_Functions._ The synovial membrane adds nothing to the solidity of the articulation. The fibrous capsules and the lateral ligaments alone serve this purpose. The smooth surface which the articular extremities derive from this membrane, favours their motions; it can even in this way assist the muscular action; thus the portions of synovial membrane which are found at the knee behind the cruræus, at the elbow under the triceps, at the phalanges under the flexors, &c. perform in respect to these muscles, the same functions as the tendinous synovial bags. They are to their tendons, what the cellular sac which separates the tendons of the psoas and the iliacus from the crural arch, is to them. The principal use of the membrane of which we are treating is in relation to the synovia. It exhales by numerous orifices this fluid which remains there for some time, and afterwards re-enters the circulation by absorption. Its parietes are then the seat of exhalation, as the kidneys, for example, are that of the secretion of urine. The reservoir of the exhaled fluid is the sac without opening which it forms, as the bladder is that of the urine that comes from the kidneys. The excretory vessels of this same fluid are the absorbents which carry it into the mass of blood, as the urethra carries the urine from the bladder. There is under these different relations more analogy than there at first seems to be, between secretion and exhalation. The phenomena of the continuance of the synovia in this membranous reservoir, have relation to the synovia itself or to the articular surfaces. The first consist in a peculiar but unknown alteration which it undergoes between the exhalant and absorbent systems. The second contribute to facilitate the articular motions. The unctuous and slippery coat which the serous surfaces receive from the synovia, is remarkably adapted to this use, as I have observed. _Natural Development._ In the fœtus and in infancy most of the synovial membranes are much larger in proportion than in the after ages, because the articular surfaces have a greater extent in the cartilaginous than in the osseous state; but they are then extremely delicate. The synovia is not, as the serous fluids are at this age, more unctuous and of greater consistence; it appears even to be less so. Before birth it is in small quantity, no doubt because the motions are trifling. In old age I have observed that the synovial membrane becomes more dense and compact. It loses in part its white colour and becomes grey; less synovia is exhaled from it. It is not like the serous surfaces, exposed to dropsies. The rigidity it acquires makes motion painful. It never ossifies except preternaturally. The phosphate of lime which gradually invades cartilage, does not take hold of it. I do not know an instance of an old person in whom bone has been found naked in an articulation. _Preternatural Development._