What establishes a real line of demarcation between these two systems is, 1st, that the fluids which lubricate their membranes appear to differ in their composition, though there is much resemblance between them. 2d. In dropsical diatheses which affect at the same time the cellular texture and all the serous surfaces of the peritoneum, the pleura, &c. the affection does not extend to the synovial membranes, which indicates a difference of structure, though we do not know what this difference is. 3d. And reciprocally in dropsies of the articulations, an affection in general very rare, and in those of the tendinous synovial capsules, there is no concomitant affection of the membranes of the great cavities. 4th. The fluid of the articular dropsies does not resemble that which fills the great cavities in the same disease. 5th. The synovial membranes are much more rarely than the serous, the seat of those slow and tubercular inflammations which the serous surfaces so often exhibit. I have however seen two examples of it in the synovial membrane of the knee. I believe that these two systems are the only ones in which this disease is observed; so that it is by its existence a character of resemblance, and by its rarity or frequency, a distinguishing attribute. 6th. The different kinds of adhesions of which I have spoken as taking place on the serous surface, are not met with on the synovial surfaces, where we see only that which identifies these two adhering surfaces, a mode which frequently takes place in anchylosis, which is also as often occasioned by it, as by the stiffness of the parts surrounding the articulation. 7th. The synovial surfaces are not as often as the serous, the seat of those remarkable locomotions of which we have spoken; which depends on this, that the articular organs are not, like the most of those covered with serous surfaces, subject to alternate dilatations and contractions. The synovial system exhibits evidently two great divisions. To one belongs the articular system, to the other that of the tendinous grooves. Each shall be examined separately. ARTICLE FIRST. ARTICULAR SYNOVIAL SYSTEM. I believe that I first described this essential portion of the synovial system. I shall relate here what I have said of it elsewhere. I shall examine first how it is separated from the blood, afterwards the fluid itself, and then I shall describe the organ which furnishes it. I. _How the Synovia is separated from the mass of Blood._