168 ANATOMY FOE, NURSES. [CHAP. XIV. 



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 of 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. 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, and next to the submucous tissue by a thin layer of 

 plain muscular tissue termed the muscularis mucosce : this layer 

 is not always present. The connective tissue layer varies much 

 in structure in different parts ; the lymphoid variety is in cer- 

 tain places greatly increased in amount, packed with lymphoid 

 cells, and forms the solitary follicles and Peyer's patches de- 

 scribed in Chapter XII. 



The small blood-vessels conveying blood to the mucous mem- 

 branes divide in the sub-mucous tissue, and send smaller branches 

 into the corium, where they form a network of capillaries just 

 under the basement membrane. The lymphatics also form net- 

 works in the corium and communicate with larger vessels in 

 the sub-mucous tissue below. The free surface of the mucous 

 membrane is in some parts smooth, but in others is beset with 

 little eminences called papillse and villi. 



The papillce are best seen on the tongue ; they are small 

 processes of the corium, mostly of a conical shape, containing 

 blood-vessels and nerves, and covered with epithelium. 



The villi are most fully developed on the mucous coat of 

 the small intestine. Being set close together like the pile 

 of velvet, they give a shaggy or villous appearance to the 

 membrane. They are little projections of the mucous mem- 



