496 DIGESTION 



as iron-free hematin or hematoporphyrin, which is produced by treating 

 hemoglobin with concentrated sulphuric acid. 



Chemical investigation has shown that bilirubin is built up from sub- 

 stituted pyrrols, probably four such being contained in the molecule. 

 The pyrrol group is also present in indole and tryptophane, and con- 

 sists of four carbon atoms and an NH group linked together as a ring 

 (see page 604). Similar pyrrol derivatives can be produced by decom- 

 posing chlorophyl, the green coloring matter of plants. It is important 

 to remember that bilirubin is acid in nature, and, therefore, can com- 

 bine with alkalies to form salts. The relative amounts of bilirubin and 

 biliverdin vary in the bile of different animals. 



When these pigments enter the intestine they are reduced to urobilin, 

 part of which passes out with the feces, another part being absorbed into 

 the blood and excreted in the urine. Part of that excreted in the urine 

 exists, however, as a so-called chromogen named urobilinogen. The 

 urobilinogen is converted into urobilin by the action of oxygen. 



The method by which urobilin is produced from blood pigment has 

 been studied by histologic examination of the liver particularly of birds 

 and amphibia, in which destruction of blood pigment goes on rapidly. 

 Increased destruction of blood pigment can be induced by poisoning 

 with certain substances such as arseniureted hydrogen. From such 

 studies it is usually believed that the bile pigments are a peculiar product 

 of hepatic activity, being produced from blood pigments that are de- 

 rived from erythrocytes which have been broken down either in the liver 

 itself or in some other viscus (e. g., the spleen). Whipple and Hooper 20 

 have brought forward seemingly incontrovertible evidence against such 

 a view. They have found, for example, that the bile pigments are 

 formed just as readily in animals in which the circulation of the liver 

 was greatly curtailed by anastomosing the portal vein with the vena 

 cava (Eck fistula) as in normal animals. Even when the circulation 

 was limited to the anterior end of the animal (head and thorax) bile 

 pigment appeared in the blood when hemolyzed erythrocytes were in- 

 jected, and it was also formed when hemoglobin was placed in the pleural 

 and peritoneal cavities. The endothelial cells of the blood vessels and 

 elsewhere can evidently form the pigments, at least when the liver is 

 absent. When such a process occurs under normal conditions, it is quite 

 probable that the liver acts merely as an excretory organ for the pig- 

 ments in the same way as the kidney does for urea. Possessed of endo- 

 thelial cells, the liver might itself also produce some of the pigments, 

 but no more than other organs with a similar number of those cells. 



Even the derivation of bile pigments from hemoglobin is called in 

 question, for the same workers have observed that, whereas the excre- 



