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: 
Day of Experiment. | Nitrogen a Metatolized. 
Third to éighth........... arse 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. 
3 gaa nie 
ay. eight, 
a Per Head, Live Woe, 
Grms. Grms. 
PG jana ca. tonne daa aek mele 2091 “36.1 17.26 
Seventhie inosine sacs alent we aa 2002 31.8 15.90 
Ninth....... We oy Sielia Ghee oo Spa's 1907 30.3 15.90 
Tenth... .ccsucwks sees redeesiny 1864 29.2 15.65 
"TP WOLEGHY «5g 350 inne. 8S wear 8 Ash mms oes 1764 30.2 17.18 
Thirteenth .... 1731 27.4 15.81 
Fourteenth 1716 27.4 15.95 
Pifteenthives ois civ. <ceseas cies ete 1697 25.5 15.90 
* Zeit. f, Biol., 17, 214; 19, 312. 
