4. 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. 5. In thus bringing all the mucous surfaces to two general membranes, I am supported, not only by anatomical inspection, but pathological observation also furnishes me with lines of demarcation between the two, and with points of contact between the different portions of the membranes of which each is the assemblage. In the various sketches of epidemic catarrhs made by authors, we frequently see one of these membranes has been affected throughout its extent, whilst the other, on the contrary, has remained untouched. It is not uncommon to observe a general affection of the first, _viz._ that which extends from the mouth, nose, and anterior surface of the eye, into the alimentary canal and bronchi. The last epidemic observed at Paris, with which M. Pinel was himself affected, bore this character: that of 1761, described by Rayons, presented the same feature: that of 1732, described in the Memoirs of the Edinburgh Society, was remarkable for a like phenomenon. Now we do not see at the same time a corresponding affection in the mucous membrane which spreads over the organs of urine and of generation. Here is, therefore, (1) an analogy between the different portions of the first, by the uniformity of the affection; (2) a line of demarcation between them, by the healthy state of the one and the disease of the other. 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.