THE DIGESTIVE SECRETIONS 925 



insoluble form. The active agent concerned in this change is an enzyme, known as 

 rennin, rennet or chymosin. Like pepsin, this substance is produced by the chief 

 cells, being stored in them in its inactive form as prorennin or, prochymosin. 

 When passed into the ducts, it assumes its characteristic action of clotting milk, 

 but only in the presence of calcium and hydrochloric acid. Chemically, this process 

 consists in a conversion of the soluble caseinogen of milk into the insoluble casein. 

 Together with the other proteins, the latter is then subjected to the proteolytic 

 action of the pepsin. 



Rennin, however, is not the only milk-clotting enzyme. Similar bodies are 

 present in the pancreatic juice, the intestinal juice, the juices of certain fruits, such 

 as the cocoanut and the pineapple, and in many bacteria. In fact, since the 

 curdling of milk is a common phenomenon of proteolytic processes anywhere in 

 nature, some authors believe that it is not caused by a specific enzyme but by the 

 proteolytic ferment itself. Opposed to this view is the less probable one that a 

 special non-proteolytic coagulating substance is almost universally present in 

 nature. 



This controversy brings up the question pertaining to the possible identity of 

 rennin and pepsin, which must still be considered as not definitely settled. To be 

 sure, Hammarsten has shown that pepsin in its pure form possesses coagulating 

 properties, but certain facts are also at hand to prove that rennin and pepsin are 

 very unlike one another. 1 If it is assumed that the clotting of milk is a general 

 property of all proteolytic enzymes, the stomach must contain two such agents, 

 namely pepsin and rennin. The former is a product of the fundic glands and is 

 activated only in an acid medium. The latter, on the other hand, is chiefly a 

 product of the pyloric glands and acts in an acid as well as in a neutral medium. 

 In this regard, therefore, it is more like the trypsin of the pancreatic juice and the 

 erepsin of the intestinal secretion. As is usual with many biological properties, 

 these enzymes are unequally distributed in different animals and undergo changes 

 in their potency even in the same animal. Thus, we find that rennin is especially 

 abundant in the stomachs of suckling animals, and that its amount or potency gradu- 

 ally diminishes in later years and particularly in the carnivora. Its place is then 

 taken by the pepsin which acts in an acid medium at a time when the adult stomach 

 has acquired a much greater resistance than it possesses shortly after birth. 



In addition, gastric juice contains a fat-splitting ferment or lipase which pos- 

 sesses the property of splitting emulsified neutral fats into glycerin and fatty acids. 2 

 Such fats are present in milk. The non-emulsified fat, on the other hand, it allows 

 to traverse the stomach practically unchanged. This lipase exists in the mucosa 

 in the form of a mother-substance. The fact that it is of much greater importance 

 in the suckling than in the adult is in accordance with the character of the food 

 of the young, which consists largely of emulsified fat. 



The Resistance of the Stomach to the Gastric Ferments. Since 

 the proteolytic ferment pepsin acts in an acid medium, it may seem 

 strange that the gastric wall is normally exempt from its digestive 

 power, while a stomach which has been rendered abnormal by inter- 

 fering with its blood-supply, rapidly undergoes autolytic changes. 

 This is also true of the excised organ when immersed in its own juice. 

 A number of theories have been advanced in explanation of these facts, 

 all of which embody the belief that the lining cells of the stomach are 

 normally resistant against the gastric juice. Thus, it has been stated 

 that the surface of the stomach is always covered with a layer of 



1 Schmidt-Nielson, Zeitschr. fur physiol. Chemie, xlviii, 1906, 92; Fuld, Ergebn. 

 der Physiol., i, 1902; and Pawlow and Pavutschuk, Zeitschr. fur physik. Chemie, 

 xlii, 1904, 415. 



2 Volhard, Zeitschr. fur klin. Med., xlii, 1900, 414. 



