412 PRINCIPLES OF ANIMAL NUTRITION. 
OM is the critical amount of food. Beyond this amount the energy 
expended in the work of digestion will become waste energy, serving 
simply to increase the outflow of heat, and the apparent and real 
availability of the food will coincide. 
Plainly, the critical amount of food will vary with circumstances. 
If the experiment is made at or above the critical thermal environ- 
ment for the fasting animal the smallest quantity must cause an 
increase in the heat production and the critical amount will be 0 
(or, mathematically, a negative quantity). As the external con- 
ditions fall below the critical thermal environment, the point K will 
‘be further and further removed from A until finally the point of 
intersection might even lie above OX, that is, above the mainte- 
nance ration. The relative availability of the food, too, will be a 
factor in determining the critical amount. .Thus if the true availa- 
bility of the food were expressed by the line AP instead of AD, the 
point of intersection would lie at R and OR’ would be the critical 
amount of food. 
§ 3. Net Availability. 
The modified conception’ of replacement values discussed in 
the preceding section and in the introductory paragraphs of this 
chapter renders it evident that both the theory of isodynamic re- 
placement, as first announced and later modified by Rubner, and 
the rival theory of isoglycosic replacement are but aspects of the 
more general question. of the availability of the metabolizable 
energy of the food. That the several nutrients are of use to the 
~body and can replace each other in the food in inverse ratio to 
their available energy is simply a necessary consequence of the law 
‘of the conservation of energy. The important question is how 
much of their energy is really available. Rubner’s theory regards - 
all the metabolizable energy of the food as virtually available, 
directly or indirectly, for maintenance, and this view has been quite 
generally accepted. Chauveau’s theory of isoglycosic replacement 
has the merit of distinctly recognizing the fact of a possible expen- 
diture of energy in the assimilation of the digested food, but, on the 
other hand, it takes no account of the digestive work, and moreover, 
so far as maintenance values are concerned, rests, as we have seen, 
upon a rather insecure foundation. Plainly, the real question at 
