cxcvi MUCOUS MEMBRANES. 



The mucous membranes of several different or even distant parts are con- 

 tinuous, and, with certain unimportant reservations, to be afterwards ex- 

 plained, they may all be reduced to two great divisions, namely, the gastro- 

 pulmonary and genito-urinary. The former covers the inside of the alimen- 

 tary and air-passages as well as the less considerable cavities communicating 

 with them. It may be described as commencing at the edges of the lips and 

 nostrils, where it is continuous with the skin, and proceeding through the 

 nose and mouth to the throat, whence it is continued throughout the whole 

 length of the alimentary canal to the termination of the intestine, there 

 again meeting the skin, and also along the windpipe and its numerous 

 divisions as far as the air-cells of the lungs, to which it affords a lining. 

 From the nose the membrane may be said to be prolonged into the lach- 

 rymal passages, extending up the nasal duct into the lachrymal sac and along 

 the lachrymal canals until, under the name of the conjuuctival membrane, 

 it spreads over the fore part of the eyeball and inside of the eyelids, on the 

 edges of which it encounters the skin. Other offsets from the nasal part of 

 the membrane line the frontal, ethmoidal, sphenoidal and maxillary sinuses, 

 and from the upper part of the pharynx a prolongation extends on each side 

 along the Eustachian tube to line that passage and the tympanum of the ear. 

 Besides these, there are offsets from the alimentary membranes to line the 

 lachrymal, salivary, pancreatic, and biliary ducts, and the gall-bladder. The 

 genito-urinary membrane invests the inside of the urinary bladder and the 

 whole tract of the urine in both sexes, from the interior of the kidneys to 

 the orifice of the urethra, also the seminal ducts and vesicles in the male, 

 and the vagina, uterus, and Fallopian tubes in the female. 



The mucous membranes lining the ducts of the mammary glands, 

 being unconnected with either of the above-mentioned great tracts, have 

 sometimes been enumerated as a third division ; and the number might 

 of course be multiplied, were we separately to reckon the membranes pro- 

 longed from the skin into the ducts of the numerous little glands which 

 open on the surface of the body. 



The mucous membranes are attached by one surface to the parts which 

 they line or cover by means of areolar tissue, named " submucous," 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 

 and its adjoining sinuses ; in other instances, especially in cavities sub- 

 ject to frequent variation in capacity, like the gullet and stomach, it is lax 

 and allows some degree of shifting of the connected surfaces. In such 

 cases as the last-mentioned the mucous membrane is accordingly thrown 

 into folds when the cavity is narrowed by contraction of the exterior 

 coats of the organ, and of course these folds, or rugce, as they are 

 named, are effaced by distension. But in certain parts the mucous mem- 

 brane forms permanent folds, not capable of being thus effaced, which pro- 

 ject conspicuously into the cavity which it lines. The best-marked example 

 of these is presented by the valvulce conniventes seen in the small intestine. 

 These, as is more fully described in the special anatomy of the intestines, 

 are cresceut-shaped duplicatures of the membrane, with connecting areolar 

 tissue between their laminae, which are placed transversely and follow one 

 another at very short intervals along a great part of the intestinal tract. 

 The chief purpose of the valvulae conniventes is doubtless to increase the 

 surface of the absorbing mucous membrane within the cavity, and it has 

 also been supposed that they serve mechanically to delay the alimentary 

 mass in its progress downwards. A mechanical office has also been as- 



