498 PRINCIPLES OF ANIMAL NUTRITION. 
' 
of this quantity with the work done will give the coefficient 
of utilization for the particular animal and kind of work experi- 
mented on. 
The Efficiency of the Animal as a Motor. 
The relation just indicated between the work performed and the 
total energy metabolized in its performance is not infrequently re- 
garded as expressing the efficiency of the animal as a motor, but it 
should be clearly understood that this is true only in a limited sense. 
A coefficient computed in the manner outlined above takes account 
only of the loss which occurs in the conversion of the stored energy 
of the body into external mechanical work. It neither includes the 
expenditure of energy required for the digestion and assimilation of 
the food, nor does it take account of the large amount of energy con- 
tinually consumed in the internal work of the animal machine. It 
does not, therefore, furnish a direct measure of the economy with 
which the animal machine uses the energy supplied to it, but is 
comparable rather to the theoretical thermo-dynamic efficiency of 
a steam-engine. With this limitation, however, the phrase may be 
used as a matter of convenience. 
Quite extensive investigations upon this point are already on 
record. They have generally taken the form of what may be called 
respiration experiments. The respiratory exchange of carbon di- 
oxide and oxygen has been determined, first, in a state of rest, and, 
second, during the performance of a measured amount of work. 
From the difference between these two values the quantity of ma- 
terial metabolized and the amount of energy consequently liberated 
have been computed and compared with the energy recovered in 
the form of mechanical work. 
This method of experimentation has been largely developed and 
employed by Zuntz and his associates * in experiments upon man, 
the dog, and especially the horse. Since the present work relates 
especially to the nutrition of domestic animals, the results upon the 
latter animal are of peculiar interest, but their study may be ad- 
vantageously preceded by a somewhat brief consideration of the 
results upon the dog and upon man. 
* Compare Chapter VIII, pp. 251-2 
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