476 DIGESTION 



It will be seen that the most abundant secretion occurs with meat, that 

 of milk being not only smaller but also slower in starting. The digestive 

 power is greatest in the case of bread. 



THE INTESTINAL SECRETIONS 



Pancreatic Juice 



Regarding the natural secretion of pancreatic juice, little need be added 

 to what has already been said (see page 460) . The secretion begins when the 

 chyme enters the duodenum, and attains its maximum when the outflow 

 of this is greatest. By collecting the juice from a permanent fistula of the 

 pancreatic duct, it has been found that the amount varies with different 

 foods. When quantities of food containing equivalent amounts of nitro- 

 gen are fed, the greatest secretion is said to occur with bread and the least 

 with milk. Such differences are probably dependent upon the amount of 

 acid secreted in the stomach and passed on into the duodenum. It was 

 thought at one time that, besides variation in quantity, the nature of the 

 enzymes in the pancreatic juice might vary according to the kind of 

 food. This, however, has been shown not to be the case. 



Bile 



The secretion of bile runs practically parallel with that of pancreatic 

 juice. The liver is producing bile more or less continuously, since besides 

 being a digestive fluid it is also an excretory product. The bile produced 

 between the periods of digestion is mainly stored in the gall bladder. 

 When the acid chyme comes in contact with the duodenal mucous mem- 

 brane, it excites afferent nerve endings that cause a reflex contraction of 

 the gall bladder, and this expresses some of the bile into the duodenum. 

 The secretin, which the acid at the same time produces, besides affecting 

 the pancreas, acts on the liver cells, stimulating them to the increased 

 secretion of bile. Thus, by a nervous reflex operating on the gall bladder 

 and later by a hormone mechanism operating on the liver cell, the increased 

 secretion of bile is insured throughout digestion. Of the bile discharged 

 into the intestine, a certain proportion of the bile salts is reabsorbed into 

 the portal blood. When these arrive at the liver they also excite secre- 

 tion of bile, thus assisting secretin in maintaining the secretion through- 

 out the process of intestinal digestion. 



Intestinal Juice 



The secretion of intestinal juice, or succus entericus, can obviously be 

 studied only after isolating portions of the intestine and connecting them 



