DIGESTION. 685 



Bilirubin, C 16 H 18 N 2 O 3 , is a reddish-yellow pigment derived from 

 hsematin, which it resembles. It is sparingly soluble in water, 

 alcohol, and ether, readily soluble in hot chloroform and carbon 

 disulphide. 



Biliverdin, C 32 H 36 N 4 O 8 , is a green powder existing in green biles ; 

 it is formed from bilirubin by mild oxidation. 



Tests for biliary coloring-matters. A reaction known as Gmelin's 

 test may be applied in different ways : 



1. Place into a test-tube a few c.c. of a chloroform solution of 

 bilirubin, and pour down the side of the inclined tube an equal 

 volume of yellow nitric acid in such a manner that the liquids do 

 not mix. At the line of junction colored rings appear, being green 

 nearest the solution of the coloring-matter, and progressively blue, 

 violet, red, and yellow. (Plate VIII., 7.) 



2. Place on a white porcelain slab a few drops of the solution and 

 alongside of it a drop of yellow fuming nitric acid. On causing 

 the two liquids to come in contact a play of colors as above is seen 

 at the junction. 



3. Expose an alkaline solution of bilirubin to the air in an open 

 vessel ; it turns green, owing to the formation of biliverdin. The 

 latter answers also to Gmelin's test. 



Biliary acids. Glycocholic acid, C^H^NOg, and taurocholic acid, 

 C 26 H 45 NO 7 S, exist as sodium salts in the bile^bf man and most 

 animals. Both salts may be obtained as colorless crystals, which 

 dissolve in water, forming solutions with an acid reaction and an 

 intensely bitter taste. Both acids are easily decomposed by heating 

 with alkalies or with dilute acids, also by the action of putrefying 

 material or by chemical changes taking place in the intestines. In 

 all these cases are formed cholic acid, C 24 H 40 O 2 , and a second product, 

 which in the case of glycocholic acid is glycocoll, amino-acetic acid, 

 CH 2 .NH 2 .CO 2 H, and in the case of taurocholic acid, taurine, amino- 

 ethyl-sulphonic acid, NH 2 .C 2 H 4 .SO 3 H. 



These acids are formed in the liver, and very likely from some 

 protein material ; the mode of formation is, however, not known. As 

 in the case of the bile-pigments, the bile acids in part represent waste 

 material, while a part is reabsorbed by the intestine. The physio- 

 logical activity of bile acids is concerned mainly with the fats ; they 

 aid the saponification by lipase, and promote the absorption of fat 

 (probably by their solvent action). They are believed to hold the 

 cholesterin of the bile in solution. 



