Y.-MOEBID ANATOMY OF MUCOUS MEMBRANES. 



§ 348. From the mouth to the anus, there extends a system 

 ■of membranous canals, which, owing to their free surface beino- 

 always moist, and coated A^dth a thin layer of mucus, have re- 

 ceived the name of the Mucous System. The membranous walls 

 of these canals are in direct continuity with the skin ; like the 



skin, they serve to shut off the organism from its environment 



the I from the not-I ; this must always be kept in view ; we must 

 bear in mind, for instance, that whatever a man may have taken 

 Into his stomach, is still only at the gates of the organism, not 

 in its interior. Not only is the skin continuous in a general 

 way with the mucous tract at the oral, nasal, ocular and aural 

 apertures, at the anal, urethral and vaginal orifices, but each 

 individual layer of the skin may be traced into a correspondino- 

 layer of the mucous membrane ; the epidermis into the epitlie- 

 lium, the cutis into the mucosa proper, the subcutaneous into 

 the sub-mucous tissue.* Moreover each layer retains its general 

 character ; the epithelium continues to serve as a protective 

 covering which shuts olf the organism against tlie world without, 

 the mucosa represents the proper connective tissue of the mucous 

 membrane, the submucous is a lax areolar tissue which facili- 

 tates the movements of the mucosa upon the muscular coat. 

 But within these limits of homology, both the structure and the 

 fanctions of each layer are modified in accordance with the 

 |)hysiological duties of the various div'isions of the mucous tract. 

 First, as regards the epithelium. At all tlie mucous orifices 

 the horny lamina of the epidermis disappears, so that the ;itrial 



* The fourth layer of the mucous tract — the muscular coat — docs 

 nob form, any part of the mucous membrane proper ; it corresponds to 

 the locomotive apparatus of the body in its entirety, to the osseous 

 and muscular systems ; the fifth layer consists in either case of serous 

 membrane, forming on the one hand the visceral, on the other, the 

 parietal lamina of the same serous sac. 



