178 HUMAN PHYSIOLOGY. 



flattened cells, which, from their exposure to the atmosphere, are hard 

 and horny in texture ; it varies in thickness from J- of an inch on the 

 palms of the hands and soles of the feet to ? Jg- of an inch in the 

 external auditory canal. 



APPENDAGES OF THE SKIN. 



Hairs are found in almost all portions of the body, and can be 

 divided into 



1. Long, soft hairs, on the head. 



2. Short, stiff hairs, along the edges of the eyelids and nostrils. 



3. Soft, downy hairs on the general cutaneous surface. 



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

 and about ^Q 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 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 hair protects the head from the heat of the sun and from the 

 cold, retains the heat of the body, prevents the entrance of foreign 

 matter into the lungs, nose, ears, etc. The color is due to pigment 

 matter. In old age the hair becomes more or less whitened. 



The sebaceous glands, embedded in the true skin, are simple 

 and compound racemose glands, opening, by a common excretory 

 duct, upon the surface of the epidermis or into the hair-follicle. 

 They are found in all portions of the body, most abundantly in the 

 face, and are formed by a delicate, structureless membrane, lined 

 by flattened polyhedral cells. The sebaceous glands secrete a pe- 

 culiar oily matter (the sebum}, by which the skin is lubricated and 

 the hairs are softened ; it is quite abundant in the region of the nose 

 and forehead, which often presents a greasy, glistening appearance ; 

 it consists of water, mineral salts, fatty globules, and epithelial cells. 



The vernix caseosa, which frequently covers the surface of the 

 fetus at birth, consists of the residue of the sebaceous matter, con- 



