334 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
masticated, it is not necessary that it sin mid be dissolved before leaving 
the stomach. It does not fellow that the foods which are more rapidly 
dissolved are also more rapidly peptonised, nor, indeed, that those which 
are more rapidly peptonised are also more thoroughly utilised by the 
organism. 1 
" Rennin. 2 — The presence of a milk-curdling principle in the stomach 
of the calf has been known for ages, but it is only within recent times 
that it has been shown that this action is due to the presence of a 
soluble ferment or enzyme. 
This enzyme is present in neutral aqueous infusions of the mucous 
membrane of the stomach of the calf and sheep, but in the case of other 
mammalia, of birds and of fishes, the zymogen is more stable, and the 
active enzyme itself is only set free on treating a neutral infusion with 
acid. 3 
The presence of rennin in the stomachs of birds and fishes is very 
remarkable, and points to some wider function of the enzyme, at present 
unknown to us, since it cannot be supposed that in such animals the 
ferment plays any part in connection with the clotting of milk. Many 
plant juices also contain enzymes which coagulate milk, such as the latter 
of the fig tree, 4 and of Garica p&paya, and the flowers of many cynaria. 
Milk 5 is also coagulated by bacterial action with the development of 
an acid reaction due to lactic acid (in the souring of milk). A curdy 
precipitate somewhat resembling a clot is caused by the addition of acids 
to m ilk , which led to the erroneous analogy being drawn, that the 
coagulation of milk by rennet was also an acid action, due to lactic acid 
set free from the lactose of the milk by ferment action. 
The following is a summary of the proofs that milk coagulation is not an 
acid action, but due to a specific enzyme (rennin), which acts on a proteid 
(caseinogen) of the milk : — 1. When a neutral solution of rennin (rennet) is 
added to alkaline milk, and the mixture is kept at 38°-40° C, complete 
coagulation occurs in 4-10 minutes, and in the x>rocess the reaction n 
unchanged '. 2. Solutions of caseinogen prepared from milk and free /mm 
lactose coagulate in presence of calcium salts, on the addition of rennin. 
3. Purified solutions of rennin have no action whatever on lactose. 6 
Rennin is always present under normal conditions in human gastric 
juice, both at birth and in the adult, 7 The distribution of the enzyme 
and its zymogen in the gastric mucous membrane is similar to that of 
1 See "Absorption of Proteids,'' p. 441. 
2 The name is due to Sheridan Lea ; that of chymosin has been proposed by Deschamp. 
3 Hammarsten, "Lehrbuch d. physiol. Chem.," "Wiesbaden, 1S95, Ann. 3, S. 241. 
4 This also contains a proteolytic ferment, active in either alkaline, neutral, or acid 
reaction (Baginsky, Ztschr. f. physiol. Chun., Strassburg, 1SS2, Bd. vii. S. 209 ; Arch./. 
A not. u.Physiol., Leipzig, 1883, S. 276). 
5 For the chemistry of milk, see p. 125. 
6 These proofs are due to : Heintz, Journ. f. prakt. Chan., Leipzig, 1S72. K. F., Bd. vi. 
S. 374 ; Hammarsten. Jahresb. ii. d. Fortschr. d. Thier-Ghem., Wiesbaden, 1S72, Bd. ii. 
S. 118 ; 1874, Bd. iv. S. 135 ; 1887, Bd. vii. S. 158 ; "Zur Kenntniss des Caseins und der 
Wirkungdes Labfermeutes, " Upsala, 1S77 ; Al. Schmidt, Jahresb. ii. d. Fortschr. d. Thier- 
Chem., Wiesbaden, 1S74, Bd. iv. S. 154. From the fact that rennet when impure acts on 
lactose, but not after purification, Hammarsten supposed that gastric juice contained a 
third enzyme, which acted on lactose, forming lactic acid, but this has not been sub- 
stantiated. 
7 Zweifel, Gentralbl.f. d. mod. Wissensch., Berlin, 1874, Bd. xii. S. 939 ; Hammarsten, 
Bt itr. :. Anat. u. Physiol, als Festtjabc C. Ludioig, Leipzig, 1S74 ; Schumberg, Virchow's 
Archiv, 1884, Bd. xcvii. S. 260; Boas, Centralbl.f. d. med. Wissensch., Berlin, 1887, Bd. 
xxv. S. 417. 
