388 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



in tint which the bile undergoes after having been removed from the 

 body. When bile remains for a considerable time in the gall-bladder, 

 and when bile is exposed to the air, provided the reaction remains alka- 

 line, the reddish-brown coloring matter, bilirubin, absorbs oxygen from 

 the atmosphere and acquires a greenish color; it is then termed 

 biliverdin. In the bile of herbivora and most cold-blooded animals 

 this pigment exists naturally, and is present even before it passes into 

 the small intestine. Both these substances, bilirubin and biliverdin, 

 are insoluble in water, and their state of solution in the bile is to be 

 explained on the basis of their forming soluble combinations with alka- 

 lies, and partly to their being held in solution by the bile acids, in whose 

 solutions they are soluble. They are slightly soluble in ether and 

 alcohol, and readily soluble in chloroform and in alkalies. The test for 

 their detection is known as Gmelin's test, which is claimed to be suf- 

 ficiently sensitive to detect the presence of one part of bilirubin in eighty 

 thousand parts of solution. 



The test is performed by adding nitric acid which contains some free nitrous 

 acid to bile. This causes a precipitate which disappears on the addition of fresh 

 acid, and results in the formation of a series of colors, passing through green, 

 blue, violet, red, and then yellow, and is due to the different degrees of oxidi- 

 zation of the red coloring matter of the bile. Various modifications have been 

 proposed for this test. Briicke recommends the addition of dilute nitric acid to 

 the suspected fluid, and then pouring a quantity of concentrated sulphuric acid 

 carefully down the side of the tube ; as it sinks to the bottom it liberates free 

 nitric acid, which produces the characteristic play of colors ; or a concentric solu- 

 tion of nitrate of sodium may be added and then sulphuric acid. "When only 

 traces of bile and coloring matters are present, the addition of the tincture of 

 iodine causes the appearance of a green color. 



Bilirubin (C^FL^tOt)- — Bilirubin, which is also called hsematoidin, 

 occurs as an amorphous, orange-yellow powder, which, by its precipi- 

 tation out of chloroform (obtained by boiling bile with chloroform), 

 may be crystallized in red prisms. This bile-pigment is more frequently 

 obtained from biliary calculi, especially those of the ox, which are con- 

 stituted almost entirely of this pigment and cholesterin. 



The calculi should be powdered, exhausted with ether, and then with boil- 

 ing water containing a few drops of hydrochloric acid, which is added to separate 

 the bilirubin from the alkali with which it is supposed to be combined. The 

 residue is then to be washed in pure water, then dried, and then boiled with chlo- 

 roform and finally filtered. From the filtrate the chloroform may be distilled off, 

 the residue then extracted with alcohol, and ether and pure bilirubin will remain 

 behind. The amorphous, reddish powder which remains may be purified and 

 obtained in a crystallized form by re-solution in chloroform, which should be 

 allowed to evaporate spontaneously. In the preceding process the ether is em- 

 ployed to remove the fat and cholesterin, and water to remove the other soluble 

 biliary constituents. 



Bilirubin is only slightly soluble in water, readily soluble in chloro- 

 form and benzole, and sparingly soluble in alcohol and ether. It seems 

 to play the part of an acid, and unites with alkalies to form combinations 



