THE VARIABILITY OF THE SKIN 371 would lack any degree of suppleness were it not for the fact that, because of a permanent moistening with sweat, the water content of the outer skin layers is kept at such a level that a reasonable absorption through the keratin is ensured and therefore a certain elasticity. In these places, the sweat glands are six times as numerous as, for instance, on the back (200/cm•), and, moreover, the sweat secretion is independent of the heat regulation. Only on the palms of the hands, and the soles of the feet, is there a regular pattern of the papillae, as shown on most anatomical models of the skin. On other parts of the body the papillae are much less pronounced and are irregularly arranged moreover, the living and dead epithelia are much thinner. (2) The distribution of the adnexa varies. In the foetus, the hairs and sweat glands are uniformly distributed over the surface. Because the various parts grow at different rates and no new sweat glands or hair follicles are formed, their density on the head remains fairly high but is greatly reduced on other parts of the body. During the life of a person, the hair is subject to considerable changes in structure and locality. To some extent it is probably justifiable to regard this as proceeding parallel with changes in the epidermis. The hair cells of the new-born baby grow out as lanugo hairs (fine hairs without core) which are constantly replaced until the sixth year by which time the hair on the scalp has reached its ultimate form. (a) In the fourth to fifth month of embryonic development a cluster of epithelium cells unites with blood vessels from the cutis as the hair germ. The whole then forms an epithelial "string", which first grows downwards to the sub-cutis, after which it begins to keratinise at its centre, where a sharply pointed core of incompletely keratinised cells is formed. Under this core, the formation of the ultimate hair begins this keratinises completely and in due course pierces and emerges from the apex of the core. At the same point a bulge develops from the so-called outer root sheath, which forms the sebaceous gland. {b) The outer sheath is, as it were, a continuation of the cells of the stratum basale, extending from the surface of the skin to the fundus of the hair follicle around the hair papilla. Two parts can be distinguished. (i) The upper part--an extension of the normal surface epithelium which is static and not subject to rapid changes. (ii) The lower part, which does not keratinise no tonofibrils form in the cytoplasm. This part remains in a more or less embryonic state and evolves with the great changes in the hair papilla. Once the hair has emerged, it continues to grow for some time, about three years in the case of scalp hairs. The growth then ceases

372 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS abruptly for a time, the so-called catagen stage, and finally the hair papilla withers and is largely destroyed. The hair then moves upwards and is shed. In this phase, a new epithelial shoot forms at the side of the hair follicle. This grows downwards and a new cycle begins. This interaction between the upper part of the hair follicle, the sebaceous gland, and the lower part of the follicle must proceed faultlessly if difficulties are to be avoided. Interruption of this interaction by internal or external factors might explain the occur- rence of comedones and pimples in infancy, in puberty, and in old age. The conception of this interaction can be extended to apply to the entire skin. This is a symbiosis of ectodermal and mesenchymal elements of various origin (9). In addition to the hair germ and all that is related to it, there are the sweat glands (acrosyringium), less closely connected, epithelial but not keratinised, which may keratinise, however, under the influence of certain stimuli. The melanocytes are foreign intruders originating from the neural crest, which produce melanin and inject it into the epithelial cells. Finally, there are the mesenchymal structures, such as blood vessels, lymphatic vessels and nerves. The whole is fed and co-ordinated through the connective tissue which encloses these organs as a nutrient medium. If changes in this connective tissue occur, in old age or under the influence of the sun on a skin with little or no pigment, the whole becomes dis- organized (10). VARIABILITY IN THE DIFFERENT PERIODS OF LIFE The skin displays distinct variations in the different periods of life. To a small extent these variations are the results of ageing in general they are also caused by the sensitix'ity of the skin to a number of factors, e.g. to hormone stimulation and climatic influences. The epidermis of the baby is thin and readily detaches itself from the underlayer. This epidermis has very thin lanugo hairs (average thickness of 0.03 mm) and a relatively large number of sweat glands per cm 2. The heat regulation, especially on the periphery, is not entirely in equilibrium. The connective tissue has a relatively high water content. Difficulties in the discharge from the adnexa, which may possibly lead to infection, occur chiefly in the sweat glands (peri- poritis), whereas diseases of the hair follicles are relatively rare. Clinically, the view is held that the skin of the baby is "more sensitive", although, in general, "tar" (e.g. pix liquida) in high concentrations can be tolerated very well. Marcussen (11) has recently established from experi- ments with 200 children, that patch tests with nickel sulphate and formalde- hyde, which cause no irritation to adults, give rise in children of 7-8 years