GROWTH AND DEVELOPMENT OF HAIR. 



253 



Fig. 43. 



V* 



tureless membrane, corresponding to the basement membrane of other parts. 

 The Epidermic lining of this follicle, which constitutes what is known as the 

 " root-sheath," is composed of two principal layers, 

 the one (g) in contact with the corium being the 

 continuation of the stratum Malpighii, and the one 

 nearest the hair (e, /) bearing a like relation to the 

 horny layer. 1 At the deepest portion of the follicle, 

 according to Prof. Kolliker, there arises a minute 

 papillary elevation of the Corium (7), which occupies 

 the centre of the hair-bulb ; and over this we find a 

 great accumulation of cells of spheroidal form, which 

 are obviously continuous at m, with those of the 

 outer root-sheath, and which are in every respect 

 analogous to those of the Malpighian layer of the 

 Epidermis. The envelop of imbricated scales (c, d\ 

 on the other hand, which the bulb as well as the 

 shaft of the hair presents, commences deep in the 

 follicle as a double layer of nucleated cells (, o), 

 which forms a kind of duplicature of the outer or 

 horny stratum of the Cuticle. The fusiform cells 

 of the fibrous portion of the shaft are continuous 

 with those of the outer part of the hair-bulb, which 

 are seen to undergo elongation (s), as they are 

 pushed upwards by the development of new cells 

 beneath; and thus, as they are at the same time 

 narrowed, the shaft comes to be of less diameter 

 than the bulb at its base. The cells of the medul- 

 lary substance are derived with less change from 

 those of the interior of the hair-bulb ; they are at 

 first colorless (r), but gradually acquire the dark 

 aspect which is partly due to the development of 

 pigmentary matter, but still more to the production 

 of air-spaces by their desiccation. Thus we see that 

 the whole tissue of the Hair is derived from Epider- 

 mic cells, developed in peculiar abundance from the 

 surface of the papilla at the base of the follicle which 

 is itself extremely vascular; some of these cells re- 

 taining their original form, whilst others are trans- 

 formed into fibres, and others converted (like those 

 of ordinary Epidermis) into flattened cells. They all 

 have the power, however, of drawing horny matter 

 into their cavities ; and resist the solvent power of 

 chemical reagents, except when these are employed 

 in unusual strength. The Hair is constantly under- 

 going elongation, by the addition of new substance 

 at its base ; and the part which has been once fully 

 formed, and which has emerged from the follicle, 

 usually undergoes no subsequent alteration. There 

 is evidence, however, that it may be affected by 

 changes at its base, the effect of which is propagated along its whole extent : 



Hair-bulb of a well-developed Hu- 

 man Hair, with its follicle : a, me- 

 dullary substance, containing air- 

 spaces, with indistinct cells; 6. 

 fibrous cortical substance ; c, d, inner 

 and outer layers of the scaly enve- 

 lop; e, /, inner and outer layers of 

 the internal root-sheath ; g, external 

 root-sheath; h, structureless mem- 

 brane; i, transverse fibre-stratum; 

 k, longitudinal fibre-stratum ; I, hair- 

 papilla ; TO, lowest cells of the hair- 

 bulb, continuous with those of the 

 external root-sheath; n, perpen- 

 dicularly-arranged nucleated cells. 

 which, near <?, become non-nucle- 

 ated, and are continuous with the 

 inner layer of the scaly envelop; 

 o, small perpendicularly arranged 

 cells, likewise nucleated, passing 

 into the outer layer of the same; 

 p, lowest portion of the inner root- 

 sheath; r, commencement of the 

 medullary substance in the condi- 

 tion of colorless cells ; s, part where 

 the cells of the bulb begin visibly 

 to lengthen themselves, to form the 

 fusiform cells of the shaft. 



1 According to Prof. Kolliker, it is by the laceration of a layer of flattened cells without 

 nuclei, which forms the outer stratum of the inner layer of the root-sheath, that the so- 

 called "fenestrated membrane" is produced, the presence of which between structures 

 corresponding to the Malpighian and horny layers of the Epidermis, has been a subject of 

 much perplexity to Microscopists. 



