Fish, too, have great powers of colour change within a certain scale of colours, which scale does simulate very closely the colour of the bottom on which they lie. Their colour change is also wrought by melanophores, but these seem more directly to be controlled by the nervous system and less by hormones than those of the frog. Thus, if a nerve going to a particular segment of the skin be severed the possibility of colour change appears to vanish. The Chemical Nature of Melanin.—Melanin is closely re lated to two remarkable chemical substances found in the body.
The one is the hormone adrenalin (which turns a blackish colour on exposure) ; the other is a substance, tyrosine, which is one of the breakdown products of protein.
The chemistry of melanin formation has been studied chiefly in the vegetable kingdom. A familiar example is seen when an apple is cut and the cut surface is exposed to air. The melanin is formed from tyrosine in the apple by oxidation ; but the mere exposure of tyrosine to air will not produce melanin. There must also be a ferment present which effects the oxidation. This fer ment is called tyrosinase. If two solutions—(i) tyrosine and (2) tyrosinase—both of which are colourless—be mixed and shaken with air the mixture first becomes a reddish colour and gradually turns to a dark brown. The reddish pigment appears to be an intermediate body between tyrosine and melanin. Recently, Raper and his colleagues have shown that the role of the tyrosinase lies in the formation of this reddish substance, the melanin being a more complicated body derived from the union of two molecules of this red substance.
The pigments in hair are formed on similar lines to those of skin. H. Onslow, the leading authority at the time of his death, wrote : "The colour of human hair depends upon the colour and form of the pigment (i.e., whether it is diffused or deposited in granules) and upon the (air) vacuoles. . . . In light and sandy hair the pigment is chiefly diffused and of a yellowish red colour, but in darker hair the pigment is present as dark brown or black granules." Onslow recognizes three pigments in rabbits giving rise to six colours, the pigments being black, chocolate and yellow, which give rise to rabbits of those colours, and, when the pig ments are diluted, to blue, fawn and cream respectively. The pig ments may, however, be mixed, or in bands, giving rise to the agouti. With regard to whiteness, Onslow writes : "The hair of
an animal may be white for one of three reasons—(I) the absence of either chromogen or enzyme; (2) the absence of both chro mogen and enzyme; (3) the presence of an inhibitor of the enzyme." Whilst most of the pigments in the integument consist either of melanin or some substance closely allied to it, a few are of quite a different origin.
The beak and the legs of fowl, for instance, are often yellow, the pigment being one known as carrotin—the same that colours the carrot. Its presence is, in a sense, accidental. It is not manu factured in the animal body, but exists in the vegetables eaten. It is absorbed into the blood during digestion and, being soluble in fat, it accumulates in places where fat exists. More especially is this the case in the yolk of the egg Hens fed on foods com pletely free of pigment after a time lay eggs with colourless yolks; moreover, if fed on food containing other fat soluble pig ments, the yolk becomes correspondingly coloured; eggs with bright red yolks are obtained if the dye "Soudan III." be mixed into the food.
It is remarkable that haemoglobin contributes little to the pig mentation of the skin--in man only in pathological conditions. In jaundice the yellow pigment is the same as ordinarily appears in the bile, and is practically haemoglobin stripped of its iron and of its protein. Closely related to bile pigment is uro-porphyrin, which occasionally appears in the skin, and then with very dis tressing results. The skin becomes exceedingly sensitive to light so that the individual requires veiling in the open air ; otherwise he would suffer from sunburn amounting to inflammation.
Another sort of porphyrin is, however, found in the integument of some of the lower animals, as for instance, the brown line on the back of the earth-worm. This is protoporphyrin; it is the pigment which forms the basis of haemoglobin and is most fa miliar as being that which colours brown eggs.
Finally, the distinction between pigment and colour must not be forgotten. Black eyes and blue contain the same pigment melanin—black more than blue, but the blue hue is obtained from the way in which the light is reflected from the surfaces of the fibres of which the iris is made, and among which a number of pigment cells are disposed. ( J. BAR.)