558 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES Ul 



c. Ho} ny Substances (Keratin) 



Microscopic structure and birefringence of hair. The great technicaf 

 importance and the remarkable elastic behaviour of wool and other 

 hairs were the incentive to research on keratin. 



Microscopically, hairs consist of three layers, viz., a scaly and un- 

 pigmented epidermis which is covered by a very thin cuticle or 

 epidermicula (Lindberg, Philip and Gralen, 1948; Schuringa and 

 Algera, 1950), a thick, fibrous cortical layer containing pigment, and 

 a parenchymatous pith. Sometimes there is no pith, as in Merino wool. 

 The surface skin, which covers the cortex with scales that are ring- 

 shaped or like roofing tiles, may likewise disappear owing to mechanic- 

 al chafing, and yet the elastic and optical properties of the hair will 

 not radically change. Their source is, therefore, the keratin fibre cells 

 of the cortex, which consist of numerous tonofibriUae orientated in 

 parallel. In the electron microscope the fibrillae can be seen to unravel 

 into still finer subfibrillae (Reumuth, 1942). The tonofibriUae vary in 

 length between 50 f.i and a few millimetres, being about 80 pL (Hohnel,. 

 1887) in sheep. They are usually flattened. Although a hair appears 

 to be optically homogeneous, it is not comparable cytologically to- 

 a single bast fibre, but to multicellular strands of bast fibres consisting 

 of relatively short fibre cells, as they occur in Monocotyledons (sisal^ 

 Manila hemp, etc.). 



. Unlike cellulose fibres, horn fibres are extremely elastic. In cold 

 water a hair can be stretched reversibly by 50 to 70^0, whereas bastr 

 fibres of good fibrous structure break when stretched only a feu^ 

 per cent. The elastic elongation of the hairs is especially impressive 

 under the polarizing microscope (Pochettino, 1913). Although the 

 cross-sectional area of the hair decreases owing to the elongation, the 

 retardation increases considerably, and this is apparent from the sharp- 

 rise in interference colours. It is a fascinating spectacle to watch the 

 polarization colours of weakly pigmented (fair) hair changing as the 

 hair is stretched and released. Whereas photo-elastic effects of this 

 kind, however, are usually brought about by slight changes of 

 distance in the crystal lattice which are not detectable by X-rays- 

 (Wiener, 1926b), the molecular frame of keratin is completely re- 

 formed during elongation. 



In curly wool the stretched outer side takes basic dj-es (Janus green^ 

 neutral violet, pyronine) more easily, and has a lower refractive index. 



