2 o6 TEXT-BOOK OF PHYSIOLOGY. 



in ether and boiling alcohol (Fig. 86). It is held in solution in bile by 

 the bile salts. If they are deficient in amount, the cholesterin may 

 pass out of solution, collect around some foreign matter, and form 

 a gall-stone./ Cholesterin is a product of the metabolism largely of 

 nerve-tissue, from which it is absorbed by 

 the blood, carried to the liver, and excreted'. 

 In the intestine it is converted into stercorin 

 and discharged from the body in the feces. 



Bilirubin, Biliverdin. These two pig- 

 ments impart to the bile its red and green 

 colors respectively. Bilirubin is present in 

 the bile of human beings and the carnivora, 

 . biliverdin in the bile of the herbivora. As 



and Stirling.} the former pigment readily undergoes oxi- 



dation in the gall-bladder, giving rise to the 



latter pigment, almost any specimen of bile may present any shade of 

 color between red and green. Bilirubin is regarded as a derivative 

 of hematin, one of the cleavage products of hemoglobin, the coloring- 

 matter of the blood. In the liver the hematin combines with water, 

 loses its iron, and is changed to bilirubin. By continuous oxidation 

 there are formed biliverdin, bilicyanin, and choletelin. After their 

 discharge into the intestine the bile pigments are finally reduced to 

 hydrobilirubin, which becomes one of the constituents of the feces. 

 An oxidation of the bilirubin can be, produced by nitroso-nitric acid. 

 If this agent is added to a thin layer of bile on a porcelain surface, 

 a series of colors will rapidly succeed one another, commencing with 

 green and passing to blue, orange, purple, and yellow. This is the 

 basis of the well-known test for bile pigments suggested by Gmelin. 



Lecithin is one of the products of the metabolism of nerve-tissue. 

 It is removed from the blood by the liver cells and thus becomes one 

 of the constituents of the bile, in which it is held in solution by the 

 bile salts. 



Mode of Secretion of Bile. The manner in which the bile 

 flows from the liver into the main hepatic ducts, the variations in the 

 rate of its discharge into the intestine, as well as the total quantity 

 secreted daily, have been approximately determined by fistulous 

 openings either in the hepatic ducts or in the gall-bladder. Although 

 the liver presents some physiologic peculiarities, there is no reason 

 to believe that the conditions of secretion therein are different from 

 those in any other secretory organ, or that any other structure than 

 the cell is engaged in this process. As shown by chemic analysis, 

 the bile consists of compounds, some of which, like the bile salts, 

 are formed in the liver cells out of material furnished by the blood, 

 by a true act of secretion, while others, such as cholesterin and lecithin, 

 principles of waste, are merely excreted from the blood to be finally 



