360 DIGESTION 



or cystin. In certain pathological conditions cystin appears in the 

 urine (cystinuria). The source of the cholic acid which goes to form 

 the bile acids is unknown,, but it has been surmised that it may be 

 derived from cholesterin. Thus, 



[CH 2 .SH + 3O CH 2 .SO 2 .OH CH 2 .SO 2 .OH 

 CH.NH 2 = CH.NH 2 = CH.NH 2 

 COOH COOH CO 2 



Cystein. Cysteinic acid. Taurin. 



Traces of cholic acid, formed by hydrolysis from the bile-acids by 

 the action of putrefactive bacteria, are found in the intestines, especially 

 in the lower portion. 



Pettenkofer s test for bile-acids (Practical Exercises, p. 456), acciden- 

 tally discovered in examining the action of bile upon sugar, depends upon 

 three facts: (i) That cholic acid and furfuraldehyde give a purple colour 

 when brought together; (2) that the bile-salts yield cholic acid when 

 acted upon by sulphuric acid ; (3) that when cane-sugar is decomposed 

 by strong sulphuric acid, furfuraldehyde is formed. 



Since a similar colour is given when the same reagents are added to a 

 solution containing albumin, it is necessary to remove this, if present, 

 from any liquid which is to be tested for bile -acids. 



Lecithin and cholesterin, or cholesterol, are by no means peculiar to 

 bile (p. 4). They are very widely distributed in the body. Lecithin 

 (C 4 4H9oNPO 9 ) belongs to the group of phosphatides, fat-like phosphorus- 

 containing substances present in all cells. It is a compound of glycerin 

 with two molecules of fatty acid and one of phosphoric acid. The 

 phosphoric acid is at the same time united with a base cholin (C 5 H 15 NO 2 ) . 

 The fatty acid (stearic, palmitic, oleic, etc.) varies in different varieties 

 of lecithin . Heated with baryta-water, lecithin yields the corresponding 

 fatty acid in the form of a soap, along with cholin and glyceryl-phos- 

 phoric acid. Glyceryl-phosphoric acid can be further split so as to 

 yield a molecule of glycerin and one of phosphoric acid. 



Cholesterin is a substance with the empirical formula C 27 H46O. It 

 contains an alcohol group in virtue of which fatty acids can be linked 

 to it, forming esters. It is best obtained from white gall-stones, of 

 which it is the chief, and sometimes almost the sole constituent (see 

 Practical Exercises, p. 457). 



All the compounds related to cholesterin are grouped together under 

 the name of sterins. The sterins are very widely distributed both in 

 animals (zoosterins) and in plants (phytosterins). Every cell seems to 

 contain sterins and sterin esters (compounds of the same nature as the 

 compounds of fatty acids with the alcohol glycerin which constitute the 

 neutral fats) . In the vertebrates cholesterin and its products constitute 

 the chief, perhaps the only sterins, but in invertebrate animals and 

 plants there is a much greater variety of these substances. 



The chief inorganic salts of bile are sodium chloride, sodium carbonate, 

 and alkaline sodium phosphate. The phosphoric acid of the ash comes 

 partly from the phosphorus of organic compounds (lecithin and bile- 

 mucin), the sulphuric acid from the sulphur of taurocholic acid, the 

 sodium largely from the bile-salts. Iron is a notable inorganic con- 

 stituent of bile, although it exists only in traces, in the form of phosphate 

 of iron. Manganese is also present in minute amount. 100 c.c. of 

 fresh bile yields 50 to 100 c.c. of carbon dioxide, part of which is in 

 solution and part combined with alkalies. 



