14 GENERAL STRUCTURE OF THE BODY. 



the intercellular substance, which was at some time pro- 

 duced by the cells. 



In epithelial tissue there is little intercellular substance, 

 the cells being close together and arranged generally as a 

 skin or membrane covering external or internal surfaces. 

 When there are several layers of cells, the deepest are 

 columnar in shape and the others become more and more 

 flattened and scale-like as they approach the surface, 

 where they are gradually rubbed off and replaced by the 

 growth of new cells from below. This stratified epithe- 

 lium, as it is called, is found wherever a surface is exposed 



FIG. 1. Epithelium: 1, pavement epithelium; 2, columnar epithelium; 3, 

 ciliated epithelium; 4, stratified epithelium. 



to friction, as in the skin and in the mucous membrane of 

 the mouth, pharynx, and esophagus, and in that of the 

 vagina and the neck of the uterus. In simple epithelium, 

 where there is only a single layer of cells, the cells may be 

 pavement or hexagonal, columnar, glandular, or ciliated, 

 according to their different functions. The flat pave- 

 ment cells occur where a very smooth surface is required, 

 as in the heart, lungs, blood-vessels, serous cavities, etc. 

 None of these surfaces communicate directly with the 

 external surface of the body and the name endothelium is 

 substituted for epithelium. The columnar form of cell in 

 the intestine facilitates the passage of leucocytes between 

 the cells. In glandular epithelium the cells vary accord- 

 ing to the gland in which they occur, their protoplasm 



