THE BILE. 157 



globin and into hsematin, which latter then passes over into bili- 

 rubin, as shown in the equation : 



C 32 H 32 N 4 4 Fe + 2H 2 - Fe = C 32 H 36 N 4 O 6 

 Haiiuatiii. Bilirubin. 



This reaction, it will be noted, is also supposed to express the 

 formation of haematoporphyrin from hsematin (see page 337). The 

 two substances, indeed, are now quite generally regarded as isomeric. 

 As regards the size of the molecule, however, opinions differ, and it 

 is possible, as Nencki and Sieber suppose, that the molecule of bili- 

 rubin, as well as of hsematoporphyrin, is only half as large as 

 expressed above. In that case, of course, the equation would have 

 to be written : 



C 32 H 32 NAFe -f 2H 2 Fe = 2C 16 H 18 N 2 O 3 



On reduction with nascent hydrogen bilirubin is transformed into 

 hydrobilirubin, as is shown by the equation : 



C 32 H 36 NA(viz, 2C 16 H 18 N 2 3 ) + H 2 O + 2H = C^H^NA 



Hydrobilirubin. 



This actually takes place in the intestinal canal, where nascent 

 hydrogen is constantly being formed through the action of various 

 bacteria, as during the process of lactic acid fermentation. During 

 intra-uterine life, however, where no bacteria are found in the 

 intestinal tract, bilirubin appears in the feces as such (meconium). 



According to some observers, hydrobilirubin is thought to be 

 identical with the febrile urobilin of Jaffe, which is met with in the 

 urine in various febrile diseases, and exceptionally also under normal 

 conditions, but it is said to differ from the normal urobilin which 

 represents the principal coloring-matter of the urine in health. 

 This question, however, still awaits solution. The possible identity 

 also of hydrobilirubin and stercobilin, which is the principal pig- 

 ment that is found in the feces, has not been definitely established 

 (see also pages 207 and 291). 



In the crystalline state bilirubin occurs in the form of reddish- 

 yellow rhombic platelets with rounded angles, which are soluble in 

 benzol, carbon disulphide, amyl alcohol, glycerin, the fatty oils, and 

 especially in warm chloroform. In alcohol and ether they are but 

 little soluble, and in water, as also in Platner's bile, they are insol- 

 uble. On spectroscopic examination its solutions do not give rise to 

 any specific bands of absorption, but to a continuous absorption 

 which extends from the red to the violet end. 



Bilirubin as such is a weak acid, bilirubinic acid, and combines 

 with bases to form salts, which for the most part are either insoluble 

 or only slightly soluble in water and insoluble in chloroform. Its 

 salts with the alkalies, however, are soluble in solutions of the alka- 

 line hydrates and carbonates. In the bile bilirubin is largely present 

 as neutral bilirubinate of sodium, and is held in solution owing to 

 the presence of alkaline carbonates. 



