12 ANATOMY FOR NURSES. [CHAP. II. 



Cilia have been shown to exist in almost every class of ani- 

 mal, from the highest to the lowest. In man their use is to 

 impel secreted fluids, or other matters, along the surfaces to 

 which they are attached; as, for example, the mucus of the 

 trachea and nasal chambers, which they carry towards the out- 

 let of these passages. 



Ciliated epithelium is found in the air passages, the genera- 

 tive organs, ventricles of the brain, and central canal of the 

 spinal cord. 



To recapitulate: The most important situations in which a 

 covering or lining of epithelial tissue is found in the body 

 are : 



1. On the surface of the integument, or external skin. 



2. On mucous membranes, or internal skin. 



3. On the inner surface of serous membranes, and on the 

 inner surface of the heart, blood-vessels, and lymphatics. 



4. Lining the ventricles or cavities of the brain, and the 

 central canal of the spinal cord. 



Some varieties of epithelium are specially modified to form 

 protective membranes ; others to elaborate or make secretions ; 

 others, again, to form smooth linings for opposing surfaces ; 

 and yet others to keep the surfaces they cover clean, by sweep- 

 ing outwards material that would otherwise accumulate, and 

 clog important passages. 



The hairs, nails, and the enamel of the teeth, are modifica- 

 tions of epithelial tissue. 



