INTERNAL WORK. 377 
Experimental Results. 
General Methods.—It follows from what has been said above 
that two general methods, or more properly two modifications of 
one general method, may be employed to determine the total ex- 
penditure of energy due to the ingestion of food. 
First, since the energy expended in the various processes out- 
lined above is ultimately converted into heat, we may determine the 
heat production of the animal while fasting and compare with it the 
heat production during the digestion and assimilation of a known 
amount of food. The excess of heat produced in the latter case as 
compared with the former will represent the increased expendi- 
ture of energy in the work of digestion and assimilation. 
Second, we may determine the total income and outgo of energy 
in the fasting and in the fed animal by one of the methods indicated 
in Chapter VIII. In this case the extent to which the net loss of 
energy by the body has been diminished by means of the food will 
show how much of the metabolizable energy of the latter has been 
utilized by the organism in place of that previously drawn from the 
metabolism of tissue. The part of the metabolizable energy not: 
thus utilized has obviously been expended in some of the various 
operations of digestion, assimilation, etc. The two methods are com- 
plementary, in the one case the expenditure for digestion, assimila- 
tion, etc., being determined directly and in the other by difference. 
A point of some importance, at least logically, is that the deter- 
minations should be made below the point of maintenance. The 
term assimilation as above defined includes all those processes by 
which the resorbed nutrients are prepared for their final metabo- 
lism in the performance of the vital functions. When we give food 
in excess of the maintenance requirement, however, there is added 
to this the set of processes by which the excess food is converted 
into suitable forms for more or less temporary storage in the 
body. These may be presumed to consume energy, and as it would 
seem, to a more or less variable extent. At any rate, we have 
no right to assume in advance that the relative expenditure of en- 
ergy above the maintenance point in the storage of excess material 
is the same as that below the maintenance point for the processes 
of assimilation as above defined. In other words, it is not necessa- 
rily nor even, it would seem, probably the case that the resorbed 
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