EPITHELIUM. 233 



ART. IX. EPITHELIUM. 



As the external surface of the body is invested with a cuticle 

 which has received the name of Epidermis, so are its internal 

 free surfaces in like manner clothed with a delicate pellicle 

 which has been denominated Epithelium. 



Both the epidermis and epithelium are constituted of 

 cells : there is this difference, however, between them, that 

 while the former, by the intimate union and superimposition 

 of its cells, exists as a distinct and continuous membrane, 

 the latter, owing to the feeble cohesion of its constituent 

 cells, can scarcely, except in certain situations, be shown to 

 exist as a united and extended structure.* 



The epidermis and epithelium are therefore hardly to be 

 regarded as distinct structures, but rather as the same, the 

 differences observed between them being merely modifications, 

 the result of the different circumstances to which they are 

 each subject. 



The essential identity of the two may be shown by an ex- 

 amination of the epidermis at the outlets and inlets of the 

 body, where, by gradual transition, it may be traced inwards 

 into the condition of epithelium ; and this also, traced from 

 within outwards, will be observed gradually to acquire the 

 characters of epidermis ; so that, within a certain distance 

 of the termination of the cavities of the body, which open 

 externally, the epithelium may also be demonstrated as a 

 distinct membrane ; this membrane may be followed in man 

 from the lips as far backwards as the posterior part of the 



* Leeuwenhoek first discovered in the mucus of the vagina little scales, 

 which he presumed formed the internal membrane of that canal, and from 

 which he conceived they become detached by coitus. (Opera, t. i. p. 153. 

 155.) He likewise noticed that the mucus of the mouth contained scales, 

 (Ibid. t. iii. p. 51.), and he saw also the cylindrical epithelial cells of the 

 intestinal cavity. (Ibid. p. 54. 61.) 



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