84 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



vagus ; but as he did not exclude in his experiments the passage of 

 the acid chyme from the stomach into the duodenum, it is probable 

 that the pancreatic secretion he obtained was due to that circum- 

 stance and the consequent formation of secretin. 



Injection of secretin also stimulates the flow of bile. 



Secretin is an instance of the chemical messengers or hormones 

 (Starling) of the body. Evidence is accumulating to show that 

 hormones are extremely important. It has already, for instance, 

 been shown that one called gastrin is formed as the result of salivary 

 digestion, and stimulates the flow of gastric juice. Another is 

 formed from the foetal tissues, which, passing into the mother's 

 circulation, stimulates the mammae to enlarge and secrete milk. 



INTESTINAL DIGESTION 



The pancreatic juice does not act alone on the food in the intes- 

 tines. There are, in addition, the bile, the succus entericus (secreted 

 by the crypts of Lieberkiihn), and bacterial action to be considered. 



The succus entericus or intestinal juice has no action on fats or 

 native proteins, but it appears to have to some extent the power of 

 converting starch into sugar ; its best known action is due to a ferment 

 it contains called invertin, which inverts saccharoses that is, con- 

 verts cane sugar and maltose into glucose. The original use of the 

 term ' inversion ' has been explained on p. 17. It may be extended 

 to include the similar hydrolysis of other saccharoses, although there 

 may be no formation of levo-rotatory substances. There are probably 

 several inverting ferments in the succus entericus, one of which acts 

 on cane suger, one on maltose, and one on milk sugar. 



A few years ago, however, Pawlow showed that succus entericus 

 has a still more important action, which is to intensify the proteo- 

 lytic power of the pancreatic juice. Fresh pancreatic juice has 

 very little power on proteins, for what it contains is not trypsin, 

 but its precursor, trypsinogen. 



If fresh pancreatic and intestinal juices are mixed together, the 

 result is a very powerful proteolytic mixture, though neither juice by 

 itself has any proteolytic activity. The substance in the intestinal 

 juice that activates trypsinogen or, in other words, liberates trypsin 

 from trypsinogen has been called by Pawlow a ' ferment of the 

 ferments,' or enter o-kinase. 



Intestinal juice contains another ferment called erepsin (Otto 

 Cohnheim), which is capable of breaking up proteoses and peptone 

 into simple substances (leucine, tyrosine, hexone bases, ammonia, 

 &c,), and so assisting the action of trypsin. 



