332 DIGESTION. 



what appears to be its chief or only office, that of a reser- 

 voir ; for its presence enables bile to be constantly secreted 

 for the purification of the blood, yet insures that it shall 

 all be employed in the service of digestion, although diges- 

 tion 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 communicates 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 into 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 intro- 

 duction of food into the small intestine : being pressed on 

 by the contraction of the coats of the gall-bladder, and pro- 

 bably of the common bile-duct also ; for both these organs 

 contain organic 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. 



Yarious estimates have been made of the quantity of bile 

 discharged into the intestines in twenty-four hours : the 

 quantity doubtless varying, like that of the gastric fluid, 

 in proportion to the amount of food taken. A fair average 

 of several computations would give thirty to forty ounces 

 as the quantity daily secreted by man. 



The purposes served ly the secretion of lile may be con- 

 sidered to be of two principal kinds, viz., excrementitious 

 and digestive. 



As an excrementitious substance, the bile serves espe- 

 cially as a .medium for the separation of excess of carbon 



