is not available for driving the main engine. To 
recur to the illustration of the reservoir, it is as if the water, instead 
of simply flowing into the reservoir, actuated a pump or a hydraulic 
ram which lifted part of it to the required level. 
Gross AND Net AvaILABILITy.—As stated in Chapter X, the 
difference between the potential energy of the food and that of the 
excreta represents the maximum amount of energy which is avail- 
able to the organism for all purposes. This quantity has some- 
times been designated as gross available energy, but has here been 
called metabolizable energy. 
A portion of this metabolizable energy, however, as just pointed 
out, has to be expended in the various processes which have been 
grouped together under the term work of digestion and assimilation. 
This portion ultimately takes the form of heat, thus tending to 
increase the heat production of the animal by a corresponding 
amount, and becomes unavailable for other purposes in the body, 
since, so far as we_ know, the organism has no power to convert heat 
into other forms of energy. The remainder of the metabolizable 
energy of the food represents the amount which that food con- 
tributes directly towards the maintenance of the capital of potential 
energy in the body. It is the measure of the net advantage derived 
by the body from the introduction into it of the food.* From this 
point of view the energy remaining after deducting the expenditure 
caused by the ingestion of the food from its metabolizable or gross 
available energy has been called the net available energy. There 
are obvious objections to the use of the words available and avail- 
ability in two senses, but no better term for net available has 
yet been suggested, while the use of available energy in the sense 
of metabolizable energy has become quite general. It appears 
necessary, therefore, to retain for the present the modifying words 
gross and net to avoid ambiguity. 
DISTINCTION BETWEEN AVAILABILITY AND UTILIzATION.—The 
net available energy of the food in the above sense represents the 
* As will appear later, this somewhat broad statement appears to be sub- 
ject to modification in certain cases in which there is an indirect utilization 
of the heat resulting from the work of digestion and assimilation. 
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