THE KERATINIZED TISSUES 



71 



Hairs 



Most of our information concerning the properties of keratin comes from 

 a study of hair and wool which will thus, perforce, form the subject 

 matter of much of the discussion to follow. The reasons for this are 

 partly economic, funds for research being derived from the textile and 

 cosmetic industries, and partly experimental convenience, hairs being 

 easy to obtain in quantity, easy to purify, and their thread-like form 

 lends itself to many physical investigations. 



Hairs are as characteristic of mammals as feathers are of birds. When 

 fully grown they consist of a tapering tip, a shaft and a base normally 

 embedded in the skin; in cross-section, a cuticle, cortex and medulla 

 may be distinguished (Fig. 31). A great deal of variety can be produced 



Fig. 31. The parts of a hair seen in cross-section. The proportions may 

 vary greatly, and in fine hairs and wool the medulla may be missing. 



with these simple themes, every species of mammal having its own hair 

 style by which it may be characterized. 



The diameter and shape of a hair fibre and its parts changes from tip 

 to root. Often these features are characteristic and are employed in the 

 identification of the hair (Lochte, 1938; Hausmann, 1925; Wildman, 

 1955). Hairs normally cease growing after a more or less definite period, 

 thus achieving a genetically-determined length and, after a further period, 

 are shed usually as the result of a new growth. As with feathers the 

 hairs of successive generations from the same follicle may be different in 

 character. 



In what is usually taken as the primitive condition, hairs slope backwards 

 from head to tail and are rather uniform in type and length. This simple 

 pattern is approximated to in rodents; in other mammals great variations 

 involving slope reversals, whorls, tufts, etc., are common. The direction 

 of slope defines the hair-stream which is closely involved with other 

 structures present (scales and glands) and with the organization of the 

 permal fibrils. 



As with feathers, a great variety of hairs has been evolved to meet 



