OF THE TEGUMENTARY MEMBRANES IN GENERAL. 183 



traverses it, continuous with each other to the two extremities; 

 or rather as that of two canals, the one wide, the other narrow, 

 the narrow one cased, in the other, and continuous to the two 

 ends, and in the space between which the remainder of the 

 body is lodged. If we wished to employ a trivial comparison, 

 the one which is best fitted to represent this disposition is that 

 of a muff, having in fact two surfaces separated by a layer of 

 intermediate substance, more or less thick. 



242. Besides the skin and the mucous membrane of the 

 alimentary canal, continuous with each other to the two orifices 

 of this canal, every where continuous with themselves, and 

 which constitute the two principal parts of the tegumentary 

 membrane, this membrane has a great number of dependencies 

 or prolongations more or less extended and ramifiecj in the 

 thickness of the body: such are, 1st, the genital and urinary 

 membranes, which are prolonged into all the cavities of the 

 organs of generation, and the urinary depuration; 2d, the pul- 

 monary membrane which lines all the divisions of the bron- 

 chiae : 3d, the membranes which line the excretory ducts of the 

 glands, whether they terminate in the mucous membrane, or 

 like those of the mammae, in the skin ; 4th, those of the nasal 

 cavities, of their sinuses, and posterior nasal fossae, of the audi- 

 tory canals, of the tympanum, of the mastoid sinus, and of the 

 surface of the eye. 



Among these prolongations, all mucous, except that of the 

 external auditory canal, which is cutaneous, the greater por- 

 tion of them terminate in and are appendages or prolongations 

 of the mucous membrane; the external skin, on the contrary, 

 is much less complicated by appendages of this kind. 



243. The tegumentary membrane presents, in its vast ex- 

 tent, differences of appearance, of texture, and of function, 

 which might induce one to doubt of its unity and continuity. 



The skin and mucous membrane compared with each other, 

 at the first glance, seem to be very different; but in the animal 

 series, the difference is gradually effaced in the more simple 

 animals; it is also, generally, but slightly marked in the higher 

 animals which inhabit the water. In the human foetus, the 



difference, though real, is at first but slightly defined, Even,. 

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