110 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS eccrine sweat glands into all other body regions of man served solely thermoregulatory purposes. It is of interest that at their presumably original sites in the palms and soles of man, the sweat glands do not re- spond for all practical purposes to heat stimuli, but are strongly influenced by emotional tensions (12). The role of the sweat glands in dissipating body heat by evaporation of water cannot be overestimated. Atropinized cats who lose their ability to sweat only on the small areas of their paws, succumb much more easily to high-temperatures than normal cats (14). To produce artificial evaporative heat loss, animals resort to all sorts of devices: they lick their coats, immerse themselves in mud or wallow in their own urine (12). In man, the heat lost through sweating amounts to about 600 kcal./sq. m./hr. and the volume of sweat excreted may reach 1 L/hr. or more. Sweating also decreases the insulating power of the horny layer. A moist horny layer conducts heat much better and thus contributes to the lowering of the body temperature. Heat protection also includes an indirect delayed effect, as part of the acclimatization process. After heavy sweating of a few days' duration, the body cuts down on the salt lost through the sweat glands, by mobilizing adrenocortical hormones. Thus the organism is protected against de- mineralization (12). A salt-poor sweat has also the advantage of faster evaporation. 4. Against ultraviolet light we have no immediate protection. "Fog- burns, .... wind-burns," from excessive exposure to ultraviolet light in fog or wind, are clear indications of inadequate warning and protective de- vices. All protection against ultraviolet light is of a delayed nature. The first reaction is the darkening of pigment precursors by visible rays within one hour after exposure. The erythema which appears one to two hours later and reaches its maximum withih the first day, is followed by tanning. The tanning is caused by increased production of melanin by the melanocytes (15) and upward migration of pigment. A much later protective mechanism is the thickening of the horny layer which effectively screens the ultraviolet rays from reaching the cellular layers of the epi- dermis (16). It is still undecided which component in the horny layer absorbs the ultraviolet rays. The most recent candidate is urocanic acid, a metabolite of histidine and glutamic acid (17). 5. Protection against chemical agents is threefold: physical, chemical and physiological. The physical device, the most important of all of them, consists of a barrier at the base of the horny layer. This barrier, often called stratum corneum compacturn, though only one or two cell layers thick, offers effective resistance against the penetration of electrolytes (18). The skin is unable to ward off a large number of lipid-soluble substances, many of them industrial poisons (19). Against acids and alkalies the

FUNCTIONS OF THE SKIN 111 horny layer has developed very efficient buffering systems which neutralize these agents. Chemical combinations with some externally applied com- pounds, like heavy metals or sulfur (20) also take place and may serve a detoxifying purpose. When external irritants have pierced through the epidermal barrier, as in insect bites, in the corium the triple response of erythema, whealing and flare serves the purpose of walling in and washing away the toxic agent. 6. Against infections the surface offers physical and chemical protection. Drying-out and actual killing of bacteria are both effective means of dis- posing of micro Srganisms. In addition to its bacteriostatic effect, sebum also has a fungistatic action, especially pronounced against Microsporch audouini (1). PERCEPTION In order to mobilize these defensive devices, the skin had to develop an elaborate warning system, notifying the organism of impending danger. Cutaneous perception is best suited to transmit signals about mechanical (touch and pressure), cold and painful stimuli it is less ready to pick up heat stimuli, while the phylogenetically most recent stimuli, ultraviolet light and ionizing radiations, do not trigger off any warning device. Cu- taneous perception is essential for our well-being. In its absence, serious damage is inflicted upon the skin, as in impaired cutaneous sensations of all kinds. The ability to distinguish between the various forms of stimuli does not reside in the receptors themselves, but is accomplished in the conducting systems or in the center (21). HEAT REGULATION The third most important function of the skin, heat regulation, has already been briefly touched upon when protection against heat was dis- cussed. The skin participates solely in heat dissipation, not in heat pro- duction. Heat is lost from the surface in four ways: by conduction, con- vection, radiation and evaporation. Conduction is the transmission of heat to bodies directly in contact with the skin convection is transmission of heat in currents of air radiation is the emission of electromagnetic waves at the speed of light. At comfortable temperatures most of the heat is lost by radiation at high temperatures and humidities evaporation accounts for the bulk of heat loss (22). During breakdowns in the evaporative mechanism, as in the sweat retention syndrome (plugging of sweat ducts), or in heat exhaustion, the body temperature rises with all its undesirable sequelae (1). UPTAKE Percutaneous uptake of water and metabolites, essential for the survival