STRUCTURE OF THE HUMAN BODY. 31 



being composed of condensed cellular membrane. Accord, 

 ing to Rudolphi, serous membranes line not only the closed 

 cavities of the body, but the interior of the vessels also, 

 and the canals which open outwardly, as the alimentary 

 canal and the air passages, forming a cuticle over the mu- 

 cous membranes, which line these passages. \he uses of 

 the serous membranes are to separate different organs, to 

 diminish friction, and to facilitate the motion or gliding of 

 these parts upon one another, by means of thin smooth, 

 moist, and polished surfaces?! 



15. Mucous Membranes.-?YhesQ membranes are also 

 another form of the cellular structure, and derive their name 

 from the nature of the fluid, which it is their office to secrete 

 by means of numerous minute glands imbedded in their sub- 

 stance. As serous membranes form a shut sac, completely 

 excluding the air, mucous membranes on the contrary, line 

 the various cavities, which are exposed to the air, such as 

 the mouth, the nostrils, the windpipe, the gullet, the stomach, 

 the intestines, and the urinary organs. Their internal sur- 

 face, or that by which they are attached to the passages they 

 line, is smooth and dense, while their external surface, or that 

 which is exposed to the contact of the air, is soft and pulpy > 

 like the pile of velvet, or the rind of a ripe peach. 



