216 THE SKIN 



differ in structure from the hair shaft. It possesses the same 

 three layers, the medulla, however, being very irregularly de- 

 veloped. 



The imbricated cells of its cuticle interdigitate with the simi- 

 lar cells of the cuticle of the root sheath in the deeper half of the 

 follicle ; in its superficial half, viz., above the opening of the seba- 

 ceous gland, a narrow space intervenes between the cuticle of the 

 hair and that of the root sheath. 



The axis of the hair root is always inclined at an angle to the 

 epidermis ; it therefore makes with the epidermis an obtuse angle 

 on one side and an acute angle on the other. The arrector pili 

 muscle is always found on the side of the obtuse angle ; it there- 

 fore, by drawing the hair follicle and its inclosed hair root nearer 

 the perpendicular, causes the erection of the hair. The sebaceons 

 gland is included in the angle between the arrector muscle and 

 the hair follicle. 



The Epidermal Root Sheath. The epidermal root sheath consists 

 of an inner and an outer portion, each of which is here and there 

 divisible into three layers corresponding to the three similar layers 

 of the horny and the mucous portions of the epidermis. In those 

 portions of the follicle and in those individual hairs in which the 

 process of cornification is less advanced these subdivisions can not 

 all be demonstrated, and it is only in the most highly developed 

 hairs that they are typically found. This is in accordance with 

 the structure of the epidermis, in which the subdivisions of its 

 horny and mucous portions are typically found only in the more 

 highly developed portions, e. g., the palms and soles. 



Inner Root Sheath. The cuticle of the inner root sheath con- 

 sists of thin horny epithelial scales which are imbricated down- 

 ward, viz., toward the hair bulb, and which interdigitate, in the 

 deeper portion of the follicle, with the similar scales of the hair 

 cuticle. The direction of the imbrication explains the removal 

 of the epidermal root sheath when the hair is artificially extracted. 



The mid-layer of the inner root sheath, layer of Huxley, one or 

 two cells thick, consists of horny cells which are somewhat flat- 

 tened, and in which the semblance of a nucleus is sometimes pres- 

 ent. It corresponds to the flattened cell layer of the epidermis. 



The outer layer of the inner root sheath, layer of Henle, is fre- 

 quently wanting or imperceptibly blended with the preceding 

 layer. Its cells are clear and highly refractive and their nuclei 

 can but rarely be demonstrated in the usual microscopical prepa- 



