NET AVAILABLE ENERGY—MAINTENANCE. 413 
issue can only be settled by experiments in which the actual availa- 
bility of the energy of the food or of its various ingredients is deter- 
mined. ce 
Determinations of Net Availability. 
Since the net available énergy of the food is equal to its metabo- 
lizable energy minus the energy expended in digestion and assimila- 
tion, the two general methods for the determination of the latter 
quantity which were outlined in the preceding chapter (p. 377) are 
also, from the converse point of view, methods for the determination 
of net availability. In our study of digestive work we considered 
chiefly the results of direct determinations of the increase in the 
heat production due to the ingestion of food; for our present pur- 
pose the results of any accurate determinations of the metabolism 
upon varying known amounts of the same food may be used. 
The experimental evidence available is far from being as full as 
could be wished, but in the following paragraphs the attempt has 
been made to summarize such data as are accessible. In consider- 
ing these results it should be remembered that, as explained on 
p. 396, the net available energy means the energy available for 
maintenance. In a considerable number of the experiments to be 
considered, more or less gain was made by the animals, but it seems 
better to give the results of each series of experiments in full, re- 
serving a discussion of the results with productive rations for a 
subsequent chapter. 
Experiments on Carnivora. —The most extensive data regarding 
the metabolism of the carnivora in its relations to the food supply 
are those afforded by the investigations of Pettenkofer & Voit and 
of Rubner. These have already been ‘considered in Chapter V 
from the standpoint of matter and chiefly in a qualitative way; 
we have now to study them quantitatively in their bearing upon 
the income and expenditure of energy by the body. 
In Pettenkofer & Voit’s experiments, and in the earlier ones by 
Rubner, the quantities of energy involved must be computed from 
the chemical data. In Rubner’s experiments upon the source of 
animal heat, cited in Chapter IX, the actual heat production of the 
animals was determined, but in no case was there a direct determi- 
nation of the total income and expenditure of energy, and in par- 
ticular the data as to the energy of the food are incomplete. For 
