724 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 bili verdin. Biliverdin is supposed to stand to bilirubin 

 in the relation of an oxidation product. Bilirubin is given the formula 

 C 16 H 18 N 2 O 3 > and biliverdin C 16 H 18 N 2 O 4 , the latter being prepared 

 readily from pure specimens of the former by oxidation. These 

 pigments give a characteristic reaction, known as" Gmelin's reaction," 

 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 pigments; starting 

 with bilirubin, the first stage (green) is due to the formation of bili- 

 verdin. 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 

 " hematoi'din," which was undoubtedly derived from hemoglobin, 

 and which upon more careful examination was proved to be identical 

 with bilirubin. This origin, which has since been made probable by 

 other reactions, is now universally accepted. It is supposed that 

 when the blood-corpuscles disintegrate the hemoglobin is brought to 

 the liver, and there, under the influence of the liver cells, is converted 

 to an iron-free compound, bilirubin or biliverdin. It is very signifi- 

 cant that the iron separated by this means from the hemoglobin 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 way to make new hemoglobin in the 

 hematopoietic organs. The bile pigments are carried in the bile to 

 the duodenum and are mixed with the food in its long passage through 

 the intestine. Under normal conditions neither bilirubin nor bili- 

 verdin occurs in the feces, but in their place is found a reduction prod- 

 uct, urobilin or stercobilin, formed in the large intestine. More- 

 over, it is believed that some of the bile pigment is reabsorbed as it 

 passes along the intestine, is carried to the liver in the portal blood, 

 and is again eliminated. That this action occurs, or may occur, has 

 been made probable by experiments of Wertheimer* on dogs. It 

 happens that sheep's bile contains a pigment (cholohematin) that 

 gives a characteristic spectrum. If some of this pigment is injected 

 into the mesenteric veins of a dog it is eliminated while passing 

 through the liver, and can be recognized unchanged in the bile. 

 * "Archives de physiologie normale et pathologique," 1892, p. 577. 



