PIGMENTS OF SKIN AND HAIR. The pigment of the skin, the pigment of the blood, the pigment of the yolk of the egg, and the pigment of the butterfly's wing may each be regarded as representative of a chemical group of colouring matters, and into those four groups the great bulk of the pigments seen in the animal world may be placed.
The Pigment of the Skin.—The skin consists of two principal layers--the epidermis or superficial portion, and the dermis or deep portion. The dermis is not pigmented in man, nor is the epi dermis in the so-called white races. In the negro the epidermis is densely pigmented, the pigment being present in granules. These granules are not uniformly distributed throughout the epi dermis, which itself consists of many layers of cells, but they are confined to the layer of cells next to the dermis, and, there fore, farthest removed from the external surface.
The black pigment of the skin, which is called melanin, is situ ated in granules in the deepest layer of endo-dermal cells, namely, that placed immediately over the dermis. All the cells of the epidermis really are developed from this layer (the Malpighian layer) the constituents of which divide and work their way towards the surface. It might be supposed that all the epidermal cells should there be pigmented, but this is not so; as they get nearer the surface, they lose their pigment. This is well shown in the case of a blister. The fluid forms in the epidermis, separating the deeper from the more superficial portion; the covering of the blister in a negro is not pigmented.
In man the pigment is present as a quite inert deposit, the person being no more negroid at one time than another; but in many lower animals this is not so. The frog, for instance, will at one time be dark, at another pale. The melanin in the frog's skin is laid down in special cells known as melanophores ; these may contract in minute balls, in which case the skin is pale, or they may expand, pushing out branches in all directions, in which case they form an almost continuous network of pigmented material.
The degree of expansion of the melanophores is regulated by circumstances, the following table being given by Hogben (abbre viated) :— Evidently their dark background, low temperature and moist ness tend to make the melanophores expand. The mechanism by
which expansion of the melanophores in the frog is effected is bound up with a remarkable body situated underneath, and con nected with the brain—the pituitary body (see art. HORMONES). This body secretes, no doubt at the bidding of the brain, a hor mone into the blood which, acting on the melanophores, causes them to expand.
The pigment cells in the frog, unlike those in man, are situated at different levels; some are in the epidermis and some in the dermis. This arrangement has been exploited very completely by the chameleon. The chameleon possesses two layers of cells, each in the dermis. As an example may be taken an animal which alters in colour from a yellow or very pale green, through green to something nearly black. The cells of the outermost layer, called variously guanophores, leukophores, ochrophores, iridocytes and interference cells, are yellowish in colour; so far as is known, these cells do not alter either in shape or tint. Underneath these are the melanophores, also a similar but reddish type of cell called erythrophores. To take the melanophores, these send pro cesses towards the surface more or less encircling the guano phores. Darkening in the colour of the chameleon is due to the migration of granules of melanin from the deeper portion of the melanophores into the more superficial tentacles which surround the guanophores. The actual colour which the animal presents is partly due to the mixture of colours which the various pigment cells present, and partly due to the scattering of light between these. The migration of the melanophore granules in the cha meleon is due largely to the hormone adrenalin. The current be lief that the chameleon takes on the colour of its surroundings is much exaggerated; the colour is influenced largely by light and temperature, and, so far as the author has been able to judge, in a very limited way by temper also ; if the chameleon were annoyed it took on a darker colour.