MUCOUS MEMBRANE.—This term has been usually and properly restricted to those large expansions of membrane, in the interior of the body, which are continuous with the external tegument: but it is impossible, in the present state of knowledge, to treat of these apart from the true glands and the skin, which form with them a great system, to which the generic term mucous will be applied in this article.
Many anatomists since the time of Bonn' have treated of the mucous membranes and skin under the common title of tegumentary membranes ; and the opinion has been gradu ally gaining ground, that all the glands fur nished with excretory ducts have a very close relation to the former, in which their ducts for the most part open. Still, it does not appear that the proofs of this alliance have been hi therto, by any author, deemed sufficient to in duce him to blend these several parts under a description common to them all. Even Muller, in whose philosophical work on the glands is contained so much new and important evidence of this relation, continues thus to sever them in the late edition of his Physiology. But, in deed, although much weight is to be granted to the arguments drawn from continuity and occasional convertibility of structure, course of developement, rough analogies of composition or of function, and sympathies under disease, it must be allowed that hitherto that most im portant of all proofs has been all but wanting, which, as I shall endeavour to show, is capable of being derived from minute anatomical ana lysis.
The researches which I have hitherto been able to make on this subject are still so incom plete, that I should have gladly delayed their publication for some time longer, had the pro gress of this work admitted of it. As it is, I shall state the conclusions to which I have been led, and the grounds they rest upon, (pointing out, as far as possible, where farther examination is demanded,) with the hope of thereby giving a clearer and more satisfactory view of the structure and relations of this im portant class of tissues than could be otherwise accomplished.
I shall point out that the skin, mucous mem branes, and secreting glands, consist of certain elements, which the anatomist may detect and discriminate, some of which are essential to their tissue, others appended or superadded,— and that the broad characteristic distinctions between these structures, appreciable to ordi nary sense, as well as the innumerable grada tions by which they every where blend insensi bly with one another, are solely due to various degrees and kinds of modifications wrought in the form, quantity, and properties of these re spective elementary parts.
The skin is the outer tegument of the body ; the mucous membranes form its internal invest ment, and are continuous with the skin. The ducts of all glands are continuous either with skin or mucous membrane, and their true secreting portion, as already described, (see GLAND,) is merely a further prolongation of the same tissue. These offsets, like the great mucous tracts, are in the direction of the inte rior of the body ; they form follicles and tubes of infinite variety, and, however complicated, may still be regarded, in a certain sense, as ex ternal to all other textures. Thus the mucous system may be described as a great and un interrupted membrane, every where perfectly closed, in which the rest of the animal, or the parenchyma, is enclosed. This membrane has two surfaces, the one free, superficial or exter nal, the other attached, deep, or parenchymal. It is on the parenchymal surface that the ap pended structures (viz. blood- and lymphatic vessels, nerves, and areolar tissue) are found in more or less profusion.
The functions of the mucous system, nume rous and diversified as they are, all bear a dis tinct reference to its really external anatomical position, and by this circumstance they are associated together: the principal are sensation, absorption, secretion, excretion, and defence of the parts lined by it against the contact of foreign bodies.