40 



STKTJCTUEAL UNITS OF HUMAN TISSUES 



composed are called epithelial cells. As we have already 

 noted, one "of the functions of epithelial tissue is protec- 

 tion. A second function, exhibited only by certain layers 

 of this tissue, is secretion. Thus the skin that covers the 

 flesh and the mucous membrane that lines the mouth, 

 both prevent injury to the underlying parts, but in addi- 

 tion to this protective action the mucous membrane • 

 secretes a liquid (the mucus) which keeps that mem- 

 brane moist and flexible. All epithelial cells consist of a 

 large oval nucleus floating in protoplasm of a granular or 

 netted appearance, while the cell walls are extremely thin. 

 Epithelial tissues are classified according to the shape and 

 arrangement of the cells which compose them. Thus we 

 speak of pavement, cubical, columnar, spheroidal epi- 

 thelium, according as the cells which compose them are 



Fig. 11 — Forms of epithelial tissue; a, 1, cylindrical cells; a, 2, squamous cells 

 showing cement connection; 6, ciliated epithelial cells ; c, statified epithelium, 

 showing, 1, a pavement cell, 3, 4, spheroidal cells ; d, columnar epithelium, com- 

 posed of ciliated cylindrical and spheroidal cells. 



flat, cubical, cylindrical, or spherical in shape (Fig. 11). 

 When the layers are more than one cell layer deep we call 

 it stratified epithelium. The cells scraped from the inside 

 of the cheek (see Ex. XXII.) illustrate the flattened or 



