440 SPECIAL HISTOLOGY. 



finally, in the plates, the nuclei have begun to retrograde, are smaller, 

 0-004-0-006 of a line long, 0-002-0-0015 of a line broad, generally flat- 

 tened and more homogeneous, without any distinct cavity or nucleolus^ or 

 containing instead several granules. With respect to its chemical rela- 

 tions, the pavement-epithelium of the mouth agrees, so far as we know, 

 in all essential points with the mucous layer of the epidermis, and 

 with the deepest layer of the horny lamina, particularly in the circum- 

 stance, that even the plates readily swell up in alkalies ; the reader 

 may therefore be referred to § 45. 



The most essential -pliysiological characters of the epithelium of the 

 oral cavity are, the continual change to which it is subjected, and its 

 relations as regards absorption and secretion. With respect to the for- 

 mer, it may be said that the epithelium undergoes a continual desqua- 

 mation, which, however, does not here, any more than in the epidermis, 

 appear to be the effect of special vital energies in the mucous membrane, 

 or in the epithelial cells, but rather to result from the manifold mecha- 

 nical disturbances to which the surface of the oral mucous membrane is 

 subjected during mastication and speaking. On the one hand, these 

 disturbances give rise to a constant detachment of the uppermost plates ; 

 and on the other, an uninterrupted regeneration of the lost parts takes 

 place, a process which I am disposed to interpret in this case exactly as 

 I have done in § 46, for the epidermis, and in § 64, for the hairs. As 

 regards the exact mode of that growth of the oral epithelium, which, 

 from what has been said, must perhaps, always be going on in one part 

 or another, we invariably find, upon the surface of the epithelium, 

 during and after very copious desquamation, large, completely flattened 

 cells (which, of course, possess no power of multiplication), never 

 younger and smaller structures, and, therefore, the reparation of any 

 loss cannot take place by the formation of new cells at the surface of 

 the epithelium ; on the contrary, everything indicates that the renewal 

 occurs in the layers of smallest cells, for although no new development 

 of cells can here be directly observed, yet the analogy with other epi- 

 dermic structures, and the frequent occurrence of two nuclei in the cells 

 of these layers, nay, even of constricted cells [Bowman, compare § 46), 

 are striking facts in favor of the multiplication of the cells which al- 

 ready exist (by division) and against their actual new formation. 



The epithelium of the oral cavity, although thick, is yet readily pei-- 

 meable, differing widely in this respect from the epidermis, which pre- 

 sents similar relations, only in the stratum Malpighii. Fluids of the 

 most different description permeate it from without, and once in contact 

 with the mucous membrane, may either be absorbed by its vessels, or 

 perceived by its nerves. Other conditions remaining the same, the 

 activity of the sensitive and absorbent powers will depend upon the 



