508 THE LIVER [CH. XXXIII. 



the results of its other activities. This, however, it will be convenient 

 to take first. 



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 immediately on the arrival of food in the 

 duodenum ; there is a second increase in secretion a few hours later. 



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. Further, 

 the active peristalsis of the intestine and the pumping action of 

 the spleen are additional factors in driving more blood onwards to 

 the liver. 



The bile being secreted from the portal blood is secreted at much 

 lower pressure than one finds in glands such as the salivary glands, 

 the blood supply of which is arterial. Heidenhain found that the 

 pressure in the bile duct of the dog averages 15 mm. of mercury, 

 which is nearly double that in the portal vein. This fact is of con- 

 siderable importance, as it illustrates the general truth that secretion 

 is not mere process of passive filtration, but that the cells exercise 

 secretory force. 



The second increase in the flow of bile that which occurs some 

 hours after the arrival of the semi-digested food (chyme) in the 

 intestine appears to be due to the effect of the digestive products 

 carried by the blood to the liver, stimulating the hepatic cells to 

 activity: this is supported by the fact that proteid food increases 

 the quantity of bile secreted, whereas fatty food which is absorbed, 

 not by the portal vein, but by the lacteals, has no such effect. 



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. 431). 



An injection of haemoglobin into the portal vein or of substances 

 like water which liberate haemoglobin from the red blood-corpuscles 

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



