406 PRINCIPLES OF ANIMAL NUTRITION. 
ing the metabolism of energy in the body, but, as was natural, his 
first. conclusions have undergone more or less modification, in part 
at his own hands. 
Digestive Worx.—The law of isodynamic replacement as 
stated above is equivalent to saying either that all the metabolizable 
energy of the food below a maintenance ration is net available 
energy or that the percentage availability of all the nutrients 
experimented with is the same. The latter supposition, however, 
appears to be negatived by the results of Magnus-Levy and others 
on digestive work. 
If, however, a fraction of the metabolizable energy of the food 
is applied to the work of digestion and assimilation, it is plain that 
this fraction cannot serve directly for tissue building. In his first 
paper, Rubner, while not denying the fact of the consumption of 
energy in digestive work, appears to regard its amount as insignifi- 
cant, although what he specifically claims is that the total metabo- 
lism below the maintenance ration is not increased by the inges- 
tion of food. In support of this view he gives the results of three 
experiments in which fat was fed; that is, the nutrient which, ac- 
cording to Magnus-Levy’s later results, causes the least digestive 
work. Of these, one on a dog, in which approximately a mainte- 
nance ration was given, showed no increase of the metabolism over 
the fasting state. In the other two experiments, one on a dog and 
one on a rabbit, more fat was consumed than corresponded to the 
fasting metabolism, and an increase of the latter was observed 
amounting to approximately 3 per cent. and 12 per cent. respec- 
tively. Feeding with bone also caused an increase of about 12 
per cent. 
In a later publication,* however, he recognizes the apparent 
inconsistency between the effects of small and large amounts of 
food, and propounds a hypothesis to explain it which, in its general 
features at least, seems in harmony with the observed facts. This 
hypothesis is outlined in the following paragraphs, although in a 
slightly different manner than by Rubner. 
Iypirect Utinization or Heat Resuttine rrom DicEstivE 
Work.—In Chapter XI we acquired the conception of the critical 
thermal environment. According to the ideas there advanced, 
* Biologische Gesetze, Marburg, 1887, p. 20. 
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