810 PHYSIOLOGY OF DIGESTION AND SECRETION. 



ponderance of one or the other of the main bile pigments;, bihrubin 

 and biUverdin. The bile of carnivorous animals has usually a 

 golden color, owing to the presence of bilirubin, while that of the her- 

 bivora is a bright green from the biliverdin. The color of human 

 bile seems to vary: according to some authorities, it is yellow or 

 golden yellow, and this seems especially true of the bile as found in 

 the gall-bladder of the cadaver; according to others, it is of a dark- 

 olive color with the greenish tint predominating. Its reaction is 

 feebly alkaline, and its specific gravity varies in human bile from 

 1.050 or 1.040 to 1.010. Human bile does not give a distinctive 

 absorption spectrum, but the bile of some herbivora, after exposure 

 to the air at least, gives a characteristic spectrum. 



Bile Pigments. — Bile, according to the animal from which it is 

 obtained, contains one or the other, or a mixture, of the two 

 pigments, bilirubin and biliverdin. Indeed, it is probable that 

 in some animals at least still other pigments, such as urobilin, 

 may be present in the bile, together with the bilirubin or biliverdin. 

 Biliverdin is supposed to stand to bilirubin in the relation of an 

 oxidation product. Bilirubin is given the formula C16H18N2O3, 

 or C32H36N4O6, and bihverdin, C16H18N2O4 or C32H36N4O8, the 

 latter being prepared readily from the former by oxidation. These 

 pigments give a characteristic reaction, known as "Gmehn's reac- 

 tion," with nitric acid containing some nitrous acid (nitric acid 

 with a yellow color) . If a drop of bile and a drop of nitric acid are 

 brought into contact, the former undergoes a succession of color 

 changes, the order being green, blue, violet, red, and reddish yellow. 

 The play of colors is due to successive oxidations of the bile pig- 

 ments; starting with bilirubin, the first stage (green) is due to the 

 formation of biliverdin. The pigments formed in some of the 

 other stages have been isolated and named. The reaction is very 

 delicate, and it is often used to detect the presence of bile pigments 

 in other liquids — urine, for example. The bile pigments originate 

 from hemoglobin. This origin was first indicated by the fact that 

 in old blood clots or in extravasations there was found a crystalline 

 product, the so-called "hematoidin," which was undoubtedly de- 

 rived from hemoglobin, and which upon more careful examina- 

 tion was proved to be identical with bilirubin. It is supposed 

 that when the blood-corpuscles disintegrate the liberated hemo- 

 globin is converted by the liver cells to an iron-free compound, 

 bilirubin or biliverdin. The bilirubin is formed from the hematin 

 of the hemoglobin by a process which involves the splitting off 

 of its iron. It is very significant that the iron separated by this 

 means from the hematin is, for the most part, retained in the 

 liver, a small portion only being secreted in the bile. It seems 

 probable that the iron held back in the liver is again used in some 



