J'. Soc. Cosmetic Chemists 15 367-375 (1964) ¸ 196• Society of Cosmetic Chemists of Great Britain THE VARIABILITY OF THE SKIN j. W. H. MALl* A lecture delivered before the Nederlandse Vereniging voor Cosmetische Chemie on 21st May 1963. In the pathogenesis of most common dermatoses the "condition '• of the skin at a given site and at a particular time is of major importance as the precipitating factor. A number of physiological parameters of skin condition, varying with season, sex or age are described. They mirror different "meta- bolic states" of the skin which, in the last analysis, determine its reaction. One might get the impression from cosmetic advertising and some textbooks on dermatology, that the skin is a more or less invariable fixed organ. In fulfilling its functions, however, a certain degree of variety is necessary. This is expressed in the anatomy of the various parts of the skin, which differs to some extent. This variety occurs not only in surface and depth but also as a fourth-dimensional quality in time, influenced by the seasons as well as by ageing. Moreover there are differences in skin structure between the sexes and the individuals. The anatomical and functional diversity is mentioned only briefly or not at all in dermatological textbooks. Emphasis is laid on gross morphologic changes or on those skin diseases in which the pathogenic factor and. the reaction of the organism as a whole determine the course of the disease. In normal practice, however, these textbook-diseases form only a part of the cases treated by the dermatologist. The majority of patients seek his help for minor or major syndromes which have to be classed as "rdactions .cutandes". These reactions are more or less disseminated, at times with a blotchy redness, sometimes itching, with an acute, sub acute or chronic inflammatory character. They can be provoked by a large number of ,external factors or by internal causes, reaching the skin via the peripheral blood flow. The reactions due to cosmetics in general fall, for the greater part, within this category. Because of its relatively simple structure, which may be represented as a number of parallel membranes, these reactions of the skin show a relatively constant pattern and are often determined more by the situation of a certain part of the skin of an individual at a certain '*Dept. of Dermatology, R.C. University of Nijmegen, Netherlands. 367

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.