EPITHELIUM. 237 



Varieties of Epithelium. 



Both the scaly and the conoidal epithelium present two varieties, 

 the simple and the compound. The conoidal epithelium is also in 

 some parts ciliated, whether simple or compound. Thus we find 



I. The simple scaly epithelium. 



II. The compound scaly epithelium. 

 III. The simple conoidal epithelium. 

 IY. The compound conoidal epithelium. 



V. Either of the two preceding may be ciliated. 



Frequently the cells of a scaly epithelium (as on serous mem- 

 branes) are matched together in such a way that its free surface re- 

 sembles mosaic or a pavement, when seen under the microscope, the 

 cells being polygonal and mostly hexagonal. This appearance has 

 given rise to the expression "pavement or tessellated" epithelium. 

 But the free surface of a conoidal epithe- 

 lium often presents the same appearance ; and 

 since it depends not on the size of the cells, 

 nor even their form, except so far as their 

 free surface is concerned, there is no sufficient 

 reason for making this appearance a distin- 

 guishing characteristic. It is indicated in 

 Fig. 139. 



Todd and Bowman have described the 

 epithelium lining the minute ducts of the 



true glands as Consisting Of globular Cells, Tessellated (scaly) epithelium 

 -, -, , -, - ,, -, -, -, ITT?? of a tubulus uriniferus. 



and hence term this "globular or glandular 



epithelium. They suppose that the secretion of the gland is secreted 

 by these cells alone. Since, however, all epithelial cells secrete in 

 proportion to their size and fluid contents, this distinction is unne- 

 cessary. Besides, these cells are not by any means uniformly glo- 

 bular. There are all intermediate phases, so far as the form of the 

 cells is concerned, between the flattened or scaly and the conoidal 

 or cylindrical. 



It will appear that different functions are assigned to these differ- 

 ent varieties of epithelium, now to be described. 



