CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 447 



pigment. Insoluble in water and cold alcohol, though soluble 

 in hot alcohol and readily soluble in ether, chloroform &c., it is 

 dissolved by the bile-salts in aqueous solution and hence is present 

 in solution in bile. Its physiological functions are obscure. 



The ash of bile consists largely of soda, derived partly from the 

 sodium chloride and partly from the bile-salts, of sulphates derived 

 chiefly if not wholly from the latter, and of phosphates partly ready 

 formed, and in part derived from the lecithin. 



245. Pigments of Bile. The natural golden red colour of 

 normal human or carnivorous bile is due to the presence of Bili- 

 rubin. This, which is also the chief pigmentary constituent of gall- 

 stones, and occurs largely in the urine of jaundice, may be obtained 

 in the form either of an orange-coloured amorphous powder, or of 

 well-formed rhombic tablets and prisms. Insoluble in water, and 

 but little soluble in ether and alcohol, it is readily soluble in chloro- 

 form, and in alkaline fluids. Its composition is C 16 H 18 N 2 3 . 

 Treated with oxidizing agents, such as nitric acid yellow with 

 nitrous acid, it displays a succession of colours in the order of the 

 spectrum. The yellowish golden red becomes green, this a 

 greenish blue, then blue, next violet, afterwards a dirty red, and 

 finally a pale yellow. This characteristic reaction of bilirubin is the 

 basis of the so-called Gmelin's test for bile-pigments. Each of these 

 stages represents a distinct pigmentary substance. An alkaline 

 solution of bilirubin, exposed in a shallow vessel to the action of the 

 air, turns green, becoming converted into Biliverdin (C 16 H 18 N 2 O 4 ), 

 the green pigment of herbivorous bile. Biliverdin is also found 

 at times in the urine of jaundice, and is probably the body which 

 gives to bile which has been exposed to the action of gastric juice, 

 as in biliary vomits, its characteristic green hue. It is the first 

 stage of the oxidation of bilirubin in Gmelin's test. Treated with 

 oxidizing agents biliverdin runs through the same series of colours 

 as bilirubin, with the exception of the initial golden red. 



246. The Bile-salts. These consist, in man and many animals, 

 of sodium glycocholate and taurocholate, the proportion of the two 

 varying in different animals. In man both the total quantity of 

 bile-salts and the proportion of the one bile-salt to the other seem 

 to vary a good deal, but the glycocholate is said to be always the 

 more abundant. In ox-gall, sodium glycocholate is abundant, and 

 taurocholate scanty. The bile-salts of the dog, cat, bear, and other 

 carnivora, consist exclusively of the latter. 



Insoluble in ether but soluble in alcohol and in water, the 

 aqueous solutions having a decided alkaline reaction, both salts 

 may be obtained by crystallisation in fine acicular needles. They 

 are exceedingly deliquescent. The solutions of both acids have 

 a dextro-rotatory action on polarized light. 



Preparation. Bile, mixed with animal charcoal, is evaporated to 

 dryness and extracted with alcohol. If not colourless, the alcoholic 

 filtrate must be further decolorized with animal charcoal, and the 



