64 Chapter III 



specific sense of taste an excellent means of discrimination in the 

 choice of foods, so the mucous membrane of the small intestine is 

 endowed with a special sensitiveness, comparable to the chemiotaxis 

 of unicellular organisms and of the cells of more highly developed 

 organisms. Hirsch and Mehring have satisfied themselves that the 

 passage of the contents of the stomach through the pyloric orifice 

 depends on a reflex mechanism which proceeds from the upper 

 reaches of the small intestine. To the researches of the school of 

 Pawlofi', however, we owe what light has been thrown on this question. 

 The duodenal mucous membrane is endowed with a well-developed 

 chemiotaxis for acid substances. The passage of the acid content 

 of the stomach into the duodenum determines this chemiotaxis and 

 brings about a secretion of alkaline juice which neutralises the acid. 

 This contest between acid and alkali forcibly calls to our mind the 

 analogous phenomena in those plants that defend themselves against 

 the alkaline secretions of parasites by the production of an acid (see 

 Chapter II). As in these lower organisms, this battle of the chemical 

 secretions is regulated by the action of living and sensitive parts. 



When the acidity of the mass which passes through the pylorus is 

 too marked, the reflex contraction starting from the duodenal mucosa 

 arrests its passage. Then takes place a neutralisation of the acid, 

 thanks to the alkaline secretion, and the pylorus is again allowed to 

 open. This mechanism thus regulates the passage of the contents 

 of the stomach into the duodenum, the passage taking place in 

 instalments. 

 [69] The sensitive intestinal mucous membrane can estimate not only 

 the degree of acidity, but also the other chemical characters of the 

 aliments which pass into the duodenum. This chemiotaxis is, as it 

 were, the starting point of the reflex action which excites the pan- 

 creatic secretion with its contained three ferments. The passage of 

 bread through the pylorus excites the secretion of a juice very rich in 

 amylase and very poor in saponase. The passage of milk into the 

 duodenum brings forth, on the other hand, a juice very much richer 

 in saponase but poorer in amylase and in trypsin. Flesh-meat 

 provokes the secretion of a pancreatic juice which is less rich in 

 amylase than the juice poured on bread, but richer in saponase. Fat 

 causes the secretion of a juice still richer in saponase than is the juice 

 poured out in the presence of bread or milk. These facts now 

 carefully established— especially by Walter^— demonstrate that the 

 ^ Arch. d. sc. biol., St.-Petcrsb., 1899, t vii, p. 1. 



