CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 357 



filtrate must be further decolourized 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 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 26 H 43 N0 6 + H 2 = C 24 H 40 5 + CH 2 . NH 2 (CO . OH) 



taurocholic acid cholalic acid taurin 



C 26 H 45 NS0 7 + H 2 = C 24 H4o0 5 + C 2 H 4 . NH 2 . SO S 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 nitrogenous 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. The 

 decomposition of the bile acids into cholalic acid and taurin or 

 glycin respectively takes place naturally in the intestine, the glycin 

 and taurin being probably absorbed, so that from the two acids, 

 after they have served their purpose in digestion, the two ammonia 

 compounds are returned into the blood. Each 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 

 characteristic spectrum. A similar colour may however often be 

 produced by the action of the same bodies on albumin, amyl 

 alcohol, and some other organic bodies. 



208. 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 

 being, generally at least, alkaline, and often strongly so, tends to 

 neutralise the acid contents of the stomach as they pass into the 

 duodenum and as we shall see so prepares the way for the action 



