Melanin occurs normally in man and the animals in the chorioid, hair and skin, also in some animals in specialized organs, as the ink bags of the cuttle fish. It occurs as the coloring matter of pigmented sarcoma in man and animals, and also in the latter as the result of certain parasites.
Its origin seems still in doubt, but most authorities agree that it is not a direct derivative from blood pigment, but the product formed by the metabolic activity of specialized cells. It is composed of nitrogen, hydrogen and carbon in the proportions of nitrogen one part to five of each of the others, with sulphur varying up to 10 per cent. and iron present sometimes in small amounts.
Melanuria is clinically of two distinct types: First, one in which the urine is black or very dark colored when passed and contains melanin by chemical tests; second, one
STEWART JC. A CASE OF BENIGN MELANOSIS.. JAMA. 1907;XLIX(3):227–228. doi:10.1001/jama.1907.25320030031002j