The serous membranes are so denominated by Bi chat, from the serous fluid which bedews their central suriace. These membranes occupy the central surface of two of the great cavities, viz. the chest, and the belly, and they inveiope the peripheral surface of several of the most important vital organs, as the brain, lungs, the heart, th, stomach, and intestines; the liver, &c. Hence there are three principal serous membranes, the arach nond coat of the brain, the pleura, and the petitoncum, each lornm.6 a uistiuctbag, that is close, or has no di rect communicatton with the surface of the body. They, however, m some measure communicate with each other, viz. the pleura with the peritoneum, through the opehings of the diaphragm; and in one instance they communicate with a mucous membrane, as the perito neum wish the mucous membrane lining the uterus through the medium of the Fallopian tubes.
The serous membranes have two surfaces, the one attached to the sides of the cavities in which they are found, but not in general adhering to it very closely; the other free, looking towards the cavities and forming more peculiarly the secreting surface. These mem branes, however, are compos«I of a single layer, which differs considerably in thickness in various parts, being of a very sensible thickness, when inveloping the liver, the heart, or the intestines ; but remarkably thin in the aracimoid coat of the brain, and that part of the perito neum which forms the omentum. They arc of a whitish colour, and a shining appearance, but not so resplendent as the fibrous membranes that constitute the ligaments and tendons. Examined more minutely, they appear to be of a cellular structure, and are furnished with exha lants, absorbents, blood-vessels, and nerves. They are capable of considerable dilatation, provided this take place in a gradual manner; as in their natural state they are not easily extended. They are possessed of a small degree of contractibility; are not very elastic, nor in their natural state, very sensible.
The mucous membranes, which have also received their name from the nature of the fluid which they se crete, are more generally diffused through the system than the serous membranes, not only lining the central cavities of most of those organs which are inveloped by the serous membranes, but also several which have lit tle or no connection with those membranes, such as the cornea and the eyelids; the lachrymal ducts, nostrils, the pharynx and gullet; the eustachian tubes, the ure ters, the urethra, the vagina and uterus. It will be seen that they differ from the serous membranes in in vesting all those cavities that immediately communicate with the surface of the body, and through it with the external air. These membranes are divided by Bichat
into two general orders, one consisting of those which invest the bronchial passages and the alimentary canal, which he calls mucous membranes; and those which invest the interior of the genital and urinary organs, he calls genitourinary mucous mem branes. These two general mucous membranes have -no immediate communication, but they resemble each other in their structure and properties.
The mucous, like the serous membranes, have two surfaces, one attached to the muscular coat of the or gans which they invest, and the other free, moistened with the mucous secretion, and generally leirnied into various plaitg' or folds. Considered in their intimate structure, the mucous membranes are compose II o: two principal layers, one which is analogous to the substance of the skin, which we have called corium and another similar to the papillary or vinous surface of the skin. The cation or the mucous membranes is in general ul considerable thickness, of a spongy texture, and of a very soft consiAence. The papillary surface, like that of the skin, scents to from the exlremities of nerves, and is possessed of considerable sensibility. These membranes, besides possessing blood-vessels, exhalants, absorbents and nerves, like the serous mem branes, are furnished with numerous glands, situated either within the substance of the corion, or betw,:en this and the muscular coat which it invests, and com municating with the papillary sot face by numerous ducts or openings.
The mucous membranes do not possess so much ex tensibility and contractibility as the numerous folds of their papillary surface Nvokild lead us to suppose, but in certain cases we find them capable of considerable This is particularly observable in the ureters and biliary ducts, which arc sometimes exceedingly en larged by the passage of calculi through them.
The secreting surfaces above described are extremely subject to inflammatory affections, but these affections arc marked in each by distinct and peculiar characters, which appear to have been first accurately distinguish ed by Pinel, in his NoNographie Philosophapic, tom. i. The inflammation of serous, or what Pinch calls the diaphanous membranes,' are characterized by acute pain, quick and hard pulse, and in general by a greater reaction of the vascular systern; and the inflamed mem branes have their blood-vessels greatly distended, and their surface remarkably red. In the inflammation of the mucous membranes there is less pain, less reaction of the vascular system, and the affections are charac terized at their commencement, chiefly by the great in crease of mucous secretion.