220 THE SKIN 



The Hair Papilla. The structure of the hair papilla is identical 

 with that of the vascular papillae of the derma except that it is 

 constructed upon a much larger scale. It consists of a conical or 

 club-shaped elevation of connective tissue which indents the ex- 

 tremity of the hair bulb. It contains an abundant plexus of capil- 

 lary 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 f ), a 

 new papilla develop, and the resulting hair grows toward the sur- 

 face in the path of the molting hair, its eruption being preceded 

 by the falling of its predecessor. 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 mentioned as the matrix 

 of the hair follicle. This matrix appears very early in the develop r 

 ment of the hair, but remains quiescent until regeneration becomes 

 necessary. 



Shed hairs are also compensated for by new formation from 

 hair germs appearing at the germinal border of the epidermis, the 

 process proceeding in the manner already described for the devel- 

 opment of the hair. 



THE SEBACEOUS GLANDS. These are compound 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 (Tyson's 

 glands), and the Meibomian glands of the eyelids. With the above 

 exceptions the distribution 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. 



* Text-Book of Histology. f Arch. f. mik, Anat., 1876. 



