Do not these various resemblances, which with some slight differences of composition only, so evidently connect the synovia with the fluid of the serous membranes, lead us to this very simple consequence, viz. that these two fluids being analogous in all other respects must be so also as to the manner in which they are separated from the mass of blood? Now it is a point in physiology at the present day generally acknowledged, that the fluid of the serous membranes is brought to them by exhalation; then we are evidently lead by induction to this which answers the question proposed above; The synovia is transmitted by exhalation to the articular surfaces.
This rigorous and accurate conclusion drawn from obvious and uniform facts will become, I think, a demonstrated truth, when to the analogies already established we shall add that of the membranous organ, the essential seat of the exhalation of the synovia.
II. Remarks upon the Synovia.
Thus separated from the mass of blood, the synovia has the appearance of a white, viscid and transparent fluid. It ropes, like some syrups, when it flows from the articulations. This unctuous property renders it peculiarly fit to lubricate the articular surfaces which rub together, and to protect them from violent shocks.
Its quantity varies; there are articulations which contain much of it; that of the ankle has always appeared to me to have the most of it. Then come the ilio-femoral, the scapulo-humeral, the humero-cubital, &c. There are others in which there is scarcely any; such are the sterno-clavicular, the sterno-costals, the costo-vertebrals, &c. It is not the smallness of the synovial surfaces that occasions in these articulations the constant dryness that is observed in them; for the synovial sacs of the larynx, which are much smaller, are much more moist.
Besides, the synovia does not vary in quantity in each articulation, like the serum in the serous membranes. Those who have opened peritoneums, pleuras, pericardiums, &c. must have seen that hardly two are similar; sometimes there is only a mere dew, at others there is a real collection of fluid. Here on the contrary there is always nearly the same quantity; which is owing to this, that the synovial surface does not feel as easily as the serous surfaces, the sympathetic influences of the other organs when diseased.
The synovia is not subject to the different alterations which the serous fluids exhibit. I have never seen upon the articular surfaces what are called false membranes from inflammation. The preternatural collections of synovia never contain those white flakes, so common in the serous collections. I do not know of an example of milky serum effused in an articulation. One of the most frequent alterations of the synovia is that, I think, in which it takes the consistence of a jelly and is of a reddish colour, analogous, if I may be allowed the comparison, to currant jelly. Now this alteration is wholly foreign to the serous fluids.
These essential differences which the synovia and the serum exhibit in their alterations, evidently suppose a diversity of nature in the principles which compose them in the natural state. The viscidity of one and the greater fluidity of the other, also declare it, as Fourcroy has observed. This diversity of nature appears to depend especially upon a peculiar substance which enters into the composition of the synovia, which few animal fluids possess, which Marguerron who has observed it designates by the name of albumen of a peculiar nature, and which should be the object of new researches.
I shall not give here the details of the analysis of the synovia; they belong to animal chemistry.