NET AVAILABLE ENERGY—MAINTENANCE. 409 
Fasting. Fed Small Amount of Meat. 
Heat Heat 
aa ke Praga any es Production, 
13.2 39.65 19.5 42.64 
19.5 35.10 18.2 41.13 
27.4 30.82 23.7 41.83 
24.8 41.10 
arising, below the critical point, from the “ chemical” regulation of 
the body temperature affords a very reasonable explanation of the 
apparent discrepancy between the law of isodynamic replacement 
as propounded by Rubner and the no less certain fact that the work 
of digestion and assimilation makes a demand on the body for 
energy, which energy finally takes the form of heat and is not 
available for other purposes. 
A consequence of this hypothesis, however, which is sufficiently 
obvious has indeed been pointed out, but hardly seems to have re- 
ceived the attention which it deserves in view of its important 
bearing on the theoretical aspects of metabolism. 
If we give increasing amounts of food to a fasting animal we 
progressively increase the evolution of heat due to digestive work, 
and this heat, according to the hypothesis, if the thermal environ- 
ment is below the critical point, is substituted for the heat pre- 
viously produced by the metabolism of tissue. There must be a 
limit to the possibility of this substitution, however, just as there 
must be to the “chemical” regulation of body temperature (p. 353), 
since otherwise there would be a ration on which all the heat of the 
body was derived from the work of digestion and the internal work 
was performed without evolution of heat. The limit is indeed the 
same in both cases and is reached when all the heat previously 
evolved by the processes of “chemical” regulation has been re- 
placed by the heat arising from digestive work. Beyond that 
point the conditions are the same as in the fasting animal above 
the critical point, and the excess of heat is gotten rid of by 
“physical” regulation. We may call the amount of food whose in- 
gestion produces the quantity of heat necessary to just reach this 
limit the critical amount of food. Below that amount the apparent 
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