THE HAIR 283 



THE EPIDERMAL BOOT SHEATH. The epidermal root sheath con- 

 sists of an inner and an outer portion, each of which at about the "mid- 

 level is 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 cannot all be demon- 

 strated, 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 consists of 

 thin horny epithelial scales which are imbricated downward, viz., toward 

 the hair bulb, and which interdigitate, in the deeper portion of the fol- 

 licle, 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 flattened, and in 

 which the semblance of a nucleus is sometimes present. It corresponds 

 to the flattened cell layer of the epidermis. 



The outer layer of the inner root sheath, layer of Henle, is frequently 

 wanting or imperceptibly blended with the preceding layer. Its cells 

 are clear and highly refractive and their nuclei can but rarely be demon- 

 strated in the usual microscopical preparations. The layer is seldom 

 more than one cell deep. It is homologous with the stratum lucidum of 

 the epidermis. 



Outer Root Sheath. The outer root sheath is continuous with the 

 stratum mucosum of the epidermis and therefore contains similar cell 

 types. The granular layer, as in the epidermis, is frequently absent or 

 rudimentary, but can be readily demonstrated in hematein-stained sec- 

 tions of the more highly developed hair follicles. It rests upon a layer, 

 several cells deep, of spheroidal prickle cells. The outermost layer of 

 the outer root sheath is formed by a basal layer of cylindrical cells. 

 It grows progressively thinner toward the hair bulb, where its cells 

 become mingled with the germinal cells of the hair matrix. 



DERMAL ROOT SHEATH. The dermal root sheath presents three 

 layers, an innermost basement membrane or glassy layer, a layer of 

 circular connective tissue fibers and a similar layer of longitudinal 

 fibers. These layers are obviously homologous with the basement mem- 



