MELANIC DEPOSIT.
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Black colouring matter appears under va rious conditions "as a morbid deposit. The only kind strictly belonging to the present head, is true melanic granule or cell-pigment, more or less closely similar to natural pigment.
Melanie pigment is essentially composed of extremely minute granules, for the most part contained within cells. The cells are of various shapes, commonly' rounded, however ; not commonly of caudate form, but often showing a tendency to prolongation in one particular direction. They very rarely con tain a nucleus.
The cells are of blackish, brownish, bistre or yellowish tint, the colour evidently depend ing on the granules. And these granules are not confined to the cells, but are commonly found, in multitudes, free ; when excessively minute they are the subjects of molecular motion. In some instances cells are not to be discovered at all.
Little is positively known concerning the development of melanic pigment, either of the mode, whether exogenous or endogenous, by which increase of cells takes place, or of the relationship in which the cells and gra nules stand to each other ; that is, whether the cells are formed around the granules, or the granules generated within the cells. But while it is certain that the cells are deficient in the attribute of permanency, and appear of secondary importance (seeing that the pig ment character may exist in perfection in-. dependently of them through ithe granules alone), it seenis very unlikely that they are truly' vegetative. Melanie cells never exhibit any tendency even to cohere much less to form the basis of a stroma.
The chemical composition of this substance is not known with accuracy. Analyses in numbers no doubt have been printed, but none of them are entitled to confidence, either because they include the composition of associated substances, organic and inor ganic, or because the black matter analyzed was not really composed of cell-pigment. It is probable, however, that the ultimate con stituents are the same, and associated in, at least very closely, the same proportions, as of the pigment of the choroid coat. Some of the more important reactions of this sub stance, as set down many years ago by Henry, may be substantiated readily, and have fre quently been confirmed by ourselves. A
" softened melanotic tumour " was experi mented on : 1. By filtering through paper, much of the colouring matter remained on the paper, and the colour of that which passed through was rendered much less in tense; 2. Boiling does not destroy the colour, not even when a little caustic potass has been added ; 3. It is not changed by acids even when heated, except by strong nitric acid, which turns it yellow ; 4. A stream of chlorine, passed through the liquid, destroy's the colour, and throws down light-coloured floc culi*; 5. A few grains of corrosive sublimate (nitrate of mercury and muriate of tin also, though more slomly,) precipitate the colour ing matter and leave the supernatant fluid clear.
Black cell-pigment occurs under two chief conditions unassociated, or associated with other materials. The former condition is ex cessively rare, and we have certainly never: seen it in the human subject, that is, we have never seen a fluid or solid accumulation of cell-pigment utterly unmingled with other fluids or solids, natural or adventitious : it ap pears, however, to occur thus in the horse. In the associated form it is of very common occurrence, exhibiting itself in the form of points, spots, layers, or masses, in the sub stance of natural textures or of adventitious products. In the latter condition it has more particularly excited attention, and been de scribed under the titles of " rnelanosis," " me lanotic tumour," "melanoma," &c. A full consideration of the modes of connection of cell-pigment with tumours will be found under the head of " Melanonia" in the sec tion " Growths." The substance we have just described being the only true black cell-pigment, appears to be the only one legitimately falling under the present head ; but it is absolutely neces sary (were it only for the purposes of diag nosis) that we should briefly consider certain other causes (most ably investigated by Dr. Carswell) of black discolouration. These causes are, (a.) Alteration of the colouring matter of the blood ; (b.) Introduction of black-coloured substances from without.