218 PRINCIPLES OF ANIMAL NUTRITION. 
of those insufficiently fed the respiratory quotient is increased by 
the performance of work, while with well-fed animals, especially 
those receiving an abundance of carbohydrates, this effect is not 
apparent. As the work is continued, there appears in many cases 
to be a tendency toward a diminution of the quotient, while in rest 
following work a still further decrease may occur. 
Nature of Non-nitrogenous Material Metabolized.—As already 
pointed out, a comparative study of the final products of metab- 
olism during rest and work does not itself afford direct evidence 
as to the nature of the material actually metabolized in a muscu- 
lar contraction, but simply shows the total effect of the contraction 
itself and of the secondary activities resulting from it upon the 
make-up cf the schematic body. When we attempt to go further 
than this, other methods of investigation are requisite, although 
experiments like those already cited may afford important con- 
firmatory evidence. 
CoNcLUSIONS FROM RESPIRATORY QUOTIENT.—The significance 
of the respiratory quotient in experiments upon work has already 
been illustrated in Chapter III (p. 76). Neglecting any slight error 
due to small changes in the proteid metabolism, the variations in 
the respiratory quotient as outlined in the foregoing paragraphs 
enable us to trace the corresponding changes in the nature of the 
carbon metabolism. 
The metabolism of a fasting animal at rest is, as was shown in 
Chapter IV, largely a metabolism of fat. Corresponding to this, 
the respiratory quotient of such an animal approaches the value 
0.7 for pure fat, although never quite reaching it, since some pro- 
tein is always metabolized. Numerous instances of this fact are 
seen in the experiments already cited. When such an animal per- 
forms work, the respiratory quotient has been found to increase 
materially, thus showing that, in addition to the fat, carbohydrate 
material is being metabolized. This is entirely in accord with the 
well-established fact that muscular exertion causes the glycogen, 
both of the muscles and of the liver, to decrease and even disappear 
entirely. With an animal at rest and liberally supplied with car- 
bohydrate food, on the other hand, the respiratory quotient ap- 
proaches or even reaches unity, showing that the metabolism is 
essentially carbohydrate in character. When work is required of 
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