Chap. VIII] SPECIAL MEMBRANES AND GLANDS 133 



of the nose. In other instances, especially in cavities subject to 

 frequent variations in capacity, like the oesophagus and the stomach, 

 it is lax; and when the cavity is narrowed by contraction of its 

 outer coats, the mucous membrane is thrown into folds, or rugae, 

 which disappear again when the cavity is distended. But in 

 certain parts the mucous membrane forms permanent folds that 

 cannot be effaced, and which project conspicuously 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^ (cir- 

 cular folds), and which 

 are doubtless provided 

 for increasing the 

 amount of absorbing 

 surface for the products 

 of digestion. In some 

 locations the free surface 

 of mucous membrane 

 contains minute glands, 

 or it is covered with 

 papillse, villi, or cilia. 



PapillcB. — The papilhe 

 are best seen on the _ _ ,, ^ 



Fig. 92. — Diagram of the Male Genito- 

 tongue ; they are small urinary Mucous Membrane showing Conti- 

 e J.1 • NuiTY of all its Parts. (Gerrish.) 



processes or the conum, ^ 



mostly of a conical shape, containing blood-vessels and nerves, 



and covered with epithelium. 



Villi. — The villi are most fully developed on the mucous coat 

 of the small intestine. They are little projections of the mucous 

 membrane, covered with epithelium, containing blood-vessels and 

 lacteals, and are favorably arranged for absorbing nutritive 

 matters from the intestines. 



Cilia. — For description of cilia see page 28. 



Function of mucous membranes. — The function of mucous 

 membranes is (1) protection, (2) support of blood-vessels and 

 lymphatics, (3) to furnish a large amount of surface for absorption. 



(1) It protects by forming a lining or inside skin for all 



' See page 266. 



