36 HUMAN PHYSIOLOGY. 



nutritive material derived from the blood diffusing itself into and 

 through the basement membrane. Chemically, the epithelial cells of 

 the epidermis hair, nails, etc. are composed of an albuminoid 

 material (keratin), a small quantity of water, and inorganic salts. 

 In other situations, e'specially on the mucous membranes, the cells 

 consist largely of mucin, in association with other proteids. The 

 consistency of epithelium varies in accordance with external influ- 

 ences, such as the presence or absence of moisture, pressure, friction, 

 etc. This is well seen in the skin of the palms of the hands and the 

 soles of the feet situations where it acquires its greatest density. 

 In the alimentary canal, in the lungs, and in other cavities, where 

 the reverse conditions prevail, the epithelium is extremely soft. 

 Epithelial tissues also possess varying degrees of cohesion and elas- 

 ticity physical properties which enable them to resist considerable 

 pressure and distension without having their physiologic integrity 

 destroyed. Inasmuch as these tissues are poor conductors of heat, 

 they assist in preventing too rapid radiation of heat from the body, 

 and cooperate with other mechanisms in maintaining the normal 

 temperature. The physiologic activity of all epithelial tissue depends 

 on a due supply of nutritive material derived from the blood, which 

 not only maintains its own nutrition, but affords those materials 

 out of which are formed the secretions of the glands, whether of 

 the skin or mucous membrane. 



Functions of Epithelial Tissue. In succeeding chapters the form, 

 chemic composition, and functions of epithelial cells will be con- 

 sidered in connection with the functions of the organs of which they 

 constitute a part. In this connection it may be stated in a general 

 way that the functions of the epithelial tissues are : 

 i. To serve on the surface of the body as a protective covering to the 

 underlying structures which collectively form - the true skin, thus 

 protecting them from the injurious influences of moisture, air, dust, 

 microorganisms, etc., which would otherwise impair their vitality. 

 Wherever continuous pressure is applied to the skin, as on the 

 palms of the hands and soles of the feet, the epithelium increases 

 in thickness and density, and thus prevents undue pressure on the 

 nerves of the true skin. The density of the epidermis enables it to 

 resist, within limits, the injurious influences of acids, alkalies, and 

 poisons. 



