DIGESTION IN THE INTESTINE 163 



c.c. per day. The secretion is continuous in herbivora, whose 

 intestines normally are well filled. In man, however, the secre- 

 tion is intermittent, and it is poured out in large amount when 

 the acid chyme is passed into the duodenum from the stomach. 



Mechanism of Flow. The factors causing a flow of pancreatic 

 juice have been much studied. It became evident that the action 

 of the acid chyme on the walls of the duodenum causes a copious 

 flow. After severing the nervous connections of the pancreas, 

 the introduction of acid into the duodenum still causes a flow 

 of juice. Apparently it is not caused by reflex nervous stimula- 

 tion alone. It was thought that perhaps the stimulation is due 

 to a local reflex, but Bayliss and Starling showed that an acid 

 extract of the walls of the duodenum, if neutralized and injected 

 into the blood stream causes the pancreas to secrete. Apparently 

 then, the action of the acid chyme on the walls of the duodenum 

 causes a substance to be given off into the blood stream. This 

 substance, when it reaches the pancreas, causes it to secrete. 

 The substance has been given the name ' ' secretin. ' ' Substances 

 of this nature undoubtedly are formed by many tissues or 

 organs in the body and sent off by way of the blood as chemical 

 messengers to arouse activity in some other organ or tissue. To 

 these substances, as yet of unknown constitution, the name 

 .hormone (derived from a Greek word meaning "I arouse to activ- 

 ity") has been given. An example of such a substance already 

 has been cited in connection with gastric secretion. The final 

 proof of the existence of intestinal secretin was obtained by 

 making a cross circulation between two dogs so that blood from 

 one also circulated in the other. Acid was placed in the -duodenum 

 of one dog, and the pancreases of both began to secrete, showing 

 that the stimulating substance had been carried in the blood. 



It is very probable that secretin is not responsible for the 

 entire secretion of the pancreas, but that a reflex stimulation 

 from the walls of the duodenum also comes into play. 



Composition of Pancreatic Juice. Human pancreatic juice is 

 a water-clear liquid, alkaline in reaction since it contains sodium 

 carbonate. It requires from 10-15 c.c. N/10 HC1 to neutralize 



