CHAPTER III 



EPITHELIUM 



WE have seen in the introductory chapter that 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 com- 



Fio. 25. From a section of the lung of a cat, stained with silver nitrate. N. Alveoli or air-cells, 

 lined with large flat, nucleated cells, with some smaller polyhedral nucleated cells. (Klein and 

 Noble Smith.) 



posed 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. 



Epithelia may be grouped into two great classes, each of which 

 may be again subdivided according to the shape and arrangement of 

 the cells of which it is composed. 



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