STRUCTURE OF A HAIR- FOLLICLE. 



441 



sometimes as far as the subcutaneous tissue. The structure of a hair-follicle is the following : 

 1. The outer fibrous layer (figs. 294, 1, 293), composed of interwoven bundles of connective- 

 tissue, arranged for the most part longitudinally, and provided with numerous blood-vessels 

 and nerves. [It is just the connective-tissue of the surrounding chorium.] 2. The inner 

 fibrous layer (figs. 294, 2, 297) consists of a layer of fusiform cells (? smooth muscular fibres) 

 arranged circularly. [It does not extend throughout the whole length of the follicle.] 3. In- 

 side this layer is a transparent, hyaline, glass-like basement membrane (figs. 292, 3, 297), 

 which ends at the neck of the hair- follicle ; while above it is continued as the basement 

 membrane which exists between the epidermis and chorium. In addition to these coverings, a 

 hair-follicle has epithelial coverings which must be regarded in relation to the layers of the 

 epidermis. Immediately within the glass-like membrane is the outer root-sheath, (figs. 294, 4, 

 297, 298), which consists of so many layers of epithelial cells that it forms a conspicuous 

 covering. It is, in fact, a direct continuation of the stratum Malpighii, and consists of many 

 layers of soft cells, the cells of the outer layer being cylindrical. Towards the base of the hair- 

 follicle it becomes narrower, and is united to, and continues with the cells of the root of the 

 hair itself, at least in fully developed hairs. The horny layer of the epidermis continues to 

 retain its properties as far down as the orifice of the seba- 

 ceous follicle ; below this point, however, it is continued as 

 the inner root-sheath. This consists of (1) a single layer 

 of elongated, flat, homogeneous, non-nucleated cells (figs. 

 294, 6, 297, / Rentes layer) placed next and within the 

 outer root-sheath. Within this lies (2) Huxley's layer (figs. 

 294, 5, 297, g), consisting of nucleated elongated polygonal 

 cells (fig. 294, x, and 3), while the cuticle of the hair-follicle 

 is composed of cells analogous to those of the surface of the 

 hair itself. Towards the bulb of the hair these three layers 

 become fused together. 



[Coverings of a hair-follicle arranged from without in- 

 wards 



1 Fihron* 1-ivptn ) (a) Longitudinallyarranged fibrous tissue. 



1. fibrous layers, j (&) circularly arrange d spindle cells. 



2. Glass-like (hyaline) membrane. 



3. Epithelial j W uter >ot-sheath. 



W < (6) Inner root-sheath. 



layers, | ^ Cuticle of the hair# 



4. The hair itself. 



The arrector pili muscle (fig. 294, A) is a fan-like arrange- 

 ment of a layer of smooth muscular fibres, attached below 

 to the side of a hair-follicle and extending towards the 

 surface of the chorium ; as it stretches obliquely upwards, 

 it subtends the obtuse angle formed by the hair-follicle and Fig. 297. 



the surface of the skin, [or, in other words, it forms an acute transverse section of a hair and 

 angle with the hair- follicle, and between it and the follicle - ts f Hi c i e ^ a outer fibrous coat 

 lies the sebaceous gland]. When these muscles contract, 

 they raise and erect- the hair-follicles, producing the con- 

 dition of cutis anserina or goosesTcin. As the sebaceous 

 gland lies in the angle between the muscle and the hair- 

 follicle, contraction of the muscle compresses the gland and 

 favours the evacuation of the sebaceous secretion. It also 

 compresses the blood-vessels of the papilla ( Unna). 



The hair with its large bulbous extremity hair-bulb 

 sits upon, or rather embraces, the papilla. It consists of (1) 

 the marrow or medulla (fig. 294, i), which is absent in woolly hair and in the hairs formed during 

 the first year of life. It is composed of two or three rows of cubical cells (H, c). (2) Outside 

 this lies the thicker cortex (h), which consists of elongated, rigid, horny, fibrous cells (H, /, f), 

 while in and between these cells lie the pigment granules of the hair. (3) The surface of the 

 hair is covered with a cuticle (k), and consists of imbricated layers of non-nucleated squames. 



Grey Hair. When the hair becomes grey, as in old age, this is due to defective formation of 

 pigment in the cortical part. The silvery appearance of white hair is increased when small air- 

 cavities are developed, especially in the medulla and to a less extent in the cortex, where they 

 reflect the light. Landois records a case of the hair becoming suddenly grey, in a man whose 

 hair became grey during a single night, in the course of an attack of delirium tremens. 

 Numerous air-spaces were found throughout the entire marrow of the (blond) hairs, while the 

 hair-pigment still remained. 



[Blood-Pigment in Hairs. The feelers of albino rabbits contain in some part of their sub- 

 stances blood-pigment {Sig. Mayer).] 



[ Henle's layer. 

 I Huxley's layer, 



with b, blood-vessels ; c, inner 

 circularly disposed layer ; d, 

 glass-like layer ; e, outer, /, g, 

 inner, root-sheath ; /, outer 

 layer of the same (Henle's 

 sheath) ; g, inner layer of the 

 same (Huxley's sheath) ; h, 

 cuticle : I, hair. 



