370 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the deeper blood vessels going to the brain. Bedford (6) observed in 2,000 subjects, working at environmental temperatures of between 12 and 24 ø C, a change in the skin temperature of the hands of 0.46ø C for each degree difference in environmental temperature, of 0.806 ø C for the feet and of only 0.139øC for the forehead. The relatively high temperature of the head makes heat loss from this part of the body marked. At very low environ- mental temperatures more than 50% of the heat generated by the body under basal conditions may thus be lost (7). The warm forehead functions as an indicator for heat regulation. Cooling of the forehead will result in a reflex constriction of the blood vessels all over the body. Seasonal variability Apart from its acute fluctuations, the peripheral blood flow is also influenced by the season. Notwithstanding the fact that man has created an artificial climate for himself by clothing and housing, summer and winter conditions have a profound effect on the circulation through the skin. This might be due to the fact that in winter, for instance, cooling is more rapid, due to the larger difference between surface and environmental temperature outdoors. In winter, the peripheral blood flow is decreased, resulting in a lower metabolic activity of the skin. On the one hand this will cause slower growth of the epidermis and its adnexa, on the other hand it seems likely that the reactivity of the blood vessels, and the cellular and nervous elements also decreases. This reactivity of the blood vessels of the skin was measured by DePalma et al (8) who determined the reactive hyper- aemia by applying a 100 g weight on the skin. As a result of this pressure, metabolites (histamine-like substances) originate in the skin causing dilation of the smaller blood vessels. They found that in winter it was necessary to increase the time of application by five to ten times of that used in summer. The different reaction of the blood vessels of the skin in summer and winter results from the fact that their condition and reactivity is not so much determined by local factors as by the need of heat regulation. Sex differ- ences also play an important role. Only in females is the more or less generalized peripheral vasoconstriction, known as perniosis or erythro- cyanosis, visible. This might be due to an inherent relative incapacity of the system of heat regulation, the cause of which is, however, unknown. VARIABILITY OF THE EPIr0ERMIS (1) As far as the structure of the stratum corneum, and of the stratum lucidum, is concerned, the palms of the hands and the soles of the feet are exceptions. In these localities the stratum corneum and the stratum lucidum are ten times as thick as elsewhere, so that the skin is less vulnerable. Since the thick stratum lucidum is practically impermeable to water, it

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