280 THE TISSUES AND MECHANISMS OF DIGESTION. 



with oxidizing agents, such as nitric acid yellow with nitrous acid, it displays 

 a succession of colors in the order of the spectrum. The yellowish golden- 

 red becomes green, this a greenish-blue, then blue, next violet, afterward 

 a dirty red, and finally a pale yellow. This characteristic reaction of bili- 

 rubin 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, 6 H. 20 N. 2 O 5 or C 16 H, 8 N. 2 O 4 , 

 Maly), the green pigment of herbivorous bile. Biliverdin is also found at 

 times in the urine of jaundice, and is prpbably 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 bili- 

 rubin in Gmelin's test. Treated with oxidizing agents biliverdin runs 

 through the same series of colors as bilirubin, with the exception of the 

 initial golden-red. 



215. 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 so- 

 lutions having a decided alkaline reaction, both salts may be obtained by 

 crystallization in fine acicular needles. They are exceedingly deliquescent. 

 The solutions of both acids have a dextro-rotary action on polarized light. 



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

 extracted with alcohol. If not colorless, the alcoholic filtrate must be further 

 decolorized with animal charcoal, and the alcohol distilled off. The dry residue 

 is treated with absolute alcohol, and to the alcoholic filtrate anhydrous ether is 

 added as long' as any precipitate is formed. On standing the cloudy precipitate 

 becomes transformed into a crystalline mass at the bottom of the vessel. If the 

 alcohol be not absolute, the crystals are very apt to be changed into a thick syrupy 

 fluid. This mass of crystals has been often spoken of as bilin. Both salts are 

 thus precipitated, so that in such a bile as that of the ox or man bilin consists both 

 of sodium glycocholate and sodium taurocholate. The two may be separated by 

 precipitation from their aqueous solutions with sugar of lead, which throws down 

 the former much more readily than the latter. The acids may be separated from 

 their respective salts by dilute sulphuric action, or by the action of lead acetate and 

 sulphydric acid. 



On boiling with dilute acids (sulphuric, hydrochloric), or caustic potash 

 or baryta water, glycocholic acid is split up into cholalic (cholic) acid and 

 glycin. Taurocholic acid may similarly be split up into cholalic acid and 

 taurin. Thus : 



Glycocholic acid. Cholalic acid. Glycin. 



C M HN0 6 + H. 2 O = C. 24 H 40 5 + CH. 2 .NH 2 (CO.OH). 



Taurocholic acid. Cholalic acid. Taurin. 



C. 26 H 45 NS0 7 + H,0 - C. 24 H 40 5 + C,H 4 .NH 2 .S0 3 H. 



Both acids contain the same non-nitrogenous acid, cholalic acid ; but this 

 acid is in the first case associated or conjugated with the important nitro- 

 genous body glycin, or amido-acetic acid, which is a compound formed from 

 ammonia and one of the " fatty acid " series, viz., acetic ; and in the second 

 case with taurin, or amido-isethionic acid, that is a compound into which 

 representatives of ammonia, of the ethyl group, and of sulphuric acid enter. 



