THE MUCOUS MEMBRANES. 495 



Hum. The basement-membrane is like that of the skin. The 

 cerium is also composed of collagenous and elastic tissue, contains 

 vessels, nerves, smooth muscular fibres, glands, papilla, and other 

 peculiar processes (villi). Beneath the corium there is, in most 

 parts, a layer of submucous areolar tissue. The development an<J 

 regeneration of mucous membranes also resemble those of the skin 

 so nearly as not to demand a separate consideration. 



The mucous membranes present marked differences in structure 

 in different situations. They will, therefore, be described separately, 

 in connection with the other structural elements of the organs, re- 

 spectively, of which they form a part, viz: 



1. In the Alimentary Canal. 



2. " Urinary Apparatus. 



3. " Genital Apparatus. 



4. " Air-passages. 



Functions of the Mucous Membranes. 



All the mucous membranes are, 1, protective of the passages lined 

 by them ; 2, they secrete mucus, of different kinds in different parts 

 (p. 195); 3, they absorb also in certain parts; e. g. the villi of the 

 small intestine, &c. ; 4, they constitute an aerating surface to some 

 extent (e. g. in air-passages) ; 5. some portions of these membranes 

 manifest the sense of touch also (lips, genital organs, &c.). 



Pathological States of the Mucous Membranes. 



1. Atrophy of mucous membrane is rare (EngeT); but is seen in' 

 the alimentary canal of the aged. Here the gastric mucous mem- 

 brane becomes less plicated and smoother ; while the peptic glands 

 are diminished in number. In the duodenum, Brunner's glands 

 become atrophied, and the villi of the small intestine become clouded 

 from the apex towards the base, pigmented, lessened in size (espe- 

 cially transversely), and even in number also. The valvulse conni- 

 ventes are also less prominent, and the Peyerian and solitary glands 

 collapse; their situation being indicated merely by a pigmented 

 border. 



2. Inflammatory exudations on the mucous membranes (except 

 the mouth, oesophagus, vagina, and palpebras), generally at once 

 detach the epithelium ; and, therefore, no vesicles form upon them, 

 except in the parts mentioned. Thus, the exudation may be at 

 once examined. It is also not so generally circumscribed, as in the 

 case of the skin ; and the submucous tissue and the glands it con- 

 tains are very frequently involved. Hence the membrane becomes 

 thickened arid swollen. Extravasation of blood is also more liable 



