892 METABOLISM. 



proteid food will now again produce an increase of tissue and of body 

 weight, until again a condition of ^-equilibrium 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 ^-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, 

 " ^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 grms. : 



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. Volt's dog, weighing 35 kilos., 

 was able to maintain N-equilibriuin with as little as 500 and as much as 2500 grins, 

 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 

 Pfliiger, 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. int., S. 111. 



