1st. We very often see these membranes abandon and cover again successively their respective organs; thus the broad ligaments, at a great distance from the womb in the ordinary state, are to it like a serous membrane during pregnancy. An intestine when distended borrows from the mesentery a covering that quits it when it contracts. The omentum is by turns, as Chaussier has well observed, a loose membrane in the abdomen and a covering of the stomach. The peritoneal envelope of the bladder often leaves it almost entirely. Has not the hernial sac of those enormous tumours of gastric viscera originally served to line the parietes of the abdomen? Now it is evident, since the different organs can exist separate from their serous membranes, that there is no connexion between their organization. 2d. It is always a loose texture, easily stretched in every direction, that serves as a means of union, and never a sanguineous vascular system, as in most of the other adhesions. 3d. The affection of an organ is not a necessary consequence of that of its serous membrane, and reciprocally the organ is often affected and the membrane does not become diseased. For example, in the operation for hydrocele, the testicle remains almost always sound in the midst of the inflammation of its tunica vaginalis. The inflammation of the mucous membrane of the intestines is not a consequence of that of their peritoneal covering; and reciprocally in the various acute catarrhal affections of the organs with a mucous membrane within and a serous one without, this last is never found inflamed. In a word, the affections of the mucous membranes are everywhere very distinct from those of the serous, though most commonly both contribute to the formation of the same organ. It is evident that a line of demarcation so great in the affections supposes one of course in the organization. The life of the serous membranes then is entirely distinct from that of their corresponding organs. Yet there are cases where these membranes do not present this loose adhesion, and where they become so united to the organs which they line, that frequently the most delicate scalpel cannot separate them. Observe the tunica vaginalis on the albuginea, the arachnoides on the dura-mater, and other membranes which form what I have called the sero-fibrous, &c.; such is the connexion of these different surfaces, that many have been mistaken to the present time for a single membrane. There is however no more identity in the organization, than where the serous membranes are more loosely attached to their respective organs, as is seen in the peritoneum, the pleura, &c. Diseases sometimes make this difference very evident. I have seen the arachnoides in a subject that had been affected with a chronic inflammation, evidently thickened on the internal surface of the dura-mater, without this having experienced the slightest alteration; it was detached without difficulty and torn with great ease.