232 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the outer atmosphere. It also functions as a seal, preventing ingress to the base of the follicles, which is the site of mitotic activity. The protection of the collagenous tissue from extraneous deleterious substances is vital. This is because extraneous materials would find little or no difficulty in diffusing through the lymph-laden structure. Further, they could rapidly alter the water imbibition of the fibres, seriously affect their functions, and cause intense irritation of the nerve endings. The collagen fibre is quickly reactive to acids and alkalis and salt concentration. The swelling curve of gelatin is indicative of this (Fig. 2). This curve is chosen because gelatin simulates collagen, but being an unorganised structure the variations in water imbibition are more clearly shown. A peak of swelling is to be noted at pH 2-3, a minimum at about pH 4-8 and a subsequent increase on the alkaline side. Because of the solubilising effect of hydroxyl ions, no real maximum is attained at high degrees of alkalinity. At pH values in the iso-electric range, it is to be noted that swelling is increased by the addition of salt (NaC1). At about pH 7, the addition of salt decreases the water imbibition quite noticeably, even at a concentration of 1.2 per cent. Although these changes are small in relation to those produced at low and high pH values, they nevertheless are significant in the instance of living tissue. Thus, a loss of salt in the blood through excessive sweating can lead to swelling of tissue such as the brain, and if the increase be only 2 per cent headaches are produced at about 3 per cent or more, death is certain. These sort of changes in water imbibition are found with collagen fibres, •'J• ItaH T 7000 L 00o 5000 $00o •ooo IOOO I I I [ I I I I I I I I o • • 3 • 5 g 7 • q •o •l I• •H Fig 2. Swelling Curve of Gelatin. Note that in the i.e.p. area the presence of sodium chloride increases the water imbibition, whereas in the region of pH 7 (the pH value of blood) the presence of sodium chloride significantly decreases the water imbibition.

PENETRATION OF SKIN--DEAD AND ALIVE 233 but are only about 1/6 in magnitude. (The •)ercentage imbibition varies from animal to animal and with age.) In the living skin the environment is controlled by the buffer system of the blood the flow of the latter and the lymph can cater for small local changes. A sudden local increase of acid• as in a sting or lactic acid produced by strong exercise--may cause rapid swelling which hinders the "washing away" process. To protect this reactive system, nature makes use of the sulphur-containing protein, keratin. Its stability t9 extraneous conditions is paralleled by the bark of trees, the chitinous carapace of the infusoria and insects, and the cuticles of leaves. The growing epidermal cells, as they are pushed away from the surface of the corium, keratinise, lose water and form a dense, hard-wearing layer. Probably due entirely to the disulphide linkages, the keratinised epidermis resists water imbibition. The swelling curve of hair shows how little is the effect of acids and mild alkalis (Fig. 3). That changes in fact do occur is •oo Fig. 3. Swelling Curve of Hair. Note the small amount of water imbibition and the smallness of any changes until the pH value of the external liquid is 11 and above. best shown by the measurement of birefringence of hair (Fig. 4). On the acid side, some freeing from strain occurs, but only at high concentrations (SN and greater). On the alkaline side there is a pronounced effect, but even here the concentration required is in the range of 0.1 to 0.SN. NaOH. • It has not to be overlooked, however, that there are two sorts of macro- scopic openings in the epidermis of the skin--sweat ducts and hair follicles. In life, the sweat duct probably resists penetration by aqueous liquids owing partly to the outward pressure of the glands themselves, but also to a potential gradient. The hair follicle, being essentially oil-wetted, because of the sebum, will resist penetration by water but not by oil. The leather and fur industries are the ones concerned with dead skin as a structure, and the tanner has to treat the skin in order to render it imputresc- ible without ruining the main attributes, i.e., suppleness, plasticity, elasticity,