878 METABOLISM. 



Special constituents of the diet. Proteids. Proteids are chiefly 

 taken in the diet in the form of egg-albumin, vitellin, myosin, casein, 

 the proteids of cereals and of leguminous seeds (mainly globulins). The 

 nutritive value of the proteids from any one of these sources is pretty 

 nearly the same, with the exception that somewhat less of the proteid 

 of vegetable food is digested and assimilated than that of animal origin, 

 and the less, the larger the amount of cellulose which is contained in the 

 food. 



Peptones and albumoses have about the same caloric and nutritive 

 value as the proteids from which they have been formed. 1 Certain 

 proteids are assimilated and have the same nutritive value, if injected 

 into the blood vessels or under the skin, as when digested and absorbed 

 from the intestines. This is the case with serum-albumin and serum- 

 globulin, and also with acid or alkali albumin (even if prepared from 

 egg-albumin) and phytovitellin. 2 Other forms of proteid are not thus 

 directly assimilable, but on injection appear at once in the urine. Such 

 are egg-albumin, 3 casein, 4 peptone, and albumoses. Haemoglobin must 

 also be reckoned with these, although if injected as blood (with the 

 blood corpuscles intact), it remains intact. If injected dissolved in 

 water or in serum, it becomes partly broken up and converted into bile 

 pigment and partly appears in the urine as haemoglobin. 



Most, if not all, proteids contain sulphur, and the nucleo-proteids 

 contain phosphorus; an increase of sulphates and sometimes of phos- 

 phates in the urine may therefore be expected, if their metabolism is 

 increased. The metabolism of proteids will be subsequently dealt with. 

 Gelatin. Gelatin, although its elementary composition is very 

 nearly the same as that of proteids, and although it becomes, like 

 proteids, converted into peptones by digestion, and after being assimi- 

 lated is oxidised into urea C0 2 and H 2 0, is different from proteids in 

 its chemical constitution (see " Chemical Constituents of Body and 

 Food, pp. 31 and 70), and cannot wholly replace proteid as an article 

 of diet. This arises from the fact that the bioplasm of the tissues 

 is unable to be produced from it. In spite, therefore, of its con- 

 taining nitrogen and all the elements of the proteid molecule, it is a 

 non-proteid food, and takes its place as such along with the fats and 

 carbohydrates. Like them also it acts as a proteid-sparer, so that a 

 certain amount of proteid can be removed from the diet and replaced 

 by gelatin ; about twice as much of this must, however, be added, as 

 proteid is removed. 5 As a proteid-sparer, gelatin acts more efficiently 

 than carbohydrates, and still more than fats. This is shown by an 

 experiment by Yoit upon a dog weighing 32 kilos., which had been 

 maintained very nearly on nitrogenous equilibrium by a daily allowance 



1 Politzer, Arch. f. d. ges. PhysioL, Bonn, 1885, Bd. xxxvii. S. 301. 



2 Plosz and Gyergai, Arch. f. d. ges. PhysioL, Bonn, Bd. ix. S. 325, and Bd. x. S. 536 ; 

 Maly, ibid., Bd. ix. S. 605; Adamkiewicz, Virchow's Archiv, Bd. Ixxv. S. 144; Stokvis, 

 Gentralbl. f. d. med. Wissensch., Berlin, 1864, S. 596; Lehmann, Virchow's Archiv, 1864, 

 Bd. xxx. S. 593 ; Ponfick, Virchow's Archiv, 1875, Bd. Ixii. S. 273 ; Forster, Ztschr. 



f. BioL, Miinclien, 1875, Bd. xi. S. 517 ; Tizzoni, Arch. ital. de biol., Turin, 1884, Bd. 

 vi. S. 395; Neumeister, Sitzungsb. d. phys.-med. Gesellsch. zu Wiirzburg, 1889, S. 64; 

 Ztschr. f. Biol., Miinchen, 1891, Bd. xxvii. S. 309. 



3 Bernard, " Le9ons sur les propr. physiol. etc.," Paris, 1859, tome ii. p. 467. 



4 Runeberg, Deutsches Arch.f. klin. Med., Leipzig, 1879, Bd. xxiii. S. 68. 



5 According to Voit, one-fifth of the ordinary amount of proteid may be so replaced. I. 

 Munk, however, in the dog got at least two-thirds of the proteid of the food replaced 

 by gelatin with maintenance of equilibrium (Arch.f. d. ges. Physiol., Bonn, 1894, Bd. Iviii. 

 S. 309, and Bd. Ixi. S. 607). 



