48 TEXT-BOOK OF PHYSIOLOGY. 



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

 situations, especially on the mucous membranes, the cells consist 

 largely of mucin, in association with other albuminoids. The con- 

 sistency of epithelium varies in accordance with external influences, 

 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 elasticity- 

 physical properties which enable them to resist considerable pressure 

 and distention 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 mechan- 



^ isms in maintaining the normal 



FIG. 5. STRATIFIED SQUAMOUS EPI- 

 THELIUM (LARYNX or MAN). 

 X 240. i. Columnar cells. 2. 

 Prickle-cells. 3. Squamous cells. 



FIG. 6. STRATIFIED CILIATED EPI- 

 THELIUM. X 560. From the res- 

 piratory nasal mucous membrane 

 of man. i. Oval cells. 2. Spindle- 

 shaped cells. 3. Columnar cells. 



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 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 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 



