PHYSICAL PROPERTIES OF MUCOUS MEMBRANES. 205 



the lachrymal canals until, under the name of the conjunctival mem- 

 brane, it "^spreads ovei" 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, sphe- 

 noidal 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-uriaanj membrane 

 invests the inside of the urinary bladder and the Avhole 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. 



Attachment. — 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 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 ru(j(r, as they are named, are effaced by distension. 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 valvules 

 connivenfes seen in the small intestine. These, as is more fully de- 

 scribed in the special anatomy of the intestines, are crescent-shaped 

 duplicatures of the membrane, with connecting areolar tissue between 

 their lamina, 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 valvule 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 ali- 

 mentary mass in its progress downwards. A mechanical office has also 

 been assigned to a series of oblique folds of a similar permanent kind, 

 though on a smaller scale, which exist within the cystic duct. 



• Physical properties. — In most situations the mucous membranes 

 are nearly opaque or but slightly translucent. They possess no great 

 degree of tenacity and but little elasticity, and hence are readily torn 

 by a moderate force. As to colour, they cannot be said intrinsically to 

 have any, and when perfectly deprived of blood they accordingly appear 

 white or at most somewhat grey. The redness which they commonly 

 exhibit during life, and retain in greater or less degree in various parts 

 after death, is due to the blood contained in their vessels, although it 

 is true that, after decomposition has set in, the red matter of the blood, 

 becoming dissolved, transudes through the coats of the vessels, and 

 gives a general red tinge to the rest of the tissue. The degree of 

 redness exhibited by the mucous membranes after death is greater in 

 the foetus and infant than in the adult. It is greater too in certain 

 situations ; thus, of the different parts of tb" alimentary canal, it is 

 most marked in the stomach, pharynx, and rectum. Again, the intensity 



