CHAP, i.] DIGESTION. 249 



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 acid, 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 H NO + H 2 = C, 4 H W 5 + C 2 H 5 N0 2 



taurocholic acid cholalic acid taurin 



C 26 H 45 NS0 7 + H 2 = C 24 H <0 5 + C,H,NS0 3 . 



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

 but this acid is in the first case associated or conjugated with the 

 important nitrogenous body glycin, or amido-acetic acid, that is a 

 compound formed out of ammonia, and one of the series of fatty 

 acids viz. acetic; and in the second case with taurin, or amido- 

 isethionic acid, that is a compound formed out of ammonia, a 

 member of the ethyl group, and sulphuric acid. The decom- 

 position of the bile acids into cholalic acid and taurin or glycin 

 respectively takes place naturally in the intestine, the glycin and 

 taurin being absorbed, so that from the two acids, after they have 

 served their purpose in digestion, the two ammonia compounds 

 are returned into the blood. Either of the two acids, or cholalic 

 acid alone, when treated with sulphuric acid and cane-sugar, gives 

 a magnificent purple colour (Pettenkofer's test) with a character- 

 istic spectrum. A similar colour may often be produced by the 

 action of the same bodies on albumin, amyl alcohol, and some 

 other organic bodies. 



Action of Bile on Food. In some animals at least bile contains 

 a ferment capable of converting starch into sugar ; but its action 

 in this respect is wholly subordinate. 



On proteids bile has no direct digestive action whatever, but 

 since it is at least often alkaline it tends to neutralise the acid 

 contents of the stomach as they pass into the duodenum and so 

 prepares the way for the action of the pancreatic juice. To peptic 

 action it is distinctly antagonistic; the presence of a sufficient 

 quantity of bile renders gastric juice inert towards proteids. More- 



