THE ORAL CAVITY. 



439 



of roundish, polygonal, more or less flattened cells. Taken altogether, 

 this epithelium is a transparent, whitish, pellicle, 0-1-0-2 of a line thick 

 on the average, very flexible, but possessing little elasticity or firmness ; 

 it may be detached in considerable flakes by macerating and scraping 

 the mucous membrane, and also by the use of acetic acid. Its elements 

 are, throughout, nucleated cells, whose arrangement and structure, 

 strongly recall those of the epidermis ; they are not, as in the latter 

 case, distinguishable into two sharply defined laminae, but constitute one 

 connected layer, more resembling the mucous layer, but representing the 

 horny layer also. The cells are thus disposed, from within outwards : 

 immediately upon the free surface of the mucous membrane, and upon 

 the papillae, rest many layers of small vesicles of 0-004-0-005 of a line 

 (Fig. 167), the deepest of which are, almost without exception, elongated 

 and larger (0-006-0-009 of a line), and disposed perpendicularly. To 

 these succeed many layers of roundish, angular, flattened cells, which 

 gradually increase in size, become flatter from within outwards, and 

 assume a more and more distinctly polygonal form (Fig. 168, b). 



Fig. 168. 



On the outer surface, finally, we meet, gradually proceeding from the 

 deeper cells, with a few layers of the so-called epithelial plates (Fig. 

 168, a), that is, large (0-02-0-036 of a line) bodies with rounded cor- 

 ners, in which the flattening has gone so far that they no longer de- 

 serve the title of vesicles. 



All these cells possess a delicate membrane, easily demonstrable by 

 alkalies and acetic acid ; clear contents, present in greater or smaller 

 quantity, according to the amount of flattening, with frequently a few 

 fatty granules, and invariably a nucleus. In the smallest cells the 

 nuclei measure from 0-002-0-003 of a line, they are elongated or round, 

 and usually without any distinct nucleolus ; in the polygonal cells there 

 are invariably one or two beautiful, clearly vesicular, usually spherical 

 nuclei, of 0-004-0-005 of a line, with clear contents, and 1-2 nucleoli; 



FIG. 168. Epithelial cells in the oral cavity of Man: a, large 5 b } middle-sized; c, the 

 same with two nuclei ; magnified 350 diameters. 



