252 DIGESTION. [CHAP. xxv. 



ductus communis choledochus, between two and three inches in 

 length, which passes behind the two upper thirds of the duodenum, 

 and opens, along with the pancreatic duct, into that intestine, in 

 or close to the angle of junction of its middle and inferior third. 



The gall-bladder is a pyriform bag, placed in a depression on the 

 inferior surface of the right lobe of the liver, and serving as a 

 reservoir for the accumulation of the bile when its flow into the in- 

 testine is interrupted or retarded. We infer, at least, that this is 

 its principal use, because it is always found full after a long fast, 

 and empty when digestion is going on. Blondlot tied the common 

 bile-duct of a dog, and established a fistulous communication be- 

 tween the gall-bladder and the external integument ; thus the bile, 

 which ought to descend into the intestine, would flow out at this 

 opening. He states that while the animal was fasting, sometimes 

 not a drop of bile would escape at the orifice, even for some 

 hours ; but in about ten minutes after the introduction of food into 

 the stomach, the bile would begin to flow freely, and continue to 

 do so as long as digestion was going on.* The process of digestion 

 in the duodenum appears to favour the flow of the bile into 

 the intestine, either by the stimulus of the food in contact with the 

 mucous membrane, acting by reflexion upon the muscular coat of 

 the gall-bladder, and causing it to contract and expel its contents; 

 or by altering its position, so as to favour the descent of the bile ; 

 or by changing the condition of the orifice of the duct, which in the 

 empty state of the bowel would be closed by the contraction of the 

 intestinal circular fibres. Indeed, it is probable that all these causes 

 would be brought into operation by the duodenum becoming filled 

 with food, and the digestive process being set up in it. 



That the gall-bladder is not an essential part of the excretory 

 apparatus of the liver, is shown by the fact that it is not universally 

 present even in the highest classes of animals. This reservoir is 

 found in the animal series first in fishes ; but it is absent from 

 many genera of that class. It exists pretty constantly in reptiles 

 and lairds, being occasionally wanting in the latter; and in mam- 

 malia it is absent from many of the genera. We do not know 

 the precise law which regulates its presence or absence, but it 

 seems that the length of time for which animals are accustomed to 

 fast, has probably a good deal of influence. Thus, in the herbivora, 

 which eat often, and at short intervals, and whose digestion is 

 slow, the gall-bladder is frequently absent ; and in the carnivora, 

 which eat at long intervals, it is almost constantly present. But 

 * Essai sur les fonctions du Foie, p. 62. 



