270 PRINCIPLES OF ANIMAL NUTRITION. 
the organism. It is only this latter portion, of course, of which the 
body can avail itself, and the term available energy has, therefore, 
very naturally been proposed for it. 
As will appear later, however, the terms available and availa- 
bility may also be employed, and have actually been used, in a more 
restricted sense to designate that part of the energy of the food 
which can be applied directly by the organism to purposes other 
than simple heat production. In order to avoid the confusion of 
terms thus arising it has been proposed to modify the term available 
by the words gross and net. The gross available energy, according 
to this terminology, signifies all of the total energy of the food 
which can be utilized by the body for any purpose whatever; 
that is, it is available energy in the first of the two senses defined 
above. Similarly, the net available energy signifies the available 
energy in the second sense, or energy available for other purposes 
than simple heat production. The term “fuel value” has also been 
employed by some writers, notably by Atwater, to designate the 
gross available energy. 
It appears to the writer desirable, however, to avoid the double 
use of the word available, even with the somewhat awkward modi- 
fying terms proposed. Strictly speaking, what is meant by gross 
available energy in the above sense is that portion of the potential 
energy of the food which the digestive and metabolic processes of 
the organism can convert into the kinetic form, and its measure, 
according to the principles enunciated in Chapter VII, is the differ- 
ence between the potential energy of the food and the potential 
energy of the various forms of unoxidized matter rejected by the 
organism. In other words, it is that fraction of the energy of the 
food which can enter into the metabolism of energy in the body. 
The writer, therefore, tentatively proposes for it the term metabo- 
lizable energy, as expressing the facts without any implication as to 
the uses made by the body of the energy thus metabolized. 
Metabolizable energy, then, may be briefly defined as potential 
energy of food minus potential energy of excreta, including under 
excreta, of course, all the wastes of the body, visible and invisible. 
The method is analogous to that of the determination of digestibility. 
In both cases it is a calculation by difference, and the result shows 
simply the maximum amount of matter or of energy put at the dis- 
