56 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



Cilia are exclusively confined to epithelial cells. 

 There are three theories to account for the motion 

 of cilia: 



1 . The contraction may be intrinsic in the wall of 

 the cilium. This theory is supported by the fact 

 that the cilium or flagellum of a spermatozoon will 

 show motility when severed from the rest of the cell. 



2. Contraction of the base where the cilia are 

 attached. The cilia will continue to vibrate if a 

 fragment of the cell protoplasm remains attached to 

 them. 



3. The cilia are supposed to be hollow tubes with 

 walls of unequal elasticity. By forcing the proto- 

 plasm rapidly into these tubes ciliary motion is 

 produced. Pseudopodia are produced in this man- 

 ner, and the morphological relation of pseudopodia 

 and cilia is a close one. 



The one great physiological action of epithelium 

 seems to be to secrete fluids. Consequently epithe- 

 lium is found lining all cysts wherever the cyst is 

 located, in the ovary, the skin, or in connection 

 with the alimentary tract. Conversely, a cyst may 

 be formed wherever epithelium is found. 



Lastly, it is of the greatest importance that stu- 

 dents should be able to recognize epithelial cells. 

 The facts to be remembered are : 



1 . That they line surfaces. 



2. That they appear in compact layers. 



3. The oval or round distinct nucleus, usually 

 rich in chromatin. 



4. The regularity of the cells, i. e., they are of 

 one pattern, either squamous, columnar, ciliated, or 

 cubical. 



