THE HAIR 279 



produces an indentation of the advancing hair column and gives rise to 

 a true dermal papilla of considerable size. 



THE HAIR BULB. Coincident with the formation of the papilla 

 there is an increased proliferation of the cells of the hair column by 

 which it is surrounded, and which therefore represents the future hair 

 bulb. Two other swellings appear in the hair column; one, the more 

 superficial, forming the anlage of the sebaceous gland, and the other, 

 the deeper, forming the so-called epithelial bed or matrix of the hair 

 which stands in close relation with the growth and future regeneration 

 of the hair. This second swelling is sometimes interpreted as simply 

 offering a point of attachment for the arrector pili muscle. 



The development of the hair papilla produces a slight evagination 

 of the epithelium of the hair bulb, which is just sufficient to redirect 

 the growth of central cells of the hair column toward the cutaneous 

 surface. It is thus that the younger cells which arise by mitosis in 

 the germinal layers of the hair bulb are pushed outward along the axis 

 of the hair column where they form the shaft of the future hair. The 

 growth of the hair from the germinal cells of the hair bulb is accom- 

 panied by beginning cornification of the newly formed cells of the primi- 

 tive hair shaft and of the intermediate cells of the hair column. The 

 growth of the shaft is, however, preceded by enlargement, vesiculation, 

 and fatty degeneration of the central cells of the hair column, thus 

 producing a central canal through which the hair may grow, and which 

 later becomes continuous with the hair canal of the epidermis. 



THE HAIR FOLLICLE. At this stage the hair column has become 

 differentiated into a peripheral follicle, the primitive root sheath, and 

 a central hair. Continued multiplication of the cells in the germinal 

 layer of the bulb pushes the advancing tip of the hair nearer and nearer 

 the surface until it forces its way into the epidermal hair canal. Finally 

 the thin cuticular covering is ruptured and the eruption of the hair 

 shaft occurs. 



Further differentiation of the cells of the epidermal root sheath and 

 the formation of a mesenchymal or dermal sheath of connective tissue 

 completes the development of the hair follicle. This process is fre- 

 quently repeated and results in the formation of new hairs not only 

 during fetal life, but also, in constantly decreasing numbers, through- 

 out childhood and adult life. 



The Mature Hair. Its development teaches that the hair follicle, 

 being formed as it were by an invagination of the epidermis, contains 

 a dermal and an epidermal sheath and that the outer portion of the 



