116 PHYSIOLOGY. 



tion of each present, the color may range from reddish-brown to 

 grass-green. They are formed from the haemoglobin of the blood 

 the mother of all the bile-pigments. In man and carnivora bilirubin 

 predominates and gives to the bile its yellow color; the green color 

 of that of herbivora is due to biliverdin. 



Bile contains neither bilirubin nor biliverdin free, but combi- 

 nations of these two substances as salts : bilirubinates and biliverdin- 

 ates of the alkalies. The bilirubinates are transformed into biliver- 

 dinates by the oxygen of the air. These bodies, bilirubin and bili- 

 verdin, act as acids. 



Bilirubin, isomeric with haematoporphyrin, represents the iron- 

 free pigment of the bile; its formula is C 16 H 1S N 2 3 . This is the 

 permanent pigment of the bile and may also appear as a calcium com- 

 pound in red gall-stones. When exposed to the air and in an alkaline 

 solution, it oxidizes very readily, changing into biliverdin; because 

 of this, bile, when standing, assumes a greenish tint. 



Biliverdin is present in all biles of a greenish color. It occurs 

 as such in the liver-secretion of herbivora, but may be obtained by 

 allowing human and carnivorous bile to oxidize slowly by exposure to 

 the air. Its formula is C 16 H 18 N 2 4 , having one more atom of oxy- 

 gen than bilirubin. 



When bilirubin arrives in the intestine the bacteria generate 

 nascent hydrogen, which reduces it and generates another pigment, 

 the coloring matter of the faeces, called stercobilin. This stercobilin 

 when absorbed and excreted in the urine is called urobilin. 



Gmelin's Test for Bile-pigments, Add to some bile some nitric 

 acid containing nitrous acid, when there will be a play of colors : 

 green, blue, purple, and yellow. These tints are due to the oxidation 

 of bile-pigments. The green is biliverdin; the blue, bilicyanin; the 

 purple, bilipurpurin ; and the yellow, choletelin. 



Cholesterin. 



Cholesterin is a monovalent alcohol. It is present to some ex- 

 tent in all protoplasmic structures, blood-corpuscles, but particu- 

 larly in bile and nervous tissues. In the latter it forms a very im- 

 portant part of myelin. In the bile it forms but a small proportion 

 of its contents from 1 to 5 per cent. It is insoluble in water and 

 dilute saline solutions, but readily soluble in ether, chloroform, alco- 

 hol, etc.; in this respect it resembles fat, though not a true fat. In 

 bile it is readily dissolved, because of the presence of bile-salts. If, 

 for any reason, the latter should be insufficient, the cholesterin passes 



