that the four substances above named are only the main steps in the hydration process, and that there are still other links in the chain which cannot as yet be separated, and which show the same general chemical behaviour.

→ peptone

Globulin → globuloses
Egg-albumin → ovalbumoses → peptone

Myosin → myosinoses → peptone

The compound proteids are firstly split up into their constituent parts, one of which only undergoes peptic transformation.

The behaviour of casein in peptic digestion, especially with regard to the phosphorus, has been studied carefully by Salkowsky; though frequently stated that the phosphorus is not dissolved, but is separated as insoluble paranuclein, he finds that much of it is dissolved, only about 15 per cent. being contained in the paranuclein; the more favourable the conditions of digestion, the less paranuclein is obtained, and paranucleic acid is obtained from the paranuclein. The phosphorus in solution is in the form of a phosphorised albumose, in which it is not combined, however, as phosphoric acid; if dry hard lumps of casein are avoided, it is completely dissolved, and this is done by making a solution of casein before adding the digestive fluid, which acts best in the proportion of 500 to 1 of casein. Lindemann confirms this, and states that caseinogen is more digestible than casein. The first product of the digestion of casein is caseoproteose, which contains all the phosphorus of the original proteid; paranucleic acid, together with paranuclein, are formed in the digestion, and Salkowski has prepared the iron salt of this acid; thus:

The decomposition of the albuminoids has been carefully studied by Chittenden, with the following results :

No peptone is formed from elastin, and keratin is not attacked at all.

Umber has obtained from crystallised egg-albumin and serum-albumin, the primary proteoses, three deuteroproteoses, A, B, C, and peptone; and Zunz finds, as primary products of peptic proteolysis, acid albumin, proto- and heteroproteose, and deuteroproteose B; as secondary products three other deuteroproteoses, two peptones, and unknown substances which do not give the biuret reaction, ammonia and amino-substances.

Pick has investigated the proto- and hetero- albumose obtained from fibrin, and finds that they differ from one another in solubility and in the amount of nitrogen in a basic form which they contain, heteroalbumose containing 39 per cent. and protoalbumose 25 per cent.; heteroalbumose yields a large quantity of leucine and glycocoll,

whereas protoalbumose yields only a little leucine and no glycocoll. They arise together from fibrin, but not from one another, and they both yield, on further peptic digestion, deuteroalbumoses A and B and peptone B. This confirms the results of Zunz.

As regards the nature of the end-products of gastric digestion, a great deal of work has lately been done. Pfaundler obtained from serum-albumin leucine and a diamino-acid, probably histidine, but from fibrin there was no leucine; he finds that the principal end-products are apparently substances intermediate between peptone and amino-acids; they do not give the biuret reaction, and contain more than one carbon nucleus, in the case of serum-albumin at least there being a leucine and diaminonucleus present. Langstein has obtained from crystallised egg-albumin the following substances: leucine, tyrosine, glutamic acid, aspartic acid, cystine, lysine (in small amount), pentamethylenediamine, hydroxyphenylethylamine, and "a polymeric carbohydrate containing nitrogen;" also a base giving skatole and two unknown acids which give the biuret reaction; he did not find the intermediate substances of Pfaundler, perhaps because the digestion had gone too far. Malfatti has found that tryptophane is formed by the prolonged action of pepsinhydrochloric acid on Witte's peptone, and also small quantities of leucine, tyrosine, and hexone bases.

V. Harlay has found a very interesting substance among the products of pepsin digestion: it is a chromogen, as it gives, on treatment with the enzyme tyrosinase, a red colour; on decomposition with trypsin it yields tyrosine and tryptophane, a body first observed by Neumeister

among the products of trypsin digestion. According to Lawroff, pepsin breaks up proteids as much as trypsin does.

Sawjaloff has recently advanced the theory that peptonisation of proteid matter is not to make it more easy for absorption, as there is a reverse process of building up of proteids of larger molecular weight from the products of peptonisation, and this action is attributed to the rennet ferment; a substance termed plastein is thus synthesised, which appears to be the same, whatever the proteid may be which is digested. It resembles Kühne's anti-albumid in properties; Krüger's results, that the digestive products inhibit the action of pepsin, help to support this view.

CHAPTER XVIII.

CHANGES OCCURRING IN ALBUMINS AS THE RESULT OF THE ACTION OF TRYPSIN.

CLAUDE BERNARD, and in 1857 Corvisart, who refers to Purkinje and Pappenheim as having made experiments with pancreatic juice in 1836, were really the first to begin the scientific study of the digestion of albumins with pancreatic juice. The former considered bile to be necessary, but Corvisart showed that the juice by itself could dissolve albumins; and that they were converted into substances similar to those of gastric digestion. These results were at first denied, but later they were confirmed by Meissner, Danilewsky, and Kühne; this last observer showed that the action was due to an enzyme to which he gave the name of trypsin. Until Kolbe discovered that disinfectants could prevent the action of those micro-organisms which are always present in the intestine, the products of digestion by trypsin were very much confused and complicated; Kühne, in his experiments, made use of this fact, and was therefore the first to investigate the real products of trypsin digestion.

Kühne discovered that the peptone which is formed is only partially further broken up by trypsin, and he gave