112 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



needed. The secretion is more active, however, during in- 

 testinal digestion than at other times. This appears to be 

 reflex, but may be simply a result of the increased amount of 

 blood passing through the portal vein to the liver during that 

 period, for the whole alimentary canal is congested while di- 

 gestive activity is in progress. Again, it is known that the 

 best cholagogue is bile itself, and some of the bile is ab- 

 sorbed in its passage down the intestine. Its presence in the 

 blood may account for the accelerated flow. 



Method of Secretion and Discharge. The bile is a pro- 

 duct of the liver cells. How they receive their normal stimu- 

 lus is obscure. But it is reasonable to suppose that a larger 

 supply of blood means a more abundant secretion. Such an 

 increase of blood supply occurs during digestion. 



The cells discharge the bile into the bile capillaries, which 

 pass it onward either to the intestine directly, or, during 

 the intervals of digestion, to the gall bladder. When food 

 enters the duodenum, a reflex influence causes the wall of the 

 gall bladder to contract and compress its contents. The only 

 outlet is through the cystic duct into the common duct, thence 

 into the duodenum. This reflex does not take place until 

 food has entered the duodenum, and of different foods it is 

 found that proteids (peptones) and fats are the most effi- 

 cient stimuli. 



The secretion of bile is not stopped by ligation of either 

 the portal vein or the hepatic artery, showing that both of 

 these vessels contain bile materials. But it would be unrea- 

 sonable to suppose that the blood of the portal vein does not 

 furnish the bulk of secreting material. 



Glycogenic Function. The formation of glycogen is con- 

 nected with nutrition, but will receive some notice here. 

 This is an internal secretion. It is produced by the liver 

 cells, and can be demonstrated in their substance by the mi- 

 croscope and by chemical reagents. It can also be shown to 

 increase markedly after eating, and to decrease notably when 

 eating is refrained from for some time. 



