THE CONNECTIVE TISSUES. 31 



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 con- 

 ductors 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 nutrition, but affords those materials out of which are 

 formed the secretions of the glands, whether of the skin or mucous 

 membrane. 



The cells lining the blood- 

 vessels, the lymph-vessels, the peri- 

 toneal, pleural, pericardial, and 

 other closed cavities are usually 



i 



FIG. 4. STRATIFIED SQUAMOUS FIG. 5. STRATIFIED CILIATED 



EPITHELIUM (LARYNX OF MAN). EPITHELIUM. X 5 6 - From the res- 



X240. i. Columnar cells. 2. Prickle- piratory nasal mucous membrane 



cells. 3. Squamous cells. (Stohr.} of man. i. Oval cells. 2. Spindle- 



shaped cells. 3. Columnar cells. 

 (Stohr.} 



termed endothelial cells. The cells in these situations are flat, irregular in 

 shape, with borders more or less wavy or sinuous in outline. 



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: 



1. 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, microorgan- 

 isms, etc., which would otherwise impair their vitality. Wherever con- 

 tinuous 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 

 influence of acids, alkalies, and poisons. 



2. To promote absorption. Inasmuch as the skin and mucous membranes 



cover the surfaces of the body, it is obvious that all nutritive material 

 entering the body must first traverse the epithelial tissue. Owing to 

 their density, however, the epithelial cells covering the skin play but a 



