156 KERATIN AND KERATINIZATION 



in the sensitivity of the follicle to some periodic systematic stimulant 

 which is perhaps brought about by a graded difference in the rate at which 

 the growth inhibitors are dissipated. Whitely's (1958) findings are similar. 

 Periodic growth is also apparent in feathers, in the formation of growth 

 bars and perhaps daily variations also occur (Lillie and Wang, 1940; 

 Liidicke, 1959). 



Zig-zags, Curls and Crimps 



In the rat and mouse, the fine hairs are not straight, they divide into 

 short lengths separated by narrow nodes to form " zig-zags " (Dry, 1926 

 and 1928). This phenomenon seems to point to the existence of small 

 localized fluctuations in inhibitor concentration which affect more the 

 finer hairs, less deeply embedded, often more closely clustered, and able 

 to exert less command over supplies and/or with less chance to dissipate 

 inhibitor concentration than the stouter more deeply-seated primary hairs. 



Zig-zags are probably related to crimps and curls. Histologically these 

 latter modifications are associated with curved follicles and it seems not 

 unlikely that the basic cyclic activity of the bulb is linked to cyclic changes 

 in a curved and asymmetric follicle which co-operate to stabilize the 

 emerging wave-form, i.e. the period of the bulb becomes related to the 

 time taken for the cells to pass through the curved tube formed by the 

 upper part of the follicle. The remarkable regularity of the emerging 

 wave-form would suggest that some kind of feed-back, possibly 

 mechanical, must integrate the entire follicular activity. 



With stout hairs the simple existence of a follicle curved in the zone of 

 hardening (Chapter 6) would seem sufficient to produce a hair of more-or- 

 less constant curvature, i.e. a helix or simple curl, since the emerging hair 

 would retain the shape of the " mould " in which it was set (Fig. 66). The 

 curly locks usually found in the fleece or pelt are of extra-follicular origin 

 and seem to result from the tendency of the hairs after emergence to adhere 

 laterally to each other in clumps or bundles (Horio and Kondo, 1953). 



Crimps are more nearly planar wave-forms and often of impressive 

 regularity (Norris, 1931). They develop typically in fine flexible hair and 

 there is a close correlation between the wavelength of the crimp and the 

 ease of bending the fibre (diameter) which is taken advantage of in the 

 practical method of judging fibre diameter by eye. The variable curvature, 

 which gives rise to " sickle tips," is discussed on p. 150. 



The periodicities of crimped wool which comprise not only wave- 

 length but rhythmic changes in diameter, shape and chemical composition 

 (Mercer, 1954) force one to suppose that the various steps in its formation 

 are interconnected in such a way that information concerning the portion 

 of a wave already produced is fed back to the keratinizing and germinal 

 layers to control current production. We appeal here to a broad principle 



