A glance at the opinions that have prevailed concerning the structure and relations of the mucous membranes, will exemplify, more clearly perhaps than any other course, how imperfect have been the means employed, until a very recent period, in researches into minute or structural anatomy. The distribution of their bloodvessels bad indeed been studied with brilliant success by Ruysch, Lieberkiihn, and others, by the aid of injections, the admirable delicacy of which no modern art has surpassed; and somewhat of their extensive connexions, general properties, and even of their texture, had been divined from rough dissection, mace ration, and observations on the mode of their developement and on their morbid states. But the ignorance that really prevailed, as to their intimate structure, is abundantly evinced by the number of disputed questions, the absence of precision of detail, and the substitution of loose and unwarranted analogies in its stead. Within the last five years discoveries have been made which throw a new and most important light on the whole subject, and when viewed in con nection with one another, must be considered to have greatly simplified our knowledge re specting it. These discoveries, due chiefly to Boehm, Boyd, and Heide, result from exami nations of recent specimens with the micro scope, and those of the last observer, which are especially valuable, were made with high pow ers employed upon a single tissue (the epithe hum) in different forms and situations. It is this kind of research that promises the most enlarged and trustworthy results to any one who will follow it in a spirit of due caution against hasty generalization, and which has already done so much in the present day to wards a complete remodelling of our ideas, both concerning the elements of organization and their union to form compound tissues.
Before proceeding to a description of the anatomical elements of the mucous system, it is necessary to premise that a great portion of the membranes, usually termed mucous, are glands of a complicated structure, arranged in a membranous form, consisting of a closely packed mass of secreting tubules, which open on the general surface, and are essentially invo lutions of it. The bloodvessels and other ap pended tissues occupy the intervals of these tubules, and so approach the surface; but, ne vertheless, they always remain on the deep or parenchymal aspect of the mucous tissue. So, the same membranes present projections, which are nothing more than hollow evolutions of the same mucous tissue, into which the appended tissues are extended. The same remarks apply strictly to many regions of the skin. Hence it becomes necessary to guard against confound ing such compound membranes with the simple mucous tissue, of which these and all other por tions of the mucous system consist.
Of the ultimate structure of the mucous system.—It has been already stated that the mucous tissue is essentidly an uninterrupted membrane in which the other tissues of the animal are contained. A very cursory attempt will serve to skew how much more easily it admits of being separated and examined in certain situations rather than in others. This variety depends chiefly on the difference of its arrangement and connexions in different regions.
In the testis and kidney, the capillary vascular rete, spread over its parenchymal surface, has no intimate attachment to it, and the appended areolar tissue is in very small quantity. In the testis especially, this latter may be said to be almost wanting as an investment to the individual tubules, being, as it were, disposed as a collective covering to the entire organ, sending partial septa irate its interior, and bearing the name of Tunica Albuginea. In the kidney, a more intricate vascular rete in a great measure supplies the place of areolar tissue. Hence, in these viscera, the simple mucous element allows of being isolated with remarkable facility. In the liver, its isolation is almost impracticable, owing to its lying in the interstices of a capillary plexus that may he termed solid, from its being extended uniformly in every direction. The intricacy of the inter lacement of the mucous and vascular elements in this organ is sufficient to explain the total ignorance that prevails concerning the mode of termination of the biliary ducts, and con cerning their size, shape, and connexions in the lobules of the gland. In many other true glands, the mucous tissue may be submitted to examination without much difficulty; examples of which may be seen in the pancreas and salivary glands, with those allied to them, such as the duodenal glands of Brunner, the buccal, palatal, arytenoid, tracheal, &c.; and in the sudorific glands of the skin. In the compound membranes also, as that of the alimentary canal below the cardia, and the more highly developed parts of the skin, the mucous ele ment may be generally distinguished from the tissues in connexion with it in a satisfactory manner. But in the plane expansions of the simple membrane which line the bony cavities of the nose and car, its isolation and the demon stration of its structure are far more difficult, for reasons which will be afterwards explained, and our knowledge of it here still rests partially on the ground of analogy.
In the mucous tissue there are two structures that require to be separately described, viz. the b isenrent membrane and the epithelium. The basement membrane is a simple homoge neous expansion, transparent, colourless, and of extreme tenuity, situated on its parenchymal surface, and giving it shape and strength. This serves as a foundation on which the epithelium rests. The epithelium is a pavement composed of nucleated particles adhering together, and of various size, form, and number. The fol lowing general observations on these elementary parts will receive illustration as we advance. Neither the one nor the other is peculiar to the mucous tissue in the sense either of being invariably present in it, or of not being found elsewhere. There are certain situations of the mucous system where no basement membrane can be detected, and others from which the epithelium is absent. Both, however, are never absent together. Again, a structure apparently identical with the basement membrane is met with in numerous textures besides the mucous, and all internal cavities, whether serous, syno vial, or vascular, or of anomalous kind, (as those of the thymus, and thyroid body) are lined by an epithelium.