INTERNAL WORK. 375 
ular, and fermentative actions indicated above, it does not call for 
any additional expenditure of energy and so does not, from the 
statistical point of view, constitute part of the “work of digestion.” 
If, however, at any time the warming of the ingesta requires more 
heat than is produced by these processes—if, for example, a large 
amount of very cold water is consumed—it is evident that the 
surplus energy required will be withdrawn from the stock otherwise 
available for other purposes, and to this extent will increase the 
expenditure of energy consequent upon digestion. 
THE EXPENDITURE OF ENERGY IN ASSIMILATION.—While our 
knowledge of the changes which the nutrients undergo after re- 
sorption is very meager, we may regard it as highly probable that 
they undergo important transformations before they are fitted to 
serve directly as sources of energy for those general vital activities 
of the body represented in gross by the fasting metabolism. 
Thus the proteoses and peptones produced in the course of 
digestive proteolysis are synthesized again to proteids, while the 
proteids, when the supply is large, undergo, as was shown in Chap- 
ter V, rapid nitrogen cleavage, leaving a non-nitrogenous residue 
as a source of energy. According to some authorities, as we have 
seen, the resorbed fat undergoes conversion into dextrose in the 
liver before entering into the general metabolism of the body. 
Even the carbohydrates, at least so far as they are not directly 
resorbed as dextrose, seem to undergo more or less transformation 
before entering into the general circulation. 
In brief, there seems good reason to believe that the crude mate- 
rials resulting from the digestion of the food undergo more or less 
extensive chemical transformations before they are ready to serve 
as what Chauveau calls the “potential” of the body—that is, as 
the immediate source of energy for the vital functions. Of the 
nature and extent of these transformations we are largely ignorant. 
So far as they are katabolic in their nature, a liberation of energy is 
necessarily involved. Any anabolic processes of course would 
absorb energy, but the energy so absorbed must come ultimately. 
from the katabolism of other matter, and in all probability there 
would be more or less escape of kinetic energy in the process. 
Moreover, as was pointed out in the opening paragraphs of 
Chapter II in discussing the general nature of metabolism, as well 
Digitized by Microsoft® 
