The Mucous Membranes occupy the interior of those cavities, which, by various openings, communicate with the skin. Their number, at the first view, appears very considerable; for the organs within which they are reflected are numerous. The stomach, bladder, urethra, uterus, ureters, the intestines, &c., borrow from these membranes a part of their structure: nevertheless, if it be considered, that they are continuous throughout, that everywhere they are observed to be extended from one organ to others, arising, as they did at first, from the skin, their number will appear to be singularly limited. In fact, in thus contemplating them, not as insulated in each part, but as continued over various organs, it will appear that they are reducible to two general surfaces.
The first of these two surfaces, entering by the mouth, nose, and anterior surface of the eye, lines the first and second of these cavities: from the first it extends into the excretory ducts of the parotid and submaxillary glands; from the other it is continued into all the sinuses, it forms the tunica conjunctiva, descends by the puncta lacrymalia through the canal and lacrymal sac to the nose. It descends into the pharynx, and there furnishes the inner surface of the Eustachian tube, and thence it penetrates and lines the internal ear. It sinks into the trachea, and spreads itself over all the air passages. It enters the œsophagus and stomach. It extends into the duodenum, where it furnishes two branches, one destined to the ductus communis choledochus, to the numerous rami of the hepatic duct, to the cystic duct and gall bladder; the other to the pancreatic duct and its various ramifications. It is continued into the small and large intestines, and finally terminates at the anus, where it is identified with the skin.
The second general mucous membrane enters, in men, by the urethra, and thence spreads from one part through the bladder, ureters, pelves, calices, papillæ, and uriniferous tubes; from the other it sinks into the excretory ducts of the prostate gland, into the ejaculatory ducts, the vesicula seminales, the vassa defferentia, and the infinitely convoluted branches from which they arise. In women, this membrane enters by the vulva, and from one part penetrates the urethra, and is distributed, as in men, through the urinary organs; from the other part it extends into the vagina, which it lines, as it also does the uterus and the fallopian tubes, and through the apertures at the extremities of these ducts it comes in contact with the peritoneum. This is the only example in the economy, of a communication between the mucous and serous surfaces.
This manner of describing the track of the mucous surfaces by saying that they extend, sink, penetrate, &c., from one cavity to another, is certainly not conformable to the march of nature, which forms in each organ the membranes that belong to it, and does not thus extend them from one to the other; but our manner of conceiving is best accommodated by this language, of which the least reflection will rectify the sense.