198 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 60 50 2O 420 460 500 540 580 620 660 700 WAVELENGTH Fig. 1. Reflectance of normal skin and its component pigments. a lesions. Anatomical variations also modify skin color in other ways. Where the epidermis is thin and more transparent the color of blood pigments in the small vessels of the dermal papillae becomes more prominent. The degree of development of the granular layer is another factor which regulates the degree of transparency or opacity of the epidermis. The water content of the skin can also modify its color, as illustrated by the general pallor associated with oedematous skin and by the whiteness of macerated skin. Changes in the amount and degree of oxygenation of blood in the upper portions of the skin greatly affect its color. If oxygenated blood near the surface is increased in amount by dilatation of superficial capillaries, as occurs in blushing or acute inflammatory processes, the skin becomes pinker or red because the bright red color of oxyhemoglobin pigment becomes more prominent. When the amount of blood near the surface is decreased, as when a person is about to faint, the skin, of course, becomes pale. If the blood near the surface is not sufficiently oxygenated, then the bluish color of reduced hemoglobin becomes evident and the skin is said to appear cyanotic.

SKIN AND HAIR PIGMENTATION 199 The yellow pigment, carotene, is present in the subcutaneous fatty tissues and also in the closely packed horny scales that comprise the outer- most dead layers of the skin. The skin, as a consequence, has a more pronounced yellowish hue wherever the fatty tissues just below the skin are abundant, or where the horny layer is particularly thick, such as over the palms and soles. Women, in general, show much more carotene through their skins than do men, probably largely because of their better developed layer of subcutaneous fat. If the dietary intake of carotene is excessive, such as from unusually heavy consumption of carrots, obviously pronounced generalized yellowing of the skin, especially on the palms and soles, may occur, and this is called carotenemia. The nature of melanoid pigment is still rather controversial. It is most often regarded as a rather pale degradation product of melanin. The rela- tion of melanoid to the well-known horn-pigment • which develops as a dark oxidation product in keratin as is evident in black heads, on tops of warts, or in the long persisting scales in ichthyosis, is also not settled, although these pigments may likely be identical. It is claimed in studies by reflectance spectrophotometry that melanoid production in large quantities is consis- tently stimulated in the skins of eunuchs by ultraviolet irradiation, although melanin production is not. 2 Melanin is the most characteristic pigment of the skin and hair. It is present in all races, and racial differences in skin color are based almost exclusively on differences in melanin content. Only in a few individuals is melanin pigment completely absent. This abnormal condition is called total albinism, and it is a recessively inherited anomaly. Although no truly more precise definition of the term melanin can be given than "dark pigment," its modern use biologically has been more or less limited to the brown or black polymers that originate as oxidation products from tyrosine or related orthodihydroxyphenyl compounds. In a broader chemical sense any dark pigment formed by the oxidation and polymerization of a polyhydroxy (or polyamino) aromatic compound with hydroxy or amino groups in the ortho or para positions could be considered as a kind of melanin. Biological types of melanin are found widely distributed throughout the plant and animal kingdoms, where their primary function seems to be simply that of providing coloration. By some animal species this pigmen- tary function has been elaborated into complex and important mechanisms for camouflage and sexual attraction. The elaborateness of some of these mechanisms can be illustrated by the "ink" of squids, by the various striped and spotted and seasonally changing coat patterns of animals, and even more so by the remarkable adaptive color changes possible in certain fishes, amphibians and lizards. In some flounders the high degree of control over melanin distribution pattern by precise nervous and hormonal mechanisms,