THE VARIABILITY OF THE SKIN 369 Local variability The function in respect to heat loss of different parts of the skin is not the same. The relation of the skin temperature at different times of the day over the body to average skin temperature and its variation (3), in 185 subjects in a moderate climate (10-20 ø C) showed that the skin could be sub-divided as follows: (a) Forehead and trunk (b) Arms and thighs (c) Hands, feet and forelegs average temperature 34.4 ø C varying between 36.4-32-4. average temperature 32.41 ø C varying between 35-4-28.4. average temperature 30-37 ø C varying between 35-3-22.3. We might represent the system of heat loss of the human body as a system of three radiators, arranged in parallel. (i) Hands and feet can very quickly lose the greater part of the heat conveyed to them by the bloodstream because their volume is small in relation to their surface. This is why smaller fluctuations in the body temperature (e.g. those arising after a good dinner) are mainly regulated by changes in the blood flow to the hands and feet. This special function finds it anatomical counterpart in a very massive meshwork of blood vessels, which may constitute a bloody sponge in the cutis (4), and its physiological counterpart in the fact that blood flow is regulated mainly by release of the vaso-constrictor tone. (ii) The trunk is subject to less variation in surface temperature because of its large heat content, and because most heat producing organs (e.g. the liver) are enclosed within it. The density of the meshwork of blood vessels in this part of the skin is only a fraction of that of the rest of the body. (iii) The density of the meshwork of blood vessels in the arms and legs lies between those of the hands and feet and the trunk. Here the blood flow is presumably mainly regulated by active vasodilatation due to bradykinin, which is liberated in the tissues during sweat gland activity (5). At ambient temperatures between 18 and 25 ø C, the heat regulation in unclothed, resting subjects is mainly carried out by changes in the peripheral blood flow. In the morning, the temperature of the feet is similar to that of the environment, to rise substantially (10 ø C) in the course of the day. The face has a special function as its temperature is more constant than that of other parts, mainly due to the close relation between its vessels and

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