EXCRETION 479 



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 followed 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 the skin. It consists of water, epithelium, pro- 

 teids, fat, cholesterin, and inorganic salts. 



The sebaceous glands are simple and compound racemose glands 

 opening by a common excretory duct on the surface of the epidermis or 

 into the shaft of a hair-follicle (Fig. 211). These glands are extremely numer- 

 ous 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 

 epithelium. 



The sebum is not produced by an act of true secretion, but is formed by 

 a proliferation and degeneration of the gland epithelium. When first 

 poured on the surface, the sebum is oily and semiliquid in character, but 

 soon hardens and acquires a cheese-like consistence. It serves to lubri- 

 cate the hair and skin and prevent them from becoming dry and harsh. 



The surface of the fetus is generally covered with a thick layer of seba- 

 ceous matter, the vernix caseosa, which possibly keeps the skin in a normal 

 condition by protecting it from the effects of the long-continued action of 

 the amniotic fluid in which the fetus is suspended. 



