180 MANUAL OF PHYSIOLOGY. 



raals, and is not soluble in chloroform, but readily so in alcohol. 

 Bilirubin is supposed to be identical with hsematoidin, a deeply 

 colored material found by Virchow in old extravasations of blood 

 within the body, and hence the bile pigment is said to be derived 

 from the coloring matter of the blood. Probably the haemoglobin 

 of some red corpuscles which have been broken up in the spleen 

 is converted into bile pigment by the liver. 



Under the influence of decomposition bilirubin undergoes a 

 change, taking up water and forming hydro- bilirubin ; this oc- 

 curs in the intestine, and the bilirubin is thus eliminated as the 

 coloring matter of the faeces (stercobilin), which is probably iden- 

 tical with the urobilin of the urine. 



IV. Fatty matters, the principal of which are lecithin (see p. 

 79), palmitin, stearin, olein, and their soda soaps. 



V. Cholesterin (C 26 H 44 O) is an alcohol, and crystallizes in clear 

 rhombic plates, insoluble in water, but held in solution by the 

 presence of the bile salts. It can be obtained from gall stones, 

 the pale variety of which are almost entirely composed of it. The 

 cholesterin leaves the intestine with the faeces. 



VI. The inorganic salts are sodium and potassium chloride, 

 calcium phosphate, some magnesia, and a considerable quantity 

 of iron. 



Tests for Bile. The most important constituents of the bile, 

 viz., the bile acids and pigment, may be detected by appropriate 

 tests, which are of great practical use : 



1. Pettenkofer's test for the bile acids: To a fluid contain- 

 ing either or both bile acids add some cane sugar, and 

 then slowly drop by drop strong sulphuric acid. The 

 solution turns to a cherry red and then changes to pur- 

 ple. As other substances, such as albuminous bodies, 

 give under this treatment a similar color, in order to 

 make the reaction a trustworthy test for bile salts, the 

 two characteristic bands given by the spectroscope 

 should also be observed. 



The following is said to be a more characteristic test : 

 Rinse out a porcelain capsule successively with the fluid 

 to be tested, then with weak sulphuric acid, and finally 



