396 PRINCIPLES OF ANIMAL NUTRITION. 
net contribution which it makes to the demands of the vital func- 
tions for energy or, in other words, its value as part of a mainte- 
nance ration. This must be clearly distinguished from its value 
for the storage of additional energy in the body—that is, its value 
for productive purposes. In the latter case it is quite possible that 
the conversion of the digested nutrients into suitable forms for 
storage (fat of adipose tissue, ingredients of milk solids, proteids of 
new growth, etc.) involves a greater expenditure of energy than is 
required to convert them into forms fitted to serve as sources of 
energy to the body cells (work of assimilation). The consideration 
of this question belongs in the succeeding chapter, but meanwhile 
it is important to bear in mind that the net available energy, in 
the sense in which the term is here employed, is a distinct con- 
ception from that of the utilization of energy in fattening, milk 
production, etc., and has reference to the availability of the energy 
of the food for maintenance. 
It is evident from the above paragraphs that the value of a 
feeding-stuff to the animal is not measured solely by its metaboliz- 
able energy, since materials containing the same proportion of the 
latter may require the expenditure of very unequal amounts of 
energy for their digestion and assimilation and, therefore, may 
contain very unequal amounts of net available energy. Plainly, 
then, it is a matter of much importance to know the net avail- 
ability of the metabolizable energy of the various nutrients and 
feeding-stuffs, and thus to learn the proportions in which they may 
replace each other. 
§ xz. Replacement Values. 
We have already seen (Chapter V, p. 148) that, aside from a 
certain minimum of proteids, the several nutrients can mutually 
replace each other to a very large if not to an unlimited extent, 
either one or all serving, according to circumstances, to supply the 
demand for energy. 
In 1882 v. Hésslin * published an extended discussion of Petten- 
kofer & Voit’s respiration experiments from this point of view, 
using such data regarding the potential energy of the nutrients as 
were then available. He calls attention to the wide range of re- 
* Virchow’s Archiv, 89, 333. 
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