32 HUMAN PHYSIOLOGY. 



body. The external investment (the skin) and the internal investment 

 (the mucous membrane, which lines the entire alimentary canal and its 

 associated body cavities) are both formed, in all situations, by the homo- 

 geneous basement membrane, covered with one or more layers of cells. All 

 materials, therefore, whether nutritive, secretory, or excretory, must pass 

 through epithelial cells before they can enter into the formation of the blood 

 or be eliminated from it. The nutrition of the epithelial tissue is maintained 

 by the 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 situatious, 

 especially on the mucous membranes, the cells consist largely of mucin, in 

 association with other proteins. The consistency of epithelium varies in 

 accordance with external influences, such as the presence or absence of mois- 

 ture, 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 aso possess varying degrees of cohesion and elasticity 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 radia- 

 tion of heat from the body, and cooperate with other mechanisms in main- 

 taining 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 considered in 

 connection with the functions of the organs of which they cons^ute 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 under- 

 lying 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 pre- 

 vents undue pressure on the nerves of the true skin. The density of the 



