PHYSIOLOGICAL CHEMISTRY 401 



water. Hold the test tube as nearly horizontal as possible, and allow 

 some fuming nitric acid to run down it, so that this forms a layer under 

 the bile. Where the two fluids are in contact, a play of colours is 

 produced. This test can be rendered still more delicate by filtering a 

 little diluted bile through white filter paper, then removing and 

 opening out the filter paper and placing a drop of fuming nitric acid 

 on it. 



Bilirubin is the least oxidised bile-pigment, and its empirical formula 

 is C 32 H 3G N 4 O 6 . If we compare this with the formula of haematin 

 C 32 H 32 N 4 4 Fe we see that it must be from this body that it is 

 derived, the change being the abstraction of iron and the addition of 

 two molecules of water. This is also the formula of iron-free haematin 

 or haematoporphyrin, and of haematoidin, a pigment which crystallises 

 out in old blood clots in the tissues. Although the same empirically, 

 these bodies vary somewhat in their physical behaviour, and neither of 

 them gives Gmelin's test, so that we may assume that they have 

 different constitutional formulae. 



When it reaches the intestine, the bile pigment is converted by 

 bacteria to another pigment called stercobilin. Some of this pigment 

 is absorbed into the portal blood along with the bile salts. This 

 ^reabsorbed stercobilin is mainly re-excreted in the bile, but a small 

 quantity is excreted in the urine, where it goes by the name of urobilin 

 (see Urine). Stercobilin forms the principal colouring matter of the 

 faeces. 



EXPERIMENT VI. Bilirubin can be extracted from pigmented gall- 

 stones. The gall-stones are ground to a rough powder and extracted 

 by heating with 95 % alcohol, to which a few drops of strong hydro- 

 chloric acid have been added. (The acid is necessary to decompose 

 the compound of bile pigment with calcium present in the stones.) The 

 coloured extract is then cooled. The crystals of cholesterol, which 

 separate, are filtered off, washed with alcohol and examined. (See 

 p. 318.) The filtered extract is placed in a dish, and pure nitric acid 

 run in, drop by drop, when a brilliant Gmelin's test is obtained. 



Lecithin (C 44 H 90 NP0 9 ) and Cholesterol (C 27 H 45 OH) (see Chapter VI.). 

 These two bodies are kept in solution in the bile by means of the 

 bile salts. For their separation, see p. 316. 



EXPERIMENT VII. Place some bile in a test tube, and add one or 

 two crystals of cholesterol to it and gently warm. The cholesterol 

 dissolves. Before doing this show that the crystals will not dissolve in 

 water. 



Both lecithin and cholesterol are excretory products. The tissues 

 which contain the highest percentage of them are the nervous, so that 



2c 



