6. We observe also, that irritation on any one point of these membranes frequently produces a pain in another point of the same membrane, which is not irritated; thus a stone in the bladder causes a pain at the end of the glans, worms in the intestines produce an itching at the nose, &c. &c. Now in these phenomena, which are purely sympathetic, it is extremely rare that the partial irritation of one of these two membranes produces a painful affection in a part of the other.
7. We ought, therefore, from inspection and observation, to consider the mucous surface in general as formed by two grand membranes, spread over several organs, and having no communication with each other but by the skin, which is intermediate, and which, being continuous with both, thus concurs with them to form a general membrane, entire throughout, enveloping the exterior of the animal, and extending to the interior over most of its essential parts. It should seem, that there exists important relations between the internal and external portions of this unique membrane, and this we shall soon be shown by ulterior researches.
[SECTION II.]
OF THE EXTERIOR ORGANIZATION OF MUCOUS MEMBRANES.
8. Every mucous membrane presents two surfaces; the one adhering to the adjacent organs; the other free, beset with villosities, and always moist with a mucous fluid: each of them deserves a particular attention.
9. The adherent surface is attached to muscles almost throughout its extent. The mouth, the pharynx, the whole of the alimentary canal, the bladder, the vagina, the uterus, and part of the urethra, &c. present a muscular bed, embracing the exterior of their mucous coat. In animals that have the panniculus carnosus, this disposition perfectly coincides with that of the skin, which, as we shall see, is in other respects analogous in structure to mucous membranes. In man the cutaneous organ presents here and there traces of this exterior muscle, as we observe in the platysma myoides, the palmaris brevis, the occipito frontalis, in most of the muscles of the face, &c. This disposition of mucous membranes places them under the influence of those habitual changes of contraction and dilatation, which are favourable to their secretion, and various other functions.
10. This muscular bed is not immediately inserted into the exterior surface of the mucous membranes, but rather, according to Albinus, into a dense layer of cellular tissue, which all the ancient authors have denominated, in the stomach, intestines, and bladder, the nervous coat; but when well examined it presents no character analogous to that which the name indicates. The experiment of inflation, by which it is brought into its primitive state, is not so easy as Albinus and others have pretended; which led me to think that its nature might not be cellular, but that it was probably of a fibrous texture, formed by a web of extremely delicate and scarcely visible tendons, offering points of origin and insertion to all the fleshy fibres of the muscular bed, which, as we know, never describe entire circles, but rather different segments of that curve. I confess that this conjecture, though very likely, is not founded upon any decisive and rigorous experiment.