JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS to the pattern and relative thickness of the medulla. (3) Examination of cuticular scale pattern. Knowledge 'of fibre characteristics resulting from such examination is of value to biologists, taxidermists, archaeologists, forensic scientists, while the microscopic examination of human hair is of importance in certain pathological scalp conditions. Histochemical studies carried out on fibre sections have yielded valuable information about the physico-chem- ical properties of cuticle, cortex and medulla 9'•ø (Figures 5 and 6), the significance of which in the formula- tion of permanent waving solutions has been described elsewhere. TM HAIR PIGMENTATION The pigmentation of hair has been studied extensively by geneticists, typical investigations being those of Onslow, Wright and Koller." Such work clearly shows that pigment formation results from the action of enzymes on a basic chromogen to give the basic colours, viz.: black, and red or yellow, the intensity and dis. tribution of which are governed by factors chiefly genetic in origin. The resulting coat patterns in animals behave as genetic characters while subject to variation which can often be modified by selection. In albin- ism (recessive whiteness) there is a complete failure to inherit the enzyme system, while in dominant whiteness an enzyme inhibitor is present and prevents formation of melanin. These views explain the changes in coat colour which can be produced experimentally in mature animals by shaving, or by local subjection to low temperatures, since the formation of enzyme and chromo- gen appears to be controlled by skin temperature, which constitutes an intermediate link in the chain of genetic causation. The absence of pigment from the coats of snow- field fauna doubtless results from a similar suppression of the chemical reactions leading to melanin forma- tion. The chemistry of melanin forma- tion has been examined by Raper, et aI., •3 who have described three stages in the oxidation of tyrosine to the black pigment. In the first stage tyrosine gives rise to a red pigment, which is followed in the second stage by formation of a colour- less substance. In the third stage the colourless substance is oxidised to melanin. The first stage is the only one requiring the presence of the enzyme (tyrosinase), and, since the second stage occurs in an atmosphere of nitrogen, it cannot be oxidative. At the end of the second stage the presence of 3:4 dihydroxyphenyla- lanine, 5: 6 dihydroxyindole and 5: 6 dihydroxyindole-2-carboxylic acid can be demonstrated, and in order to explain these facts Raper put forward the scheme shown on the next page. The mechanism of conversion of 5:6 dihydroxyindole and its 2-car- boxylic acid into melanin is obscure, and the structure of the pigment remains unknown. This classic work of Raper was followed by study of melanin precursors, first by Burton 162

HAIR --• / - • o//'x/7H.COOH ' NH• NH• MELANIN and more recently by others. The results of this synthetic work indi- cate that oxidative polymerisation of 5:6 dihydroxyindole to melanin appears to proceed via the 4:7 positions. Further light has been thrown on to the problem by Stoves' investigation of catalytic bleaching of hair. In this work the iron con- tent of pigmented and non- pigmented fibres was measured be- fore and after treatment with stabil- ised aqueous solutions of a ferrous salt. Specimens of black and white hair from the dog (Canis domesticus familiaris) were extracted with ether, alcohol and distilled water. The purified hair was treated for 16 hours at 25 ø C. with 1 per cent ferrous sulphate heptahych-ate plus 0.1 per cent potassium bitartrate buffered to pHS, a bulk liquor ratio of 1: 30 being used. After being washed for 24 hours in running water, the speci- mens were dried in vacuo over P,.O, at 78 ø C., and a known weight of hair dissolved in a mixture of nitric, sulphuric and perchloric acids. The iron content of the solution was determined colorimetrically using ammonium thioglycollate. Table I summarises the results obtained. Two facts emerge from the above figures. Firstly, the greater fixation of iron by pigmented hair, and secondly the greater natural iron TABLE I Per cent Iron Content of Hair Specimen ]Pigmented Non-pigmented -- Before treatment: A 0.0091 0.0042 713 0.0071 0.0047 After treatment: A 0.3367 0.1200 B 0.2656 0-1134 163