THE HAIR 281 



dermal and a dermal root sheath which together form the hair follicle. 



THE HAIR SHAFT. Sections of the hair shaft present a thin cuticle 

 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 flattened and very much 

 elongated horny epithelial cells, which frequently retain the remnant 

 of a nucleus, and whose keratized cytoplasm is often much pigmented; 

 or the axis of the coarser 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 

 paucity of pigment, produce the lighter shades of hair peculiar to certain 

 individuals. 



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 BOOT. The root of the hair, except for the fact that it 

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

 in structure from the hair shaft. It possesses the same three layers, the 

 medulla, however, being very irregularly developed. 



The imbricated cells of its cuticle interdigitate with the similar 

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

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

 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 epi- 

 dermis; 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 therefore, by drawing the hair 

 follicle and its inclosed hair root nearer the perpendicular, causes the 

 erection of the hair. The sebaceous gland is included in the angle be- 

 tween the arrector muscle and the hair follicle. Contraction of the 

 muscle may aid also in the expulsion of the sebum. Extreme contraction 

 of the arrectores pilorum muscles may result from fright, causing the 

 hair to 'stand on end' ; in a similar manner cold air may effect, through 

 the pilomotor fibers, the erector muscles of the small hairs distributed 

 over the body and cause the so-called 'goose-flesh.' 



