286 THE SKIN 



the hair bulb. It contains an abundant plexus of capillary blood-vessels 

 and a rich supply of non-medullated nerves. It also contains an undue 

 proportion of connective tissue cells. 



REGENERATION OF THE HAIR. Hairs are being continuously shed 

 and regenerated, the average life of a hair of the scalp being stated as 

 sixteen hundred days (Stohr). The shedding of a hair is first heralded 

 by an atrophy of its papilla and a cornification of its bulb. Growth 

 ceases, and the hair, firmly adherent to its root sheath, is gradually 

 carried, by the continued growth of the latter, nearer and nearer the 

 surface of the skin. 



Its excursion leaves behind a narrowed cell column which still unites 

 the hair with its former papilla. 



From this rudiment a new hair germ may form (Unna), a new papilla 

 develop, and the resulting hair grow toward the surface in the path of 

 the molting hair, its eruption being preceded by the falling of its pred- 

 ecessor. The formation of the new hair germ very probably occurs at a 

 point nearly corresponding with the insertion of the arrector pili muscle, 

 where there is a swelling of the root sheath which has been already men- 

 tioned as the matrix of the hair follicle. This matrix appears very 

 early in the development of the hair, but remains quiescent until regen- 

 eration becomes necessary, when the cells are said to proliferate and 

 grow downward filling the space between the atrophic hair and the new 

 bulb. 



In infancy and youth shed hairs are also compensated for by new 

 formation from hair germs appearing at the germinal border of the epi- 

 dermis, the process proceeding in the manner already described for the 

 development of the hair. 



THE SEBACEOUS GLANDS 



These are branched saccular glands which may be subdivided into 

 two classes, (1) those whose ducts open into the hair follicles, and (2) 

 those whose ducts open upon the free surface of the epidermis. The 

 former are by far the more numerous; the latter occur in the skin of 

 the face, red margins of the lips, labia minora, glans penis and prepuce, 

 and the tarsal glands of the eyelids. With the above exceptions the dis- 

 tribution of the sebaceous glands is coextensive with that of the hairs. 

 They are therefore absent from the palms of the hands and soles of 

 the feet. 



A sebaceous gland consists of a dilated saccular fundus, a constricted 



