Biology of Man 461 



which is a fusion of epidermal cells supplied with keratin (a horny, pro- 

 tein material). When a hair is being formed, it first appears as a tiny 

 elevation below the skin surface, later to be erupted. As more is pro- 

 duced, the hair is shoved farther from the skin surface. The part of 

 the hair within the folHcle is called the root, while the remainder is 

 called the shaft. All skin is provided with folHcles, except the palms of 

 the hands, the soles of the feet, and the last portion of the fingers and 

 toes. The consistency of the hair depends upon the structure of the 

 follicle; a round follicle (in cross section) gives rise to straight hair, an 

 oval follicle to curly hair, and a rather flat, ribbon-shaped follicle to 

 wavy (kinky) hair. The color of the hair is determined by the quantity 

 and quality of pigments present and their relation to the transparent air 

 spaces within the hair. Loss of hair may be due to inheritance, certain 

 diseases, or other environmental factors. Certain types of baldness are 

 due to heredity. Fewer women than men are bald because the former 

 more rarely inherit the necessary baldness-producing determiners. The 

 base of each hair is supplied with a nerve and blood vessels for its nour- 

 ishment. Smooth muscle fibers in the dermis are attached to the hair 

 follicle so that the hair can be moved. 



Sebaceous (oil) glands are formed by the invagination of the Mal- 

 pighian layer of the epidermis into the dermis and are nearly every- 

 where associated with the hair follicles, being especially numerous on 

 the face and scalp. The oily secretion passes from the glands into the 

 hair follicle from which it passes to the surface, where the oil keeps 

 the hair and skin from becoming dry and brittle and prevents undue 

 evaporation or absorption of water and other liquids by the skin. Some- 

 times the glands become infected with pus-producing bacteria. 



Nails are produced from closely packed epithelial cells along the 

 furrow at the base of the nail. The nail is formed by a fusion of clear, 

 dead, horny, keratinized cells to produce a solid plate. 



Teeth are derived embryologically from epithelial tissues and are im- 

 bedded in the upper and lower jawbones for support and strength. 

 The part of the tooth above the gum is called the crown and is covered 

 with very hard enamel. The remainder of the tooth is composed of 

 softer dentine with its central canal (pulp cavity) which in turn con- 

 tains blood vessels and nerves. The teeth are attached to the jawbone 

 by a substance called cementum. Man, like all mammals, has a tem- 

 porary, "baby" set of teeth, twenty in number, which appear between 

 six months and two and one-half years of age. The permanent set, 

 thirty-two in number (Figs. 230, and 231), is composed (on each side 



