1st. Sometimes the costal and pulmonary portion are so identified at many points or in every part, that they make but a single membrane, and are united as closely as the two edges of the lip in a hare-lip that has been operated upon with success. 2d. At other times the adhesion is so slight, that the least effort is sufficient to destroy it. I have many times noticed this fact in the pericardium. I saw it once in the tunica vaginalis of a man who had been operated upon for hydrocele by means of injection, at the time I was surgeon for operations at the Hôtel Dieu. Separated then from each other, the two surfaces were uneven; they lost their polish. 3d. Frequently between the costal and pulmonary portion of the pleura, between the surfaces of the peritoneum, &c. there are several elongations of various lengths, which form a kind of loose bridles, traversing the serous cavity, having the same organization and polish as the membrane of which they appear a kind of fold, containing in their interior a species of small canal, because they are formed by two layers united together, resembling very much the elongation of the synovial membrane of the knee, which goes from the posterior part of the patella to the space between the condyles of the femur, having also an appearance analogous to the different natural folds of the peritoneum. We can hardly conceive that these filaments so regularly organized can result from inflammation. I am inclined to believe that they are owing to an original conformation. 4th. Frequently between the two portions of the pleura, there are seen many other elongations wholly different, which are not smooth, and do not form canals, but which appear to be flocculent and really analogous to the cellular layers; so that where they exist it may be said, that the membrane is entirely changed into this texture, which is besides, as we shall see, the essential base of its organization. 5th. I do not speak of the adhesions produced by false membranes, by albuminous flakes, intermediate to the two portions of a serous surface, &c. These adhesions are to a certain point foreign to these surfaces. II. _Adherent Surface of the Serous System._ The external surface of the serous membranes adheres almost everywhere to the neighbouring organs; it is rare in fact to see these membranes detached on both sides. The arachnoides at the basis of the cranium, and some other examples are exceptions. This adhesion of the serous membranes to their respective organs, is wholly different from that of the fibrous membranes. In this last, the passage of the vessels so unites the two parts, that their organization, seems to be common, and when one is removed, the other almost always dies, as is seen in the periosteum in relation to the bones, &c. On the contrary, every serous membrane is almost foreign to the organ it surrounds; their organization is different. The following are proofs of it;