348 DIGESTION 



that the milk-curdling property not only of the gastric juice, but also 

 of the pancreatic juice and of the secretion of Brunner's glands, is 

 associated with the proteolytic ferment. 



He asserts that when the comparison is instituted under proper 

 conditions there is an exact parallelism between the proteolytic and 

 the milk-curdling power of these secretions, no matter what the 

 circumstances may be in which they are collected, or the influences 

 to which they are exposed after collection. He has found it im- 

 possible to separate from any one of them a fraction which has 

 milk-curdling power without proteolytic power. On the other hand, 

 the majority of investigators maintain the separate identity of 

 rennin. Hammarsten especially states that he can destroy the 

 peptic activity without destroying the milk-curdling power of gastric 

 extracts, and vice versa. According to Burge, when a solution dis- 

 playing both peptic and milk-curdling power is electrolyzed, the 

 pepsin action is abolished at a certain stage, while the rennet action 

 is unaffected. It would seem, then, that the balance of evidence 

 is in favour of the separate identity of the rennet enzyme. 



However this may be, the curdling of milk by the gastric ferment 

 includes two processes: (i) An action on caseinogen in the course of 

 which it acquires new properties, becoming changed into casein. 

 This substance is not capable of being converted into casein, 

 and remains in solution in the whey. (2) The altered casein- 

 ogen or casein combines with soluble calcium salts and in the 

 presence of these is precipitated as the curd. The change which 

 occurs in the caseinogen has been the subject of much discussion, 

 which has not yet led to a definite conclusion. According to some 

 observers, the change consists in a decomposition of caseinogen, in 

 the course of which a new substance, whey-protein, not previously 

 present in the milk, is split off. This substance is not capable of 

 being precipitated by lime salts, and remains in solution in the whey. 

 According to others, the molecule of caseinogen is simply changed 

 into two molecules of casein (van Slyke). Dilute acid will of itself 

 precipitate caseinogen, and the presence of acid, and particularly 

 hydrochloric acid, in the gastric juice helps its milk-curdling action. 

 But that a ferment is really concerned is indicated by the fact that 

 the juice, after being made neutral or alkaline, still curdles milk, and 

 that this power is destroyed by boiling. The optimum temperature 

 is the same as that of the other ferments of the digestive tract, about 

 40 C. (p. 331). The persistence of the milk-curdling activity in the 

 presence of OH ions, while for peptic activity free H ions are neces- 

 sary, is a further and, indeed, a strong argument in favour of the 

 separate existence of the rennet ferment. 



As to the exact function of the milk-curdling ferment of the 

 gastric juice in digestion, we have no precise knowledge. It seems 

 superfluous if we suppose that the free acid is able of itself to do all 



