CHAPTER III 



EPITHELIUM 



THE elementary tissues of which the organs of the body are built 

 up may be arranged into four groups : epithelial, connective, muscular, 

 and nervous. The first of these, the epithelial tissues, follows 

 naturally on a study of the animal cell, as an epithelium may be 

 defined as a tissue composed entirely of cells united by a minimal 

 amount; of cementing material. As a rule, an epithelium is spread 

 out as a membrane covering a surface or lining the cavity of a hollow 

 organ. 



These epithelia may be grouped into two great classes, each of 

 which may be again subdivided according to the shape and arrange- 

 ment of the cells of which it is composed. The following table gives 

 the principal varieties : 



CLASS 1. Simple epithelium; that is, an epithelium consisting 

 of one layer of cells only. Its subgroups are as follows : 



a. Pavement epithelium. 



b. Cubical and columnar epithelium. 



c. Ciliated epithelium. 



CLASS 2. Compound epithelium; that is, an epithelium consist- 

 ing of more than one layer of cells. Its subgroups are as follows : 



a. Transitional epithelium. 



b. Stratified epithelium. 



This classification does not include the more specialised forms of 

 epithelium found in secreting glands, or in the sense organs, nor 

 structures like hair, and enamel of tooth, which are epithelial in 

 origin. These will be considered in their proper place later on. 



Pavement Epithelium. 



This consists of a layer of thin cells, arranged like flat pavement- 

 stones accurately fitting together and united by a small amount of 

 cementing material. The structure of the cells and their outlines 

 may be best demonstrated in the following way : 



A portion of the fresh tissue is taken and immersed for a few 



