DIGESTION. 281 



There seems to be a close relationship between the color-matters of 

 the blood and of the bile, and it may be added, between these and that 

 of the urine (uroMlin), and of the faeces (stercoUliri) also; .it is probable 

 they are, all of them, varieties of the same pigment, or derived from the 

 same source. Indeed it is maintained that UroMlin is identical with 

 HydroMUrubin, a substance which is obtained from bilirubin by the 

 action of sodium amalgam, or by the action of sodium amalgam on alka_ 

 line haamatin; both urobilin and hydrobilirubin giving a characteristic 

 absorption band between b and F. They are also identical with sterco- 

 bilin, which is formed in the alimentary canal from bile pigments. 



The Test (Gmelin's) for the presence of 'bile-pigment consists of the 

 addition of a small quantity of nitric acid, yellow with nitrous acid; if 

 bile be present, a play of colors is produced, beginning with green and 

 passing through blue and violet to red, and lastly to yellow. The spec- 



Fio. 213. Crystalline scales of cholesterin. 



trum of Gmelin's test gives a black band extending from near b to 

 beyond F. 



(c) Fatty substances are found in variable proportions in the bile. 

 Besides the ordinary saponifiable fats, there is a small quantity of Cho- 

 lesterin, a so-called non-saponifiaUefat, which is really an alcohol, and, 

 with the free fats, is probably held in solution by the bile salts. It is a 

 body belonging to the class of monatomic alcohols (C 26 H 44 0), and crystal- 

 lizes in rhombic plates (Fig. 213). It is insoluble in water and cold 

 alcohol, but dissolves easily in boiling alcohol or ether. It gives a red 

 color with strong sulphuric acid, and with nitric acid and ammonia; 

 also a play of colors beginning with blood red and ending with green on 

 the addition of sulphuric acid and chloroform. Lecithin (C 44 H 90 NPO H ), 

 a phosphorus-containing body and Neurin (C 5 H 15 N0 2 ), are also found in 

 bile, the latter probably as a decomposition product of the former. 



(d) The Mucus in bile is derived from the mucous membrane and 

 glands of the gall-bladder, and of the hepatic ducts. It constitutes the 

 residue after bile is treated with alcohol. The epithelium with which it 

 is mixed may be detected in the bile with the microscope in the form of 



