GASTRIC DIGESTION. 347 



no precipitate upon boiling, and but slight precipitation on neutraliza- 

 tion. Since no other substance will produce this result with fibrin, it is 

 characteristic of the presence of pepsin with a dilute acid. 



(b) Milk-Curdling Ferment. — As is well known, when milk is 

 brought into contact with the mucous membrane of the stomach, or 

 when an infusion of the mucous membrane of the stomach is added 

 to milk, it coagulates. This process is made use of in the manufacture 

 of cheese, and was formerly attributed to the acid of the gastric juice 

 or to the production of acidit}- in the milk from the development of 

 lactic acid from milk-sugar. It has, however, been shown that milk, 

 while completely neutral, may be coagulated by an infusion of gastric 

 juice, or by a neutral infusion of the mucous membrane; and since this 

 specific action of the gastric juice in curdling milk is destroyed by 

 boiling, it also is attributable to a specific -ferment, which is termed the 

 milk-curdling ferment, or rennet. 



This ferment produces coagulation of the casein of milk without 

 calling in in any way the action of the acid, and will produce its char- 

 acteristic results in solutions of casein which are entirely free from milk- 

 sugar and which are perfectly neutral. 



Solutions of the milk-curdling ferment may be obtained by digesting 

 the mucous membrane of the stomach with glycerin. A few drops of 

 this glycerin extract, which also, of course, contains pepsin, will cause 

 a hundred cubic centimeters of fresh milk to coagulate within a few 

 moments if heated up to 40° C. 



Several other methods have been proposed for the extraction of 

 milk-curdling ferment, but in all pepsin is nearly always present. 



Hammarsten has found that by precipitating with carbonate of magnesium or 

 acetate of lead solution, a solution of milk-curdling ferment might be obtained 

 which is perfectly free from pepsin ; for although both ferments are carried down 

 by this precipitate, all the pepsin remains in the precipitate, while a considerable 

 amount of the milk-curdling ferment passes through the filter. By this means 

 Hammarsten was enabled to obtain solutions which would coagulate fresh milk in 

 one to three minutes at the temperature of the body, even in neutral fluids, while 

 when acidulated they were entirely incapable of dissolving the smallest particles 

 of fibrin. 



Little is known as regards the chemical reactions of the milk-curd- 

 ling ferment. It does not coagulate, when in watery solutions, by boiling, 

 nor is it precipitated by alcohol, nitric acid, iodine, or tannin. It is 

 precipitated by the basic acetate of lead; it does not give a yellow- 

 color with hot nitric acid; it does not diffuse through parchment- 

 paper, and only with difficulty through unglazed earthenware. Milk- 

 curdling ferment is a less stable substance than pepsin and is destroyed 

 at a lower temperature than pepsin ; thus, if a solution which contains 

 both pepsin and milk-curdling ferment is heated about forty-eight hours 

 to 37° or 40° C. in a .02 per cent. HC1. solution, it loses all power of 



