514 THE GASTRIC JUICE [CH. XXXIII. 



action is mainly produced by a regurgitation of the contents of the 

 duodenum mixed with pancreatic juice ; but even after the pylorus 

 has been ligatured and regurgitation prevented, the gastric juice 

 itself produces a small amount of fat-splitting, and therefore con- 

 tains lipase. It is a remarkable fact that the administration of fat 

 in the food increases the regurgitation from the duodenum. 



4. It curdles milk. This is due to the action of the rennet 

 enzyme or rennin. The conditions of this action we have already 

 discussed under milk (see p. 482) ; but it may here be added that 

 Pawlow has advanced the view that rennin is not a distinct and 

 separate enzyme, but milk-curdling is only one of the activities of 

 pepsin. This hypothesis has been accepted by numerous physio- 

 logists ; but, on the other hand, there is a number of equally eminent 

 observers who still maintain that pepsin and rennin are two separate 

 enzymes. Whichever view is correct, the curd of casein formed 

 from the caseinogen is subsequently digested as other proteins are. 



5. It is proteolytic; this is the most important action of all. 

 The proteins of the food are converted by the pepsin-hydrochloric 

 acid into peptones. 



This action is a process of hydrolysis; and peptones may 

 be formed by other hydrolysing agencies, such as superheated 

 steam or heating with dilute mineral acids. The first stage in the 

 process of hydrolysis is that of acid meta-protein, formerly called 

 acid-albumin or syntonin; the next step is the formation of 

 propep tones or proteoses. The word " proteose " includes the albumoses 

 (from albumin), globuloses (from globulin), vitelloses (from vitellin), 

 etc. Similar substances are also formed from gelatin (gelatinoses) and 

 elastin (elastoses). Then peptone (probably a mixture of polypeptides) 

 is produced. The products of digestion of protein may be arranged 

 according to the order in which they are formed, as follows : 



1. Acid meta-protein. 



f (a) Proto-proteose . / Th , P rim *ry proteoses, i.e., 



2. Propeptone I (6) Hetero-proteose * ho . se whlch are formed 

 or proteoses 1 ((j) DeuterQ _ Qr secondary < 



proteose 



3. Peptone. 



It has been stated that the prolonged action of gastric juice leads 

 to the further splitting of the peptone into amino-acids. But accurate 

 work has shown that pepsin-hydrochloric acid does not split any of 

 the known polypeptides into their ultimate cleavage products. 



1. Acid meta-protein. The general properties of the meta- 

 proteins, the first degradation products in the cleavage of the 

 proteins which occurs during digestion are described on p. 433. We 

 shall find later that, in pancreatic digestion, an alkali meta-protein is 

 formed instead of the acid modification. 



