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,

234 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS .• i i i i i i ! i i i i I ' i i •..-o ,08 . ß •$C "X Fig. 4. The Effect of Acids and Alkalis on the Birefringence of Hair. Note the comparative resistance of hair to acid even at relatively high concentrations. On the other hand, in a 0.5 N solution of sodium hydroxide the birefringence is com- pletely lost. which are due to the fibrous structure and the three-dimensional weave. He can, by various processes, modify the weave pattern but he must never destroy it. To accomplish his aim, he must make the tannins penetrate and ensure that the oils used in finishing permeate the structure according to the particular requirements. Except for furs, very few leathers are made "in the hair." The presence of the impermeable epidermis brings about many difficulties. Furs are. always thin pelts and suitable tanning materials are applied to the flesh side. Of course, to be fair, there is little urge to attempt to penetrate the, epidermis because the reagents may spoil the fur. In ordinary leather manufacture, removal of hair and epidermis is the- first step, followed by gentle squeezing of the skin to get rid of the sebum, as its presence will hinder penetration and give an uneven grain surface. Although commercially clean, penetration of the tan into thick hides is a slow process, but if the conditions are wrong, complete tannage is almost impossible. In the end, when the properly tanned leather is taken out of the liquor, it will contain about 60 per cent water which, on drying, falls to. about 14 per cent, depending on the tannage and, of course, on the relative humidity of the atmosphere. Because of the fixing of the fibres by the, tannins, the drying causes no significant loss in thickness. Just like this, however, the leather would not be saleable. It must be oiled. It cannot merely be wiped over with an oily rag. If this is done the oil will darken,