APPENDAGES OF THE SKIN. 89 



tints in different individuals and in different races of men ; the more super- 

 ficial cells are large, colorless, and semi-transparent. The latter, the corne- 

 ous layer, is composed of flattened cells, which, from their exposure to the 

 atmosphere, are hard and horny in texture ; it varies in thickness from y& 

 of an inch on the palms of the hands and feet, to the ^-Q 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 (i) 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, which is oval in shape, and 

 about the ? ^ 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 papillae 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 cold, retains the 

 heat of the body, prevents the entrance of foreign matter into the lungs, 

 nose, ears, etc. The color is due to the pigment matter, which, in old age, 

 becomes more or less whitened. 



The Sebaceous Glands, imbedded 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 peculiar oily matter, the sebum, by which the 

 skin is lubricated and the hairs softened ; it is quite abundant in the region 

 of the nose and forehead, which often present a greasy, glistening appear- 

 ance ; it consists of water, mineral salts, fatty globules, and epithelial 

 cells. 



The Vernix caseosa which frequently covers the surface of the foetus at 



