In certain sarcomas and cancers a brown pigment is present in the cells of the tumours, giving to the tissue a brown or black colour. The pigment here has the chemical characters rather of melanin than of blood-pigments and is presumably elaborated by the cells. These tumours mostly take origin in structures where pigment cells normally exist, as in the choroid of the eye, the superficial layers of the cutis, the pia mater (Virchow), and the conjunctiva of the eye, where it passes into the cornea. Such tumours also originate in pigmented naevi, in the rete Malpighii, and in the cornea (especially in horses). In some of these cases the melanin passes into the urine, where it may deeply colour that secretion (Tennent and Coats).