334 CHEMISTR Y OF THE DIGESTIVE PROCESSES, 



masticated, it is not necessary that it should be dissolved before leaving 

 the stomach. It does not follow 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 ori^ 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 Carica pepaya, 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 milk, 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 process the reaction remains 

 unchanged. 2. Solutions of caseinogen prepared from milk and free from 

 lactose coagulate in presence of calcium salts, on the addition of rennin. 

 3. Purified solutions of rennin have no action whatever on lactose. 6 



Eennin 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. 1895, Aufl. 3, S. 241. 



4 This also contains a proteolytic ferment, active in either alkaline, neutral, or acid 

 reaction (Baginsky, Ztschr. f. physiol. Chem., Strassburg, 1882, Bd. vii. S. 209 ; Arch. f. 

 Anat. u. Physiol. , Leipzig, 1883, S. 276). 



5 For the chemistry of milk, see p. 125. 



6 These proofs are due to : Heintz, Journ. f. praJct. Chem., Leipzig, 1872, N. F., Bd. vi. 

 S. 374 ; Hammarsten, Jahresb. il. d. Fortsclir. d. Thier-Chem., Wiesbaden, 1872, Bd. ii. 

 S. 118 ; 1874, Bd. iv. S. 135 ; 1887, Bd. vii. S. 158 ; "Zur Kenntniss des Caseins und der 

 Wirkungdes Labfermentes, " Upsala, 1877 ; Al. Schmidt, Jahresb. il. d. Fortsclir. d. Thier- 

 Chem., Wiesbaden, 1874, 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, Cenlralbl. f. d. med. Wissensch., Berlin, 1874, Bd. xii. S. 939 ; Hammarsten, 

 Beitr. 2. Anat. u. Physiol. als Festgabe C. Ludwig, Leipzig, 1874 ; Schumberg, Virchow's 

 Archiv, 1884, Bd. xcvii. S. 260 ; Boas, Centralbl.f. d. med. Wissensch., Berlin, 1887, Bd. 

 xxv. S. 417. 



