EPITHELIUM. 235 



conoid. These two forms may be distinguished by the appel- 

 lations of Tesselated or Pavement Epithelium, and Cylindrical 

 or Conoidal Epithelium. (See Plate XX. fig. 1. and 2.) 



The Conoidal Epithelium admits of subdivision into Naked 

 Conoidal Epithelium and Ciliated Conoidal Epithelium. 



TESSELATED EPITHELIUM. 



Form. The cells of this description of epithelium form many 

 layers, are flattened, and either circular, polygonal, or irregular 

 in outline : the younger cells are mostly of the first shape, and 

 are thicker than the older ones, which are irregular in form, 

 thin and membranaceous, while the polygonal cells are en- 

 countered more particularly in certain situations, as on the 

 choroid plexus, pericardium, and serous membranes in general. 

 The polygonal shape is produced by the mutual compression 

 exerted by the cells upon each other, and consequent adapta- 

 tion. 



Size. The size of the cells of pavement epithelium varies 

 both according to age and locality ; the younger and deeper 

 seated cells are of course smaller than the older and more 

 superficial ones ; the larger cells are met with in those situa- 

 tions where the epithelium is continuous with the epidermis., 

 as in the mouth and oesophagus, the vagina, urethra, and 

 bladder, the commencement of the rectum, the inferior di- 

 vision of the nares, lining the eyelids, and covering the 

 cornea. (See Plate XX. Jig, 1.) On the contrary, the 

 epithelium of the pericardium, ventricles, aorta, and of most 

 of the closed cavities is composed of cells which are very 

 much smaller in size than those of the localities previously 

 enumerated. (See Plate XXII.) 



Structure. Epithelial cells illustrate faithfully the doc- 

 trine of cell development, each consisting of a nucleus, cell 

 wall, and intervening space enclosing fluid, both the nucleus 

 and the cell wall exhibiting a granular composition. 



It has been observed, that the younger cells are thicker 

 than the older and fully developed ones, which become re- 

 duced to mere membranous expansions, from which it fol- 



u 4 



