490 TEXT-BOOK OF PHYSIOLOGY. 



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. 226) . 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 vernioc 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. 



