228 ELEMENTARY PHYSIOLOGY 



The mucous membrane of the stomach and the intestine, and 

 the ducts of the glands which lie embedded in it, are lined by a 

 layer of elongated cylindrical or prismatic cells, arranged with 

 their long axes at right angles to the free surface. Such cells 



Fig. 221. Columnar Epithelium from the Intestinal Villus of a Babbit. 



form what is called columnar epithelium, and may exist in a 

 single regular layer, or in several layers more or less irregularly 

 arranged. 



The free surfaces of the epithelial cells of certain parts are 

 clothed with minute hair-like processes called cilia, there being 



Fig. 222. Ciliated Epithelial Cells from the Trachea of a Cat. 



from ten to thirty attached to each individual cell. These cilia 

 are perpetually in motion, lashing to and fro in such a manner 

 that all those of the same surface tend to propel any particles 

 which come into contact with them in one direction. Thus, if the 

 ciliated epithelium in question happens to line a cavity, all 

 foreign matter resting on the surface is urged on towards the 

 orifice of the cavity. 



