CHAP. II.] THE MILK-CURDLING ENZYME. 147 



5. Action of Pepsin and Hydrochloric Acid on Keratin and 



Uh'itin. 



The keratin- and chitin-yielding structures are not attacked by 

 pepsin and hydrochloric acid. 



6. Action of Pepsin and Hydrochloric Acid on Oxy- 

 haemoglobin. 



Oxy-haemoglobin is rapidly decomposed by natural or artificial 

 gastric juice, with the formation, on the one hand, of albumoses 

 and peptone, and on the other of haematin which undergoes no 

 further decomposition 1 . 



SECT. 11. THE MILK-CURDLING ENZYME (OR RENNET-ENZYME) 



OF THE STOMACH. 



(Syn. Rennin, Lea, Foster. Ghymosin, Deschamps.) 



It has long been known that the mucous membrane of the 

 fourth or true stomach of the calf possesses the property of curdling 

 milk, and various preparations of this mucous membrane have, under 

 the name of ' rennet/ been employed to coagulate casein in the 

 manufacture of cheese. It has also long been known that the 

 gastric juice, when added to milk, curdles it; this action has been 

 ascribed, by some, to the pepsin, and, by others, with greater justice, 

 to the free acid which it contains. 



It was, however, shewn, first of all by Heintz 2 , that the mucous 

 membrane of the stomach possesses the property of curdling milk 

 when the reaction is neutral, and even alkaline. The recent re- 

 searches of Hammarsten 3 have demonstrated that the milk-curdling 

 property depends upon the presence of an enzyme of which the 

 zymogen is present in the gastric mucous membrane. 



1 Hoppe-Seyler, Physiolog. Chemie, p. 233. 



2 W. Heintz, ' Ueber die Ursache der Coagulation des Milchcaseins durch Lab und 

 liber die sogenannte amphotere Reaction.' Journ. f. prakt. Chemie, Neue Folge, 

 Vol. vi. p. 374. 



3 Hammarsten, ' Ueber die Milchgerinnung und die dabei wirkenden Fermente der 

 Magenschleimhaut. ' Maly's Jahresbericht, Vol. n. (1872), p. 118. 'Ueber den che- 

 mischen Verlauf bei der Gerinnung des Caseins mit Lab.' Maly's Jahresbericht, Vol. 

 iv. (1874), p. 135. ' Zur Kenntniss des Caseins und der Wirkung des Labferments,' 

 Upsala, 1877, and Maly's Jahresbericht, Vol. vn. (1877), p. 158. The articles in Maly's 

 Jahresbericht are by Hammarsten himself, and give a very full account of the original 

 Swedish papers. The third paper was published in extenso in German. 



102 



