NET AVAILABLE ENERGY—MAINTENANCE. 397 
placement possible, quoting also Lawes & Gilbert’s conclusions * 
on the same point drawn from their experiments on fattening swine, 
and asserts that the nutrients replace each other according to their 
content of available energy. Danilewsky ¢ also advanced similar 
views, but Rubner { appears to have been the first to investigate 
the subject experimentally. 
Isopynamic VaLures.—We have already seen that the total 
metabolism of a fasting animal is approximately constant, repre- 
senting the rate at which the store of matter and energy in the body 
is drawn upon to support the necessary internal work. If we deter- 
mine the total metabolism of such an animal and then give it a 
known quantity of some nutrient, as fat, e.g., the loss of tissue will 
be diminished by a certain amount, which will represent the net 
available energy of the nutrient and which may be compared with 
the amount fed. Similarly, a second and third nutrient may be fed 
and thus their relative values for the prevention of loss of tissue be 
determined. For example, a dog after fasting for six days was 
given on the seventh and eighth days 720 and 760 grams respect- 
ively of fresh lean meat. The average nitrogen and fat metab- 
olism for the fifth and sixth days (fasting) and the seventh and 
eighth days was as follows:§ 
Total Nit Fat 
Food. “Exeretion | Metabolized | Temperature, 
Grms. Grms., Ba: 
Nothing (fifth and sixth days).... 3.16 75.92 18.0 
Meat (seventh and eighth days).. 20.63 30.72 19.2 
Difference: ,. pws se sieea Sea ge ere oe +17.47 —45.20 +1.2 
The result of the feeding with meat was, of course. a great in- 
crease in the proteid metabolism. The increase of 17.47 grams in 
the nitrogen excreted was equivalent to 113.38 grams of dry matter 
of the meat. The metabolism of this amount of proteid matter, 
therefore, enabled the organism to diminish the metabolism of fat 
* Phil Trans , 160, 541 
+ Die Kraftvorrite der Nahrungsstoffe; Arch. ges. Physiol., 1885, p. 230. 
t Zeit. f. Biol , 19, 313. 
§ The original account of the experiments is contained in Zeit. f. Biol., 
19, 313; these figures are the corrected values given in ibid., 22, 45. 
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