368 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS moment, than by the character of the •etiologic agent or the specific reaction of the organism. GENERAL FUNCTIONS OF THE SKIN Analogous to the membrane of a single cell, the function of the skin is to protect the autonomy of the organism and prevent its drying out. More- over, it must protect the body against intrusion of foreign matter, and must maintain a constant energy level of the inner core of the organism, by regulating the heat loss from the skin. The anatomical structure of the skin reflects these functions. The stratum corneum and stratum lucidum constitute a structure which is relatively inert to a large number of chemical substances. The most important function of the skin, however, is restriction of water loss from the body. This is achieved by a barrier zone between the living and dead part of the epidermis which, when compared with an unprotected area, reduces the water-loss by a factor of 50 (1). As a consequence of the outward growth, not only of the actual epidermis but also of its adnexes (sweat glands and hair follicles), penetration of noxious substances is difficult. The thick underlayer (dermis) which feeds the exterior membrane (epidermis) is composed of a fibrillar connective tissue and lies on the subcutaneous fat. FUNCTIONAL VARIABILITY OF THE I)ERMIS Although the insulating ability of the dermis is large (thermal conducti- vity is as low as 0.01 g cal/cm • rain/1 ø C) this insulation is counteracted by a thick meshwork of blood vessels, arranged parallel to the surface, which receives blood by shunts from the deeper blood vessels through the fat layer. If the blood vessels are fully dilated, the heat conductivity of this part of the skin may rise to values of 0.4 cal/cmymin/løC (2). Only a very minor part of the blood circulating through the skin functions for its own meta- bolism the largest part serves for the trm•sport of heat from the body to the surface. Changes in the volume of blood, circulating through the skin, bring about changes in heat transfer in the connective tissue. This is prompted by the mechanism of heat regulation, which is one of the most important regulatory mechanisms of the body. Its function is to keep the inner core of the body at a constant temperature of 37 ø C. If this tempera- ture rises by an increase in heat production in the body, the regulating centre in the central part of the cerebrum is informed by the rise in blood tempera- ture, and through nervous signals. From the cerebrum, signals are given via the nerves of the sympathetic system, which may partially lessen their grip on the skin vessels, especially those of the hands and feet. The blood flow through the skin and its heat conductivity rise, resulting in a rise in skin temperature and an increase of heat loss to the environment.

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