HAIR 



397 



est are immediately above the bulb. The outer epithelial sheath grows in 

 a radial direction from the inner surface of the hyaline membrane toward 

 the shaft. 



Shortly before and after birth, there is a general shedding of hair, sub- 

 sequent to which the loss and replacement of individual hairs is constantly 

 taking place. A hair of the scalp is said to last 1600 days, but the duration 

 of other hairs has not been definitely determined. The process of removal 

 begins with a thickening of the hyaline membrane and circular fiber 

 sheath. The matrix cells cease to produce, first the inner epithelial sheath, 

 and then the cuticulae and shaft. The hollow bulb becomes a solid corni- 

 fied "club." The matrix cells increase without differentiating into hair 

 cells or sheath cells, and the clubbed hair, with its inner sheath, is forced 

 outward to the level of the orifice of the sebaceous gland, where it may 

 remain for some time (Fig. 406, D). The lower part of the outer epithelial 

 sheath, which has become empty, forms an epithelial strand which shortens 

 and draws the papilla upward; but the connective tissue sheath remains 

 behind, forming the "hair stalk." After some time, the columnar cells 

 of the epithelial bed proliferate, causing the epithelial cord to return to its 

 former depth (Figs. 407 and 408), and a new hair develops in the old sheath 

 upon the old papilla. The new hair in growing toward the surface com- 

 pletes the expulsion of its predecessor, which is dislodged together with 

 cells of the adjacent epithelial bed. 



SEBACEOUS GLANDS. 



The sebaceous glands are simple, branched or unbranched alveolar 

 structures situated in the superficial layer of the corium and usually ap- 



Epidermis. J f, ' = 



Corium. 



Cell with shrunken 

 nucleus. 



Cell with well-devel- 

 oped drops of secre- 

 tion. 



l{_ Cell with developing 

 drops of secretion. 



Cuboidal cell 



Pig. 400. A, PROM A VERTICAL SECTION THROUGH THE ALA NASI OF A CHILD. X 40. C. Stratum cor- 

 neum; M, stratum genninativum; t, sebaceous gland consisting of four sacs, a, duct of the same; w, 

 lanugo hair, about to be shed; h, sheath of the same, at the base of which a new hair, z, is forming. 



B, FROM A VERTICAL SECTION OF THE SKIN OF THE ALA NASI OF AN INFANT. X 240. Sac of a sebaceous 

 gland containing gland cells in various stages of secretion. 



pended to the sheath of a hair (Fig. 398). In connection with the lanugo, 

 a large gland may be associated with a very small bair (Fig. 409), and in 



