DIGESTION 433 



meals instead ol liiiiiled but frequent ones. When solid masses of 

 food are presented by peristaltic action, the pyloric sphincter does 

 not open. Nor does the valve operate normally when insufficient 

 hydrochloric acid has been secreted to lower the pH. These properties 

 insure a suitable consistency and a reasonable extent of gastric diges- 

 tion. 



Clhyme enters the duodenum, a portion of the small intestine about 

 11 in. long in man, and comes progressively under the influence of 

 three digestive juices. Pancreatic juice and bile are secreted from 

 separate ducts but into the same region of the intestine, so that their 

 activities are interdependent. Moreover, intestinal juice also acts 

 conjointly though this fluid is secreted by several types of small glands 

 located throughout the intestinal mucosa. For convenience the com- 

 positions and functions of the three digestive juices are considered 

 separately even though they intermingle and act together. 



Pancreatic Juice 



The pancreas is a tubular gland lying along the duodenum and 

 secreting intermittently into the intestine. Its activity is stimulated by 

 contact of the acid chyme with the walls of the duodenum, which in 

 turn produces hormonal material initiating pancreatic secretion. The 

 basis for this statement lies in the fact that maceration of duodenal 

 tissue with hydrochloric acid produces an extract which stimulates 

 the flow of pancreatic juice when injected into the blood stream of 

 another animal. If intestinal extract that has not been treated with 

 HCl is injected, no pancreatic stimulation occurs. Furthermore, acid 

 extracts of other organs have no effect. 



Proof of the transfer of secretin by the blood was shown by a most 

 ingenious experiment. The blood streams of two dogs were connected 

 in such a manner that the blood supplies intermingled extensively. 

 Tubes were placed in the pancreatic ducts of both animals, and a 

 dilute acid solution was introduced into the duodenum of one of the 

 dogs. The acid caused pancreatic secretion simultaneously in both 

 dogs, proving that secretion of secretin took place in one dog and was 

 transferred by the blood to the other animal. 



More recently, participation of at least two hormones has been 

 demonstrated. Of these secretin has been purified and shown to be 

 a complex polypeptide. It specifically stimulates the production of 

 fluid containing sodium bicarbonate. A second hormone, pancre- 

 ozymin, induces enzyme production, and the two hormones, perhaps 

 with additional unknown stimuli, result in the secretion of normal 

 pancreatic juice. The two known hormones come solely from the 



