80 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS could be further from the truth. Actually, the outer layer of the skin(epi- dermis) is constantly reproducing and transforming into an impervious horny layer which is gradually shed (the process of cornification). At the base of the horny layer lies the main protective barrier, an extremely dense impenetrable zone only 10 to 20 microns thick. The barrier is constantly be- :.2.? .i' e½-.':: -:-,-: - , Fig. 1.--Diagram of the mlcroanatomy of human skin.

BIOLOGICAL FACTORS AFFECTING PERCUTANEOUS ABSORPTION 81 ing manufactured and pushed outward to become stratum corneum, which is more spongy than the dense barrier zone. Closely related to cornification is wound repair, which starts automatically upon injury and ceases when the horny layer has been reconstituted. It would seem then that the vital forces in the skin strive constantly to exclude environmental matter. Must not the cosmetic chemist, then, be always aware of the need to design products which do not interfere with these natural protective devices, but do gently assist the skin while im- proving its external appearance? Should not a better understanding of the biochemical anatomy and physical chemical properties of the skin be help- ful in attaining this goal? CUTANEOUS ANATOMY Let us consider diagrammatically the anatomical zones a penetrating substance meets in transit from the air directly through the skin to the blood vessels (Fig. 1). In sequence, it encounteIs a surface film of emul- sified lipides, the horny layer (stratum comeurn), the barrier, living epi- dermis (stratum germinativum) and finally the dermis in which reside the !Poss !:B'LE A 9 •"N 0"'•.::g" THROUGH THE PEN ET:RAT::i 0 N [ Ntio UNBROKEN: S'K .I N A:N-C 'Between: t•e :(: ells :Of t'hb' Strut U m 'Cø•neum, • ' ThrOugh the Walls Through the • • the SeboCeous Throuc lh• : SWeet • .K. • •" : 'r•ro.c• ..the -of" the Stratum. "Cot neum. Fig. 2.--Possible avenues of penetration into and through the unbroken skin. blood vessels. Other avenues of penetration through the skin are shown diagrammatically in Fig. 2. The pathway through the epidermis is much more likely the main ave- nue of penetration than are the sebaceous glands or sweat glands, simply