4 oo CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
From this variable material products showing minute variations are 
produced, of which we only know that they are more soluble than the 
mother substance, less easily thrown out of solution by various precipit- 
ants, and to a certain slight extent are capable of diffusing through 
membranes. 
Here our knowledge at present stops. In spite of most laborious 
researches on the subject by a host of observers, we know no more of 
the structure of any save the final decomposition products of proteid 
digestion than we do of the proteids themselves. Certain products have 
been isolated at various intervals in the progress of digestion of proteids, 
which show that the process gives rise to several intermediate bodies, 
ever increasing in solubility towards precipitants as they are formed 
nearer the end of the process ; and it may be — it is a probable inference 
from analogy — that these substances are simpler than the proteids from 
which they originate, but as yet the simplest of them is too complex 
for our fragmentary knowledge to give any indication of its structure. 
Nor is there any knowledge of the relationship of these several stages of 
proteid digestion to one another. 
It is very probable that the process of proteid digestion, like all the 
other digestive processes, is one of continuous absorption of the elements of 
water or hydrolysis. 
This is shown by the following observations : — (1) One of the commonest 
agents employed in organic chemistry for the purpose of hydrolysing a 
substance is boiling with a dilute mineral acid, or subjecting in closed 
vessels to the action of superheated steam. On submitting proteids to the 
prolonged action of these reagents, products closely resembling or identical 
with those produced by the action of the proteolytic enzymes are obtained. 
(2) A small but decided increase in weight has been observed in the formation 
of peptone from proteid. 1 (3) Peptones can be converted artificially back 
into proteids by the use of reagents which are essentially dehydrating in 
their action. If fibrin- peptone be heated for an hour with acetic anhydride, 
the excess of anydride distilled off along with acetic acid formed in the pro- 
cess, and the residue treated with hot water, the greater part of it dissolves. 
When this solution is dialysed, there remains behind in the dialyser a solution 
which coagulates on boiling, and is precipitable by nitric acid or potassium 
ferrocyanide. Also, peptone heated for some time to 140° C. yields a substance 
which on solution in water shows more of the properties of a native albumin 
than of a peptone. - 
Other theories regarding the digestion of proteids are — (1) That the proteids 
are polymers of the peptones, and that the process of digestion is a process of 
depolymerisation, s (2) that proteids and peptones are simply different isomeric 
forms of the same substance, and (3) the micellar theory, 4 according to which 
the proteids are composed of micelli, which are a kind of second order of 
molecule much more complex in structure. On peptonisation, the proteid first 
breaks up into its constituent micelli, then the micelli fall into molecules, in 
the chemical sense of the word, and these molecules are the peptone molecules. 
1 A. Danilewski, CcntralbJ.f. d. mcd. Wisscnsch., Berlin, 1880, No. 42, S. 769. 
2 Henninger, Comjit. rend. Acad. d. sc, Paris, 1878, tome lxxxvi. p. 1464; Hofmeistcr, 
Ztschr. f. physiol. Chan., Strassburg, 1878, Bd. ii. S. 206; Keumeister, Ztschr. f. Biol., 
Mimchen, 1887, Bd. xxiii. S. 394. 
s Maly, Arch. f. d. ges. Physiol., Bonn, 1874, Bd. ix. S. 585: ibid., 1879, Bd. xx. 
S. 315 ; Hertb, Ztschr. f. physiol. Ohem., Strassburg, Bd. i. S. 277 ; Monatsh. f. Ghem., 
Wien, Bd. v. ; Poeld, Ber. d. deutsch. chcm. Gcscllsch., Berlin, 1881, S. 1355; 1883, S. 
1152 ; Loew, Arch.f. d. gcs. Physiol., Bonn, 1883, Bd. xxxi. S. 393. 
4 Griessmayer, Jahresb. ii. d. Fortschr. d. Thier-Chem., Wiesbaden, Bd. xiv. S. 26. 
