892 METABOLISM. 
proteid food will now again produce an increase of tissue and of body- 
weight, until again a condition of N-equilibriuni is established. And 
this may apparently be carried up to the limit of the power of digestion 
of the animal for proteid food, so that ultimately fifteen times as much 
proteid may be metabolised as in the condition of inanition. 1 On the 
other hand, diminution of the amount of proteid food tends in the same 
way to gradually establish N-equilibrium on a lower level, and with a 
diminished body weight; the animal losing flesh until such equilibrium 
becomes established, and then maintaining itself, provided the N ingested 
be constant, at a constant but lower level of N-equilibrium. In short, 
" N-equilibrium is possible with the most different amounts of proteid 
in the food." 2 
The fact that the amount of urea excreted is directly dependent 
upon the amount of proteid ingested, is well illustrated by the 
following observations of Voit upon a dog fed on lean meat; the 
numbers are grins. : — 
Meat per diem 
. 300 
600 
900 
1200 
1500 
1800 
2000 
2500 
Urea per diem 
. 32 
49 
68 
88 
106 
128 
144 
173 
About 80-85 per cent, of the ingested proteid is usually oxidated 
and eliminated, and only about 15-20 per cent, is laid on. 
On the Building-up and Breaking-down of the Bodystuffs. 
The food of animals consists, besides water and a certain amount of 
inorganic salts, of organic constituents, nitrogenous (some of which must 
be proteid) and non-nitrogenous. The food of the higher plants, on 
the other hand, consists normally of inorganic materials, some of .which 
must be nitrogenous ; and, as has been long recognised, plants have the 
power of building up from these materials complex organic substances, 
such as proteids, carbohydrates, and fats, whereas animals have not this 
power ; the materials built up by plants serving as the food of animals. 
Hence arose the belief that it was an essential difference between the 
plant and animal organisation, that the one possessed extensive 
powers of effecting syntheses, whereas the other had practically no 
powers of synthesis, but must receive its materials already synthetised, 
either directly from plants or indirectly from plants through the bodies 
of other animals, such materials being subsequently broken down into 
simpler materials, which, after being oxidised within the tissues, are got 
rid of in such simple forms as urea, water, carbon dioxide, and salts. 
These views have undergone considerable modification of late years, 
since we are now familiar with numerous instances of syntheses occur- 
ring in animals. The first well-established case of the kind was 
determined by Wohler in 1824. Wohler found that when benzoic acid 
is taken with the food, it appears as hippuric acid in the urine. Now, 
hippuric acid is formed synthetically from benzoic acid and glycine. 
1 C. Voit, Hermann's "Handbuch," Bd. vi. S. 105. Voit's dog, weighing 35 kilos., 
was able to maintain N-equilibrium with as little as 500 and as much as 2500 grms. 
flesh, containing 548 grms. dry proteid. With larger amounts than this, digestion was 
interfered with. The same fact is still more strikingly shown by the experiments of 
Pfiiiger, who kept a large dog in a condition of nitrogenous equilibrium on an almost 
exclusively proteid diet. A man weighing 70 kilos, is, as a rule, unable to digest more 
than 1500 grms. of lean meat per diem. 
2 C. Voit, loc. cit., S. 111. 
