CHAPTER XXXIX 

 THE COVERING OF THE BODY 



234. The Skin. The delicate and sensitive tissues of the 

 body are everywhere covered with a protective layer called 

 the skin. Not only is the entire surface of the body thus pro- 

 tected, but the mucous and serous membranes, the first named 

 lining all the passages of the interior to which air has access, 

 and the serous membrane lining the closed cavities, are essen- 

 tially modified skin. Even the teeth, the hardest struc- 

 tures in the body, are modifications of this tissue, as are also 

 the hair and the nails. The most important function of the 

 skin is to protect the body from germs and mechanical injury, 

 but it also aids in excretion and in regulating the temperature 

 of the body. Certain of the special senses are also located in 

 the skin. 



235. Structure of the Epidermis. There are two layers of 

 the skin known usually as the epidermis or cuticle and the 

 dermis, respectively. The epidermis is on the outside and con- 

 sists of many layers of cells, flattened near the surface and 

 more rounded deeper in the tissue. It has neither blood ves- 

 sels nor nerves, and derives its materials for growth and repair 

 from the dermis. It is constantly wearing out, as may be 

 inferred from the rapidity with which a stain disappears from 

 it, and is as constantly renewed by the living cells in contact 

 with the dermis. The epidermis varies in thickness on various 

 parts of the body, being thickest on the palms of the hands and 

 the soles of the feet. It may become thicker elsewhere when 

 an unusual amount of wear is brought upon it. Corns and 

 callouses are due to the attempts of the epidermis to protect 



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