230 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Perhaps one may quote Macaulay's well-known phrase to start with, and having said "As every schoolboy knows," accept this as governing all the statements which are "old news." The fertilised ovum consists of three layers, the thin outer one or cuticle being the embryonic epidermis. As it grows it develops into the complexly structured epi. dermis, which includes the hair, nails, and the glands which secrete sweat and fats. In addition, the cells differentiate themselves into those which cover the lips, the eyes, and the surfaces of the mouth, and other portions of the oral and nasal passages. The second layer of the ovum builds up into the cutis vera or corium, which is mainly collagenous. This portion is the major constituent of the whole skin and acts as the integument enclosing the whole •)f the living body, with its multiplicity of organs and bones, etc. The thickness of this layer, as with the epidermis, varies according to its particular site on the body. The skeletal structure, the flesh, muscles and organs are derived from the inner portion of the ovum. From these the arteries, capillaries, blood vessels and nerves extend into the corium, maintaining the true skin as a living entity. The purpose of the skin is to enclose the living body, holding it together and at the same time protecting it from physical force--a sort of cushion. The corium is mainly composed of collagen fibres that appear to be funda- mentally fibrils which are gathered together into fibres and are held together as small bundles of varying thickness. The fibres are further held together in more or less parallel groupings into larger fibres or fibre-bundles. In general, the fibre-bundles constitute the corium below the level of the hair roots, merging off into fibres of less massive character towards the epidermal layer. The fibres appear to be held together by reticular tissue 2 (Fig. 1). In the young animal the reticular tissue is almost complete, but decreases in strength with age. The fibres and bundles of the corium interweave in all directions, but the angle, notably in a direction vertical to the surface, becomes less in those parts where flexibility is required. Running through the interweaving fibre structure are the elastin fibres which give added strength to the structure, notably round the arteries and the hair follicles. All this structural detail ensures the perfect flexibility that is necessary to permit movement of the body--an elasticity adequately tempered by plasticity. The three-dimensional structure is unique in character and properties. No man-made substitute has yet, if ever, simulated all its virtues, and it is easy to understand why, when it has been made imputrescible by the tanner, the product, leather, is supreme. It is one of nature's master- pieces in design to fit the job. In the living animal this functional, highly hydrated structure has to be

ß PENETRATION OF SKIN--DEAD AND ALIVE 231 i". "-•' &' :--'•: '-•',.-•½' ..r -', ,. -..•.• ..... . .•..,,, "'.2 ,•:•- •.'4½.½*',,. •' '• ,•: •--:' •.'• . .•. Fig. 1. Collagen Fibres swollen in 0-01 N Hydrochloric Acid. (Note the encircling bands of reticular tissue which hold the fibrils in fibres and also . hold the fibres in fibre-bundles.) protected against the external environment. It must be protected against water loss and against the inroads of bacteria and deleterious substances. This is the task of the epidermis. The epidermal appendages, the sweat glands, control the amount of water which has to be excreted in order to help regulate the body temperature, notably when the temperature of the body rises because of the external heat, or because of a high rate of muscular effort, or because of a high temperature due to fever. At the same time, evaporation of moisture through the epidermis occurs in the form of vapour--the insensible perspiration. The function of the sebaceous glands is to work in conjunction with the sweat glands and control the rate of loss of water as vapour. Sebum is able to hold water within itself and to allow it to diffuse at a controlled rate into