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, in parts of 

 the generative 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 ; and in the 

 recesses of secreting glands. 



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. 



5. Epithelial cells, variously modified, are also found in the 

 sensory terminations of the organs of special sense. 



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 ; 

 others to keep surfaces moist ; and yet others to keep the 

 surfaces they cover clean by sweeping 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. 



