DIGESTION. 193 



becomes quite frothy. If now sulphuric acid be carefully added, the red 

 and purple colors present themselves at once in the white froth an indica- 

 tion that the bile salts are distributed through it. 



Cholesterin. Cholesterin is a constant ingredient of bile, though it is 

 not confined to this fluid, as its presence has been determined in the crystal- 

 line lens, blood-corpuscles, nerve-tissue, and various pathologic fluids. It 

 is an organic non-nitrogenized compound resem- 

 bling the fats in some particulars, but differing 

 from them in not being capable of saponification 

 with alkalies. It presents itself in the form of 

 thin transparent rectangular crystals, insoluble 

 in water but soluble in ether and boiling alcohol 

 (Fig. 84). It is held in solution in bile by the bile 

 salts. If they are deficient in amount, the choles- FIG. 84. C HO LEST ERIN- 

 terin may pass out of solution, collect around some CRYSTALS. (Landois and 

 foreign matter, and form a gall-stone. Choles- 

 terin is largely a product of the metabolism of nerve-tissue, from which it 

 is absorbed by the blood, carried to the liver, and excreted. In the intes- 

 tine it is converted into stercorin and discharged from the body in the feces. 



Bilirubin, Biliverdin. These two pigments 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 the 

 former pigment readily undergoes oxidation 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 intes- 

 tine the bile pigments are finally reduced to hydrobilirubin or an allied sub- 

 stance, stercobilin, 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. Lecithin is regarded, because of its physical properties and 

 chemic composition, as a complex fat. When pure it presents itself gener- 

 ally as a white crystalline powder, though very frequently as a white waxy 

 mass which is soluble in ether and alcohol. Its chemic formula is C 44 H 90 - 

 NPO 9 . Lecithin is widely distributed throughout the body, being found in 

 blood, lymph, red and white corpuscles, nerve-tissue, yolk of egg, semen, 

 milk, and bile. It is readily decomposed, yielding with various reagents 

 glyco-phosphoric acid, a fat acid (stearic), and the alkaloid, cholin. Leci- 

 thin has been regarded as one of the decomposition products of nerve-tissue, 

 removed from the blood by the liver and thus becoming one of the constitu- 

 ents of the bile, in which it is held in solution by the bile salts. 



The Mode of Secretion and Discharge of Bile. The manner in which 

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



