CH. XI.] BILE PIGMENTS. 267 



The Bile Pigments. 



Bilirubin, C 32 H 36 N 4 O 6 , is a reddish-brown pigment 

 most abundant in the bile of carnivora. It is readily 

 oxidised by the oxygen of the air into biliverdin, C 32 H 36 N 4 O 8 , 

 the green pigment found mostly in the bile of herbivora. 

 These compounds are formed in the liver cells from the 

 products of disintegration of haemoglobin. Haematin is 

 CH tt N/) 4 Fe, and haematoporphyrin is isomeric with 

 bilirubin. 



They are weak acids, forming sodium and calcium 

 salts, the latter being insoluble in water. Free bilirubin 

 is soluble in ether and chloroform : the sodium compound 

 is insoluble, as is free or combined biliverdin. 



By oxidation bilirubin is converted, through a num- 

 ber of ill-defined bodies, such as bilicyanin, and bilifuscin, 

 into choletelin, the end product of Gmelin's reaction. 



By further oxidation a compound, haematinic acid 

 (C 8 H 8 O 5 ), is formed, identical with the product obtained by 

 the oxidation of haematin or haematoporphyrin. 



By reduction with sodium amalgam in alcoholic solu- 

 tion the bile pigments are converted into hydrobilirubin, 

 which is also formed by the action of more powerful reduc- 

 ing agents on haematin or haematoporphyrin. 



These facts all indicate the close relationship between 

 haematin and the bile pigments. 



In the bowel the bacteria first reduce bilirubin to 

 hydrobilirubin. This is then attacked, two nitrogen 

 atoms being probably removed, the result being the for- 

 mation of urobilin, which is mainly excreted in the faeces, 

 being sometimes called " stercobilin." A certain amount 

 however, is absorbed into the blood, and excreted by the 

 liver into the bile, whilst a small amount is excreted by 

 the kidney in the form of urobilinogen. (See p. 278.) 



318. Huppert-Cole test for bile pigments. 



Boil about 15 cc. of the fluid in a test-tube. Add two drops of a 

 saturated solution of magnesium sulphate, then add a 10 per cent. 



