442 



DIGESTION 



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. 



Fig. 152. — Loop of intestine after tying off the portions, cutting the nerves running to the middle 

 portion, and returning the loop to the abdomen for some lime. (From Jackson.) 



Intestinal Juice 



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

 studied only after isolating portions of the intestine and connecting them 

 with fistula? of the abdominal walls. It appears here again that both a 

 nervous and a hormone mechanism exist. Mechanical stimulation of the 

 intestinal mucous membrane causes an immediate outflow of intestinal 

 juice, the purpose of Avhich under normal conditions is evidently to assist 

 in moving forward the bowel contents. This mechanically excited juice 

 • lues not contain any enterokinase and only small amounts of the other 

 enzymes. Further evidence for nervous control of the secretion of intes- 

 tinal juice has been obtained by isolating three pouches of intestine be- 



