THE DIGESTIVE SYSTEM. 315 



Protein 



I 

 Soluble globulin 



Alkali metaprotein 



I 

 Primary proteoses 



Secondary proteoses 



I 

 Peptones 



^ 

 Polypeptides and Amino-acids. 



The earlier stages of tryptic digestion of protein are most efficiently 

 carried out in a slightly alkaline medium, but the ferment is active in 

 either an alkaline or neutral solution. Under natural conditions, the 

 -alkalinity of the pancreatic juice is largely neutralised by the acid con- 

 tents from the stomach, and the contents of the small intestine through- 

 out its length are almost neutral. The activity of trypsin diminishes 

 during the course of normal digestion, since the ferment enters into com- 

 bination with the products of its own activity, that is, with amino- 

 acids and peptones, and in this way it becomes inactive. The intestinal 

 contents taken from the lower end of the ileum show little trace of 

 tryptic activity. 



The Action of Pancreatic Juice on Starch. The action of pan- 

 creatic juice upon starch depends on the presence of an enzyme, amylase. 

 By means of this ferment starch is converted into maltose, as in the 

 case of salivary digestion (p. 293), but the action of pancreatic amylase 

 is more rapid and powerful than that of the ptyalin of saliva. If some 

 pancreatic extract is added to dilute starch paste kept at a temperature 

 of 37 C., the soluble starch stage is reached in a few seconds and 

 ^rythro-dextrin may be detected in half a minute. Moreover, the 

 pancreatic juice is capable of digesting unboiled starch, on which saliva 

 has no action. 



As maltose is formed it is further hydrolysed, partly by a ferment 

 maltase, contained in the pancreatic juice, and partly by a similar 

 ferment found in* the intestinal juice. Under the influence of maltase 

 ach molecule of maltose takes up a molecule of water and is split into 

 two molecules of dextrose. 



Pancreatic juice differs, however, from intestinal juice in having no 

 similar action on the other disaccharides, lactose and cane sugar. 



The Action of Pancreatic Juice on Fats. If perfectly neutral fat, 



