Introduction 



23. Varieties of Epithelium. The various kinds of 

 epithelial tissues are usually classified as squamous, glandu- 

 lar, and columnar. 



The squamous, or pavement epithelium consists of very thin 

 flattened scales which form the outer layer of the skin and 

 the linings of the mouth. 1 



The glandular epithelium 

 forms the essential parts of 

 glands such as the liver and 

 the glands of the skin. 



The columnar epithelium 

 consists of pear-shaped or 

 elongated cells, found fre- 

 quently on the mucous mem- 

 brane of the stomach and 

 intestines, and on the lining 

 of the large air tubes. FIG. 6. Various Kinds of 



The ciliated epithelium (a Epithelial Cells. 



.' r ,, , x . A. columnar cells of intestine : B. poly- 



variety of the columnar) is ' hedral cells of the conjunctiva ; c> 



ciliated conical cells of the trachea ; 

 Z>, ciliated cell of frog's mouth ; E, 

 inverted conical cell of trachea ; F, 

 squamous cell of the cavity of the 

 mouth, seen on its broad surface; 

 G, squamous cell, seen edgeways. 



marked by the presence of 



very fine hair-like processes 



called cilia, which develop 



from the free end of the cell 



and exhibit a rapid whip-like 



movement as long as the cell is alive. This motion is 



always to and fro in the same direction, and serves to 



carry away mucus and even foreign particles in contact 



with the membrane on which the cells are placed. 



This kind of epithelium is especially common in the air 

 passages, where it serves to keep a free passage for the 

 entrance and exit of air. 



1 A variety of the smooth pavement epithelium, called endothelium, lines 

 the heart and blood vessels and is the characteristic tissue of serous 

 membranes, like the pleura and the peritoneum (sees. 167 and 213). 



