572 LECTURE XXIV. 



pigments, often serve for the formation of concretions and calculi in 

 human beings. It is not easy to say what causes the formation of 

 these concretions, which often lead to such severe symptoms. Appar- 

 ently the primary cause is usually a catarrh of the bile-ducts. Crys- 

 tals of the above-mentioned substances are then, as a secondary effect, 

 deposited upon the diseased mucous membrane. Naturally changes in 

 the conditions of solubility and in the relations of concentration also 

 play a part in their formation. The pigment-stones usually contain the 

 calcium compound of bilirubin. Often mixed stones are found, although 

 quite frequently we meet with stones of pure cholesterol which show, in a 

 fractured surface, beautiful clusters of crystals. 



The bile-pigments appear in the faeces to some extent as such. For 

 the most part, they succumb to the putrefactive processes in the ali- 

 mentary canal, especially those of the large intestine. There is a reduc- 

 tion of the bilirubin, and urobilin is formed. This last substance may be 

 obtained directly by the reduction of hematin, or of hematoporphyrin. 1 

 It is also formed by oxidation, when hemopyrrole is allowed to stand in 

 contact with the air. 2 Finally, it has been established that by feeding 

 hemopyrrole to rabbits an elimination of urobilin results. Urobilin, 

 like hematin, is supposed to contain four molecules of hemopyrrole and 

 to have the following empirical formula: C32H 40 O7N4. It is not abso- 

 lutely known whether urobilin is identical with the so-called hydrobili- 

 rubin which is obtained by the reduction of bilirubin. Urobilin is 

 absorbed in the intestine and passes into the urine. It helps to give 

 to urine its yellow color. Different urobilins have been described as 

 present in urine, and on the other hand it has even been asserted that 

 urobilin is not originally present as such in the urine, but is formed by 

 the action of light. We need not stop to discuss this question, because 

 at present it is all a matter of conjecture, and we are forced to base our 

 assumptions in part upon some very doubtful observations. It is ex- 

 actly the same with the question as to the origin of urobilin. No one 

 doubts that it is formed in the intestines as a result of putrefaction, but 

 it is questioned by many that all the urobilin in the urine arises from this 

 source. Many assume that urobilin is formed outside of the intestines 

 and in the tissues from bilirubin. 



Closely related to urobilin is urochrome, the principal yellow pig- 

 ment contained in urine. Its chemical nature is but little known. 

 Uroeryihrin also frequently occurs in urine, and is likewise of un- 

 known composition. It is the red pigment which causes the color 

 of the sediment in urine, and is also known as Sedimentum later- 

 ilium. 



1 Nencki and Sieber: Arch, exper. Path. Pharm. 18, 401 (1884). 

 8 Nencki and Zaleski: Ber. 34, 997 (1901). 



