CHAP. XIV.] ALIMENTATION. 167 



of foreign substances introduced into the body, such as air 

 and food, and also to the contact of secreted matters ; hence 

 their surface is coated over and protected by mucus, a thicker 

 and more sticky fluid than the lymph which moistens the 

 serous membranes. The mucous membranes of different parts 

 are continuous, and they may nearly all be reduced to two 

 great divisions ; namely, the gastro-pnenmonic and the genito- 

 urinary. 



The gastro-pnenmonic mucous membrane covers the inside 

 of the alimentary canal, the air-passages, and the cavities com- 

 municating with them. It commences at the edges of the lips 

 and nostrils, proceeds through mouth and nose to the throat, 

 and thence is continued throughout the entire length of the 

 alimentary canal to the anus. At its origin and termination it 

 is continuous with the external skin. It also extends through- 

 out the windpipe, bronchial tubes, and air-sacs. From the inte- 

 rior of the nose the membrane may be said to be prolonged into 

 the lachrymal passages, and under the name of conjunctival 

 membrane, over the fore part of the eyeball and inside of the 

 eyelids, on the edges of which it again meets with the skin. 

 From the upper part of the pharynx a prolongation extends, on 

 each side, along the passage to the ear ; and offsets in the ali- 

 mentary canal go to line the salivary, pancreatic, and biliary 

 ducts, and the gall-bladder. 



The genito-nrinary mucous membrane lines the inside of the 

 bladder, and the whole urinary tract from the interior of 

 the kidneys to the meatus urinarius, or orifice of the ure- 

 thra ; it also lines the vagina, uterus, and Fallopian tubes in 

 the female. 



The mucous membranes are attached to the parts beneath 

 them by areolar tissue, here named "submucous," and which 

 differs greatly in quantity as well as in consistency in different 

 parts. The connection is in some cases close and firm, as in 

 the cavity of the nose. In other instances, especially in cavities 

 subject to frequent variations in capacity, like the gullet and 

 stomach, it is lax; and when the cavity is narrowed by con- 

 traction of its outer coats, the mucous membrane is thrown into 

 folds or rugoe which disappear again when the cavity is dis- 

 tended. But in certain parts the mucous membrane forms 

 permanent folds that cannot be effaced, and which project con- 



