FOOD AND DIGESTION. 395 



was discharged for several hours; but that, in about ten minutes after 

 the introduction of food into the stomach, the bile began to flow abun- 

 dantly, and continued to do so during the whole period of digestion. 



The bile is formed in the hepatic cells; thence, being discharged 

 into the minute hepatic ducts, it passes into the larger trunks, and from 

 the main hepatic duct may be carried at once into the duodenum. 

 This probably happens only while digestion is going on, i.e., for 5 to 7 

 hours after the introduction of food into the stomach; during fasting, 

 it regurgitates from the common bile-duct through the cystic duct, into 

 the gall-bladder, where it accumulates till, in the next period of diges- 

 tion, it is discharged into the intestine. The gall-bladder thus fulfils 

 its office, that of a reservoir; for its presence enables bile to be con- 

 stantly secreted, yet insures its employment in the service of digestion, 

 although digestion is periodic, and the secretion of bile constant. 



The mechanism by which the bile passes into the gall-bladder is 

 simple. The orifice through which the common bile-duct communi- 

 cates with the duodenum is narrower than the duct, and appears to be 

 closed, except when there is sufficient pressure behind to force the bile 

 through it. The pressure exercised upon the bile secreted during the 

 intervals of digestion appears insufficient to overcome the force with 

 which the orifice of the duct is closed; and the bile in the common 

 duct, finding no exit in the intestine, traverses the cystic duct, and so 

 passes into the gall-bladder, being probably aided in this retrograde 

 course by the peristaltic action of the ducts. The bile is discharged 

 from the gall-bladder and enters the duodenum on the introduction of 

 'food into the small intestine : being pressed on by the contraction of 

 the coats of the gall-bladder, and of the common bile-duct also ; for both 

 these organs contain unstriped muscular fibre-cells. Their contraction 

 is excited by the stimulus of the food in the duodenum acting so as to 

 produce a reflex movement, the force of which is sufficient to open the 

 orifice of the common bile-duct, which is closed by a sphincter. 



Bile is not pre-formed in the blood. As just observed, it is secreted 

 by the hepatic cells, although some of its constituents may be brought 

 to them almost in the condition for immediate secretion. The blood 

 from which the liver cells secrete the bile is that supplied to them by the 

 portal vein. This is shown by the alterations which occur in the pro- 

 cess on the alteration of the pressure in the portal system. If the portal 

 vein be obstructed, the amount of bile secreted diminishes, and is ulti- 

 mately suppressed, death resulting. It has, however, been shown that 

 under extraordinary circumstances bile may be secreted by the aid of 

 the blood from the hepatic artery, since if a branch of the portal vein 

 be tied, the part of the liver supplied by it continues to secrete bile, 

 though in diminished quantity. When the discharge of the bile into 

 the intestine is prevented by an obstruction of some kind, as by a gall- 



