256 HUMUS PIGMENTS. UKINAKY MELANIN. 



4. Humus pigments. 



When carbohydrates are treated with acids or alkalis, among 

 the numerous products which arise are certain pigmentary bodies 

 of a more or less dark -brown colour. A similar colouration is 

 well known as occurring in fruits when bruised or exposed to 

 the air, 1 and generally in decaying vegetable tissues. These sub- 

 stances are known under the name of ' humus.' When urine is 

 treated with acids in presence of oxygen it acquires a markedly 

 darker colour, and since carbohydrates in small amount are prob- 

 ably present in all urines, 2 there is at once a possibility that some 

 at least of the observed colouration is due to the production of 

 humus-pigmented substances by the action of the acids on the 

 carbohydrates. In accordance with this view certain so-called 

 humus pigments have been prepared from urine, but our knowl- 

 edge of them is as yet very incomplete. They are stated to be 

 practically insoluble in any solvents other than amyl-alcohol, 

 strong ammonia, and caustic alkalis : the solutions show no 

 absorption bands when examined spectroscopically. They are 

 further said to account for the usually dark colour of normal 

 herbivorous urine and of urine after the cutaneous absorption 

 of carbolic acid and several other aromatic compounds. 3 



It is very probable that several dark-coloured pigments such as the 

 uromelanius of Flosz and Thudichum obtained by the action of acids 

 on urinary pigments or chromogens are allied to if not identical with 

 these humus substances. 



5. Urinary melanin. 4 



Certain tumours are not infrequently observed which from 

 their extremely dark pigmentation are spoken of as ' melanotic,' 

 the colouring-substance being known as melanin. 5 The urine of 

 patients suffering from these tumours is either dark-brown or 

 black when voided, or speedily assumes this colour after brief 

 exposure to the air or by the action of nitric acid or other oxidis- 

 ing agents, the pigment to which the colour is due being ap- 

 parently identical with that present in the tumour. This action 

 of oxidising agents indicates that here also, as in the case of 

 other urinary pigments, there is primarily some chromogenic 

 forerunner (melanogen) of the actual pigment. This chromogen 



1 Hoppe-Seyler, Zt. f. phi/siol. Chem. Bd. xin. (1889), S. 66. 



2 SeeWedenski, Ibid. S/122. E. Salkowski, Ibid. S. 270. 



8 Udranszky, Ibid. Bde. xi. (1887), S. 537, xn. (1888), S. 33. Contains very full 

 references to other works. 



* Morner, Zt. f. physiol. Chem. Bde. xr. (1887), S. 66, xn. (1888), S. 229. Gives 

 list of literature to date. See also Zeller, Langenbeck's Arch. Bd. xxix. (1884), S. 2, 

 and later Brandl u. Pfeiffer, Zeitsch.f. Biol. Bd. xxvi. (1890), S. 348. 



5 The name melanin is more usually applied as a generic title for the dark- 

 brown or black pigments such as occur in the hair, epidermis, retinal epithelium, 

 choroid, &c. 



