378 PRINCIPLES OF ANIMAL NUTRITION. 
portion of a maintenance ration is first converted into the same 
materials (particularly fat) that are deposited in the body when 
excess food is given, and that these materials are then metabolized 
in the performance of the bodily functions. It is at least conceiv- 
able, if not likely, that a much less profound transformation, and 
one involving a smaller loss of energy, suffices to prepare the re- 
sorbed nutrients for their functions as “ potential” than is required 
for their storage as gain of tissue. 
Finally, the comparison need not necessarily be made, and in- 
deed in case of most agricultural animals cannot well be made, with 
the fasting state. While this method is the simpler when practi- 
cable, a comparison of the total heat production or of the balance 
of energy on two different rations (both being less than the mainte- 
nance requirement) will afford the data for a computation by differ- 
ence (exactly similar to that employed in the determination of 
metabolizable energy in Chapter X) of the expenditure of energy in 
the digestion and assimilation of the food added to the basal ration. 
The most important quantitative investigations upon the work 
of digestion are those of Magnus-Levy * on the dog and on man, 
and those of Zuntz & Hagemann f+ upon the horse. 
Experiments on the Dog.—In Magnus-Levy’s experiments 
the respiratory exchange of the animal was determined by means 
of the Zuntz apparatus at intervals of one or two hours during 
fasting and after feeding. The single periods were twenty-five to 
thirty minutes long, and the external conditions were maintained 
as uniform as possible. 
Fat.—Fat (in the form of bacon free from visible lean meat), 
when given in quantities not materially exceeding in heat value 
the fasting metabolism, resulted in a slight increase of the latter, 
beginning about one to three hours after eating, reaching its maxi- 
mum between the fifth and ninth hours, and disappearing about the 
twelfth hour. The maximum increase observed was 12 per cent., 
seven hours after eating. In amounts largely exceeding the equiv- 
alent of the fasting metabolism the effect of fat was somewhat 
more marked and longer continued, a maximum increase of 19.5 
per cent. being observed in one case seven hours after eating, while 
* Arch. ges. Physiol., 55, 1. 
t Landw. Jahrb., 27, Supp. III. 
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