43 8 TEXT-BOOK OF PHYSIOLOGY. 



They consist of a root and a shaft. The shaft is oval in shape 

 and about 4-^ of an inch in diameter; it consists of fibrous tissue, 

 covered externally by a layer of imbricated cells, and internally by 

 cells containing granular and pigment material. 



The root of the hair is embedded in the hair-follicle, formed by 

 a tubular depression of the skin, extending nearly through to the 

 subcutaneous tissue ; its walls are formed by the layers of the corium, 

 covered by epidermic cells. At the bottom of the follicle there is "a 

 papillary projection of amorphous matter, corresponding to a papilla 

 of the true skin, containing blood-vessels and nerves, upon which 

 the hair- root rests. The investments of the hair- roots are formed of 

 epithelial cells, constituting the internal and external root-sheaths. 



The lower portion of the hair 

 follicle is connected with the upper 

 surface of the derma by bundles of 

 non-striated muscle-fibers which are 

 termed arrectores pilorum muscles. 

 Their inclination and insertion are 

 such that their contraction is fol- 

 lowed by erection of the hair follicle 

 and hair shaft. These muscles are 

 excited to action by nerves termed 

 pilo-motor nerves. 



THE SEBUM. 



The sebum or sebaceous 



matter is a peculiar oily material 

 produced by specialized glands in 

 FIG. 2oo.-LARGE SEBACEOUS GLAND. the skin - Jt consists of water, epi- 

 i. Hair in its follicle. 2,3,4,5. thelium, proteids, fat, cholesterin, 



Lobules of the gland. 6. Excre- ^A inm-o-QrnV calfc 

 tory duct traversed by the hair. ^ ^organic Salts. 



(Sappey.} The sebaceous glands are 



simple and compound racemose 



glands opening by a common excretory duct on the surface ~bi" Fhe 

 epidermis or into the shaft of a hair-follicle (Fig. 200). These 

 glands are extremely numerous and found in all portions of the 

 body, with the exception of the palms of the hands and soles 

 of the feet, and most abundantly in the face. They are formed 

 by a delicate structureless membrane lined by polyhedral epithe- 

 lium. 



The sebum is not produ^ fry an act of true secretion T bufcJ t s 



formed by a proliferation and degeneration of the gland epithelium. 

 When first poured on the surface, the sebum is oily and semi-liquid 

 m character, but soon hardens and acquires a cheese-like consistence. 



