CHAP. XIII.] ALIMENTATION. 145 



extends, on each side, along the passage to the ear ; and off- 

 sets in the alimentary canal go to line the salivary, pancreatic, 

 and biliary ducts, and the gall-bladder. 



The genito-urinary 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 urethra; 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 rugce 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- 

 spicuously into the cavity which it lines. The best marked 

 example of these folds is seen in the small intestine, where 

 they are called valvulce conniventes, and which are doubtless 

 provided for increasing the amount of absorbing surface for the 

 products of digestion. The redness of mucous membranes is 

 due to their abundant supply of blood. 



A mucous membrane is composed of a layer of connective 

 tissue called the corium, and by a layer of epithelium which 

 covers the surface. The epithelium is the most constant part of 

 a mucous membrane, being continued over certain parts to which 

 the other parts of the membrane cannot be traced, as over the 

 air-sacs in the lungs, and the front of the cornea of the eye. 

 It may be scaly and stratified, as in the throat, columnar, as in 

 the intestine, or ciliated, as in the respiratory tract. The 

 mucus which moistens its surface is either derived from little 

 glands in the mucous membrane, or from the columnar cells 

 which cover the surface. The corium of a mucous membrane 

 is composed of either areolar or lymphoid connective tissue. 

 It is usually bounded next to the epithelium by a basement 

 membrane, 1 and next to the submucous tissue by a thin 



1 A basement membrane is formed by a layer of connective tissue cells joined 

 edge to edge. 



