MUCOUS MEMBRANES. 405 



MUCOUS MEMBRANES. 



These membranes, unlike the serous, line passages and cavities which communi- 

 cate with the exterior, as well as recesses, ducts and receptacles of secretion, 

 which open into such passages. They are habitually subject either to the contact 

 of foreign substances introduced into the body, such as air and aliment, or of 

 various secreted or excreted matters, and hence their surface is coated over and 

 protected by mucus, a fluid of a more consistent and tenacious character than that 

 which moistens the serous membranes. 



The mucous membranes of several different or even distant parts are continuous, 

 and they may all, or nearly all, be reduced to two great divisions, namely, the gastro- 

 pneumonic and genito-urinary. The former covers the inside of the alimentary 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 wind- 

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

 the lachrymal canals until, under the name of the conjunctival membrane, it spreads 

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

 meets with 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 membrane to line the 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. 



By one surface the mucous membranes are attached 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 subject 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 disten- 

 sion. But in certain parts the mucous membrane forms permanent folds, not 

 capable of being thus effaced, which project conspicuously into the cavity which it 

 lines. The best-marked example of these is presented by the valvulm conniventes 

 seen in the small intestine. These, as is more fully described in the special anatomy 

 of the intestines, are crescent-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 

 use of the valvulse conniventes is doubtless to increase the surface of the absorbing 

 mucous membrane within the cavity. 



In most situations the mucous membranes are nearly opaque or only slightly 

 translucent. They possess no great degree of tenacity and but little elasticity, and 



