EPITHELIA 



507 



their position and function, and the extent to which they maintain 

 their identity. 



If the cells make a single layer the epithelium is said to be 

 simple. According to the shape of the cells it is called pavement 

 or squamous (where they are flattened), columnar, cubical, or 

 polyhedral, but there are intermediate shapes. Columnar and 

 cubical epithelia may have cilia on their free surfaces, when they 

 are called ciliated, and columnar, cubical and polyhedral epithelia 





/■- 



>- 





Fig. 394. — Diagrams of epithelia. — From Le Gros Clark, The Tissues of the Body. 



3rd edition, 1952. Clarendon Press, Oxford. 



a.. Simple columnar ; b., pavement ; c, transitional ; d., ciliated colunmar, showing a mucus-secreting 



goblet cell (G.c). 



may be glandular, passing secretions through their walls. Pave- 

 ment or columnar and polyhedral epitheha may be associated 

 together to form three or four layers, giving transitional epi- 

 thelium, or several layers, when the result is stratified epithelium. 

 The outer layers of this are usually keratinised, and the cells 

 dead. Some of these types of epitheHum are shown in Fig. 394. 

 They may be summed up in the schema : 



Simple or 



transitional 



or 



stratified 



cubical I 



[' columnar j 



polyhedral 

 [ pavement 



cihated or 



glandular 



Examples of some functional types of epithelia will now be 

 described. 



