CH. XXXV.] BILE 531 



Bile. 



Bile is the secretion of the liver which is poured into the duo- 

 denum : it has been collected in living animals by means of a biliary 

 fistula ; the same operation has occasionally been performed in human 

 beings. After death the gall-bladder yields a good supply of bile 

 which is more concentrated than that obtained from a fistula. 



Bile is being continuously poured into the intestine, but there 

 is an increased discharge soon after the arrival of food in the 

 duodenum. 



Though the chief blood supply of the liver is by a vein (the 

 portal vein), the amount of blood in the liver varies with its needs, 

 being increased during the periods of digestion. This is due to the 

 fact that in the area from which the portal vein collects blood 

 stomach, intestine, spleen, and pancreas the arterioles are all 

 dilated, and the capillaries are thus gorged with blood. The 

 peristalsis of the intestine and the pumping action of the spleen 

 are additional factors in driving the blood onwards to the liver. 



The bile is secreted from the portal blood at much lower pressure 

 than one finds in glands such as the salivary glands, the blood supply 

 of which is arterial. Herring and Simpson, in experiments performed 

 upon numerous animals, found that the bile pressure averages 

 30 mm. of mercury, which is about three times the pressure in 

 the portal vein. This fact illustrates the general truth that secretory 

 cells exercise pressure. 



Nothing is known of any nervous agency which regulates the 

 flow of bile ; the stimulus appears to be of a chemical nature, and 

 the increased flow which occurs soon after the arrival of the chyme 

 in the intestine is chiefly due to the action of secretin, for this 

 material stimulates the liver as well as the pancreas. 



The chemical process by which the constituents of the bile are 

 formed is obscure. We, however, know that the biliary pigment is 

 produced by the decomposition of haemoglobin. Bilirubin is, in fact, 

 identical with the iron-free derivative of haemoglobin called haema- 

 toidin, which is found in the form of crystals in old blood-clots such 

 as occur in the brain after cerebral haemorrhage (see p. 464). 



An injection of haemoglobin into the portal vein or of substances 

 such as water which liberate haemoglobin from the red blood-corpuscles 

 produces an increase of bile pigment. If the spleen takes any part 

 in the elaboration of bile pigment, it does not proceed so far as to 

 liberate haemoglobin from the corpuscles. No free haemoglobin is 

 discoverable in the blood plasma in the splenic vein. 



The amount of bile secreted is differently estimated by different 

 observers ; the amount secreted daily in man varies from 500 c.c. to 

 a litre (1000 c.c.). 



