INTERNAL WORK. 345 
performance of the internal work and how it varies under varying 
conditions. Carnivorous animals, with their short and relatively 
simply digestive canal, lend themselves most readily to experiments 
of this sort although rabbits and guinea-pigs have been employed 
to some extent, as well as, for short periods, men. 
Constancy Under Uniform Conditions. — Attention has al- 
ready been called in Chapter IV to the relative constancy of the 
total metabolism of the fasting animal, particularly as compared 
with the total mass of active tissue in the body. This constancy 
has been especially emphasized by Rubner,* and forms the basis 
of his determinations of the replacement values of the several 
nutrients which will be considered in the following chapter. 
With a rabbit the following daily averages, computed per 100 
parts of nitrogen in the body, were obtained: 
" Nitrogen in Fat | 
Day of Experiment. Urine. Metabolized. 
Third to eighth................ 2.16 16.2 
Ninth to fifteenth.............. 2.19 13.8 
Since the ratio of proteids to fat metabolized did not vary 
greatly in these trials, the total amount of carbon dioxide ex- 
creted may be taken as an approximately accurate measure of 
the total metabolism. For the several days of the experiment, 
this was as follows: 
Carbon Dioxide Excreted. 
eae tive 
Day. eight, 
Grme- Per Head, Live’ Wott, 
Tms. Grms. 
Fitthy, 24 vtawsetyee aise < orice. 2091 36.1 17.26 
Seventh veccuesagciadgeesmd sas 2002 31.8 15.90 
NMG Does. jas, oh owsen ek Os 89 Hee 1907 30.3 15.90 
en Chis ssc wn eee Gan ca aware 1864 29.2 15.65 
AWLP ixisis-i acore te snd a4 Been 8 aS 1764 30.2 17.18 
Thirteenth .........-...-.006- 1731 27.4 15.81 
Fourteenth ... 1716 27.4 15.95 
Fifteenth... 00... 0025 Sa sea 8a 1697 25.5 15.90 
+ Zeit. £, Biol., 17, 214; 19, 312. - 
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