214 THE SKIN 



THE MATURE HAIR. Its development teaches that the hair 

 follicle, being formed as it were by an invagination of the epi- 

 dermis, contains a dermal and an epidermal sheath and that the 

 outer * portion of the latter, being identical with the deeper por- 

 tion of the epidermis, must possess a close structural resemblance 

 to the rete mucosum, while its inner portion, like the horny layer 

 of the skin, is more or less cornified. There is thus an outer and 

 an inner epidermal root sheath corresponding respectively to the 

 mucous and horny layers of the epidermis ; the cornified portion, 

 inner root sheath, becomes progressively thinner toward the hair 

 bulb. The hair, on the other hand, represents an excessively de- 

 veloped horny layer whose rete mucosum is found in the germinal 

 layer of the hair bulb. 



The mature hair is divisible into a hair shaft or free portion, 

 and a hair root or concealed portion. The latter is inclosed within 

 an epidermal and a dermal root sheath which together form the 

 hair follicle. 



The Hair Shaft. Sections of the hair shaft present a thin cuti- 

 cle which consists of delicate horny scales whose free edges are 

 imbricated upward, viz., toward the tip of the hair. Within the 

 cuticle the hair may consist solely of a hair cortex formed by flat- 

 tened and very much elongated horny epithelial cells, which fre- 

 quently retain the remnant of a nucleus, and whose keratized 

 cytoplasm is often much pigmented ; or the axis of the hair may 

 contain enlarged angular cells in which eleidin granules and much 

 pigment are found. In the latter case the hair is said to possess 

 a medulla. The medulla is seldom if ever present throughout the 

 entire length of the hair. When present it sometimes contains 

 numerous air bubbles which, together with the absence of pig- 

 ment, produce the lighter shades of hair peculiar to certain indi- 

 viduals. 



In the light of its development it is obvious that the several 

 layers of the hair shaft are comparable to the homologous layers 

 of the horny epidermis, the cuticle, cortex, and medulla of the hair 

 being respectively homologous with the scaly layer, the flattened 

 cell layer, and the eleidin containing layer or stratum lucid um of 

 the epidermis. 



The Hair Root. The root of the hair, except for the fact that 

 it is immediately invested with a hair follicle, does not in any way 



* The terms inner and outer as applied to the hair follicle refer respectively 

 to points nearer or farther from the axis of the hair root. 



