CHEMICAL COMPOSITION OF THE HUMAN BODY 51 



soluble in alcohol, slightly in chloroform. Bilirubin, by the 

 reduction of nascent hydrogen, takes up water and forms 

 hydrobilirubin, C^H^N^ , which is identical with urobilin 

 (a pigment of urine). Urobilin cannot be changed back to 

 bilirubin by oxidation. 



Gmelin's test. In a test-tube place some nitric acid con- 

 taining nitrous acid. Carefully cover it with an aqueous 

 solution of bile pigment. At the junction of the two liquids, 

 colored layers will be seen, which from the top downward 

 are green, blue, violet, red, reddish yellow. The pigments 

 to which the colors are due are formed by the oxidation 

 of bilirubin or biliverdin; they represent various oxidation 

 stages of the bile pigment. 



The bile pigments are made in the liver from the haematin 

 formed by the decomposition of the red blood corpuscles. 

 This haematin loses its iron: 

 C 32 H 32 N 4 O 4 Fe (haematin)+ 2 H 2 O - Fe = C 32 H 36 N 4 O 6 (bilirubin). 



9. Besides the end-products of metabolism above named, there 

 are present in the urine aromatic substances. It is not certain 

 whether these originate by metabolism or whether they are merely 

 products absorbed from the alimentary canal. By the proteid 

 putrefaction in the intestine, aromatic compounds are formed 

 (phenol, aromatic oxyacids, indol, skatol) which, in so far as they 

 are not cast out with the faeces, are absorbed by the body and are 

 in part oxidized (forming indoxyl, skatoxyl) and in part united 

 with sulphuric acid and excreted by the kidneys as such. For 

 further information, see Chapter VII and Chapter IX. 



