DIGESTION IN THE SMALL INTESTINE. 389 



which are insoluble in chloroform. Bilirubin is the most important 

 coloring matter, and from it originate the others. It is undoubtedly 

 formed in the liver-cells, though it is also formed in other localities. 

 Pathologically, it occurs in old blood-extravasations, where it was for- 

 merly described by Virchow under the name of hffimatoidin crystals ; 

 physiologically, it is found in the corpora lutea, in the ovaries, and in 

 the borders of the placenta of the dog. Bilirubin evidently originates 

 in the haemoglobin of the red blood-corpuscles. All causes which pro- 

 duce breaking down of the red blood-cells and consequent jaundice, 

 such as poisoning by ether and chloroform, lead to the appearance of 

 bilirubin in the urine. This decomposition may also be produced by the 

 action of the alkalies of the bile acids, and it is therefore probable that 

 the physiological origin of the bile coloring-matter is due to the action 

 of the bile acids on the blood-corpuscles in the liver. When oxidizing 

 agents, such as nitrous-nitric acid, are added to a solution of bilirubin it 

 displays a succession of colors identical with that seen in the applica- 

 tion of Grmelin's test. Each of these stages represent a distinct pig- 

 mentary substance. The first which results, or the greenish color, is due 

 to the appearance of biliverdin. 



Bilivei'din (C 32 H 36 X 4 8 ) occurs through the action of oxygen on 

 bilirubin, and is produced even when the solutions of the latter are 

 allowed to stand exposed to the air. This body is found in abundance 

 in the bile of cold-blooded animals, and is the principal pigment of the 

 bile of herbivora. Bilivei'din may be prepared by making an alkaline 

 solution of bilirubin and exposing it to the air in a shallow vessel ; after 

 awhile the reddish solution becomes intensely green, and biliverdin may 

 be deposited as a green, amorphous powder by precipitation with hydro- 

 chloric acid, washing with water, dissolving in alcohol, and finally pre- 

 cipitating with water. Biliverdin then forms a green, amorphous powder, 

 which is insoluble in water, ether, and chloroform ; is soluble in alcohol, 

 acetic acid, and solutions of the alkaline carbonates. When subjected to 

 the action of nitrous-nitric acid this pigment also liberates a series of 

 different colors, which pass through the same sequence as those developed 

 by the addition of this acid to solutions of bilirubin, the only difference 

 consisting in the absence of the original red color. The first change is 

 from a green into a blue or violet color, and is due to the formation of 

 choletlin, which finally becomes yellowish-brown. Each of the coloring 

 matters of the bile has a distinctive absorption of the spectrum, which 

 is yielded when the solution is treated with nitric acid. The bile of 

 carnivora is usually free from absorption bands, unless an acid be 

 added, in which case the absorption bands characteristic of bilirubin 

 appear in the spectrum. 



(d) Cholesterin (C^H^O (H 2 0)) is also an important constituent 



