JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS structure, although fully oriented, is still unstable. Urea rapidly dis- solves it moderate temperatures lead to disorientation with a fall in birefringence, but without a con- traction in the longitudinal direction of the shaft. After heating, the zone yields a disoriented Jg-pattern. The zone can be stabilised by the intro- duction of artificial cross-linking. (d) The zone of consolidation. Hard- ' ening becomes detectable about half- .way along the pre-keratinous region and is thereafter progressive. Above a fairly definite level the protein only swells in urea, is not digested b• enzymes, and is not disoriented at 90 ø C. Heating to a higher tempera- ture causes disorientation with a contraction in'length. (e) The fully hardened hair. Cortical cells are here elongated and show birefring- ence indicative of molecular orienta- tion and crystallinity, while the sulphur is chiefly in the form of cystine disulphide linkages which bridge adiacent polypeptide chains. These linkages confer considerable stability upon the fibre, which, in the fully keratinised state, has been likened to a vulcanised protein. Nevertheless, under certain condi- tions, the cystine disulphide linkage is very labile, a phenomenon which is discussed later in this paper. Cortical cells constitute the bulk of the fibre substance in human hair, and are regarded as the seat of such physico-chemical properties as stress/ strain characteristics and supercon- traction. As a result of the X-ray and physico-chemical. investigations of the Leeds school of workers, it is now accepted that the more highly organised regions of normal, un- stretched hair consist of incom- pletely extended polypeptide chains cross-linked by salt and cystine disulphide linkages, to form sheets or grids, such grids being held together by the attraction of secondary van der Waals forces. Other linkages involve hydrogen sharing between imino- and carbonyl-groups. The nature of the fold in the polypeptide chains is not at present fully under- stood. For a consideration of views of Astbury, together with the critic- isms of later workers, reference should be made to an earlier paper in this Journal. 8 Medulla cells present interesting departures from the above types. The walls of the air-filled cells contain little, if any, cystine, and the keratin is in the •-form. A lipo-protein- sterol complex is also present? and is doubtless responsible for the great resistance to attack by alkali shown by medulla cells. HXSTOLOGIC^L EXAmNATXON OF H_ax• The value of microscopic examina- tion as a basis for fibre classification has long been recognised. In general, the identification of fibres is based on the results of macro- and micro- examination. The histological exam- ination proper consists of three parts, • viz.: (1) Study of cuticle, cortex and medulla as revealed by trans- verse sections. (2) Examination of longitudinal sections with special reference 160

HAIR Figures I to 6 are shown herewith: top left, Fig. I top right, F•g. 2 middle left• Fig. 4 middle right, Fig. 5 lower left, Fig. 3 lower right, Fig. 6. Fig. I. Hair replacement, Fig. 2. Pattern of growing hair in the rat. Fig. 3. Primitive hair pattern (otter), Fig. 4. Hair patterns on lion skin. Fig. 5. Medulla stained with Safranine B. Fig. 6. Cortex stained with Kiton Red G. 161