DIGESTION IN THE SMALL INTESTINE 392 



cane-sugar and strong sulphuric acid, they give a purple colour (Petten- 

 kofer's test). This is due to the reaction of cholalic acid with the 

 furfurol formed by the interaction of the cane-sugar and sulphuric 

 acid. An even more delicate test depends upon the fact that the bile 

 salts so lower the surface-tension of any fluid containing them that 

 flowers of sulphur will no longer float when sprinkled on the top, but 

 immediately sinks to the bottom. The test is a useful one for showing 

 the presence of bile in the urine. It is sometimes known as Hay's test. 



The bile salts endow the bile with its powers of solution of fatty 

 acids, and of aiding the digestion and absorption of protein. The 

 property of bile salts of precipitating protein from acid solution may 

 be used as a test for bile salts. If a solution of bile salts be added to 

 a 1 per cent, solution of Witte's peptone acidified with acetic acid, 

 a white precipitate or milkiness, due to the precipitated protein, is 

 produced. 



In the shark and allied fishes there exists a third group of bile 

 acids, rich in sulphur, and akin to ethereal sulphuric acid. These 

 bile acids yield sulphuric acid on boiling with hydrochloric acid. 



The Bile Pigments. There are several bile pigments. The chief 

 pigments of normal bile are bilirubin and biliverdin. By oxidation, 

 derivatives of these pigments are formed, some of which have been 

 isolated from gall-stones bilifuscin, biliprasin, bilicyanin, choleprasin, 

 and choletelin. Bilirubin occurs most abundantly in carnivora, 

 biliverdin in herbivora. The pigments, however, are readily inter- 

 changeable, bilirubin being found in gall-stones of cattle, and bili- 

 verdin in the placenta of the bitch. Their presence can be detected 

 by the addition of fuming nitric acid (that is, nitric acid containing 

 nitrous acid); there occurs a play of colours due to oxidation deri- 

 vatives green, blue, purple, yellow. 



Bilirubin may be isolated as a reddish powder or as rhombic 

 plates. It combines readily with calcium to form an insoluble salt, 

 and upon standing in contact with air becomes oxidized to biliverdin. 

 It is soluble in chloroform, and in solution exhibits no absorption bands 

 in the spectrum. 



Biliverdin is an amorphous substance, insoluble in chloroform, and 

 therefore easily separated from bilirubin. Bilirubin appears to be 

 derived from the disintegration of haemoglobin in the liver, the iron 

 portion of the blood-pigment being split off and retained in the liver. 

 Its empirical formula is C 32 H 36 N 4 6 , that of hsematin is C 2 H 32 N 4 4 Fe. 

 Bilirubin has the same empirical formula as hsematoidin, which occurs 

 in old blood-clots, and haematoporphyrin, which is sometimes found 

 in urine. Neither of the latter, however, give Gmelin's test. When 

 reduced by sodium amalgam, in alcoholic solution, both it and bili- 

 rubin are converted into hydrobilirubin. Hydrobilirubin can also be 

 formed from hsematin or haematoporphyrin by the action of more 

 powerful reducing agents. 



By still further oxidation, haematinic acid (C 8 H 8 5 ) is formed 

 from. bile pigments, haematin, and haematoporphyrin. These facts 

 indicate the close relationship between haematin and bile pigments. 



