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 nuclei >-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 CO., 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 Archie, Bd. lxxv. S. 144; Stokvis, 
C entralbl. f. d. mcd. JFissensch.. Berlin, 1864, S. 596 ; Lehmann, Virchow's Archiv, 1864, 
Bd. xxx. S. 593 ; Ponfick, Virchow's Archiv, 1875, Bd. lxii. S. 273 ; Forster, Ztschr. 
f. Biol., Miinchen, 1875, Bd. xi. S. 517; Tizzoni, Arch. ital. dc hiol., Turin, 1884, Bd. 
vi. S. 395; Neumeister, Sitzvngsb. d. phys.-med. GcscUsch. zv. Wiirzburg, 1889, S. 64 ; 
Ztschr. f. Biol, Miinchen, 1891, Bd. xxviu S. 309. 
3 Bernard, " Lecons sur les propr. physiol. etc.," Paris, 1859, tome ii. p. 467. 
4 Runeberg, DcutschcsArch.f. kliu. "Mcd., 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. lviii. 
S. 309, and Bd. lxi. S. 607). 
