The increase of the fluid of the tendinous synovial membranes forms a species of dropsy which is called ganglion, a tumour which never exists in the synovial membranes of the fingers, no doubt on account of the want of extensibility of the fibrous capsules. It should not be thought however that all these tumours, which are cured by bursting them by strong pressure and thus effusing the fluid into the cellular membrane, have for their base a natural synovial membrane. Most frequently they are preternatural; they are cysts which are formed in the cellular texture. In fact these tumours are often found in the course of the great extensor of the thumb, where there is no synovial membrane. After rheumatic pains I have seen a considerable collection of fluid in the small synovial membrane of the tendo Achillis; it gradually disappeared. I have observed another analogous one in the bag of the psoas of a dead body. The fluid was reddish and of the consistence of currant jelly. The action of nitric acid immediately coagulated it into a white mass, analogous to the white of an egg hardened.
The adhering surface of the tendinous synovial membranes is spread, 1st, on the one hand upon the tendons, with which it is more or less intimately united. It is easily detached from those of the internal obturator, the psoas, &c. It is closely connected with those of the flexors. 2d. On the other hand, it commonly lines the periosteum, which, in this place, is penetrated with gelatine, and forms a fibro-cartilage. Its mode of relation is there analogous to that of the articular synovial membrane with the cartilage of the bone. Sometimes it is reflected upon a fibrous capsule after having lined the tendon; such are those which are in the neighbourhood of the scapulo-humeral articulation. In some cases, after having lined the tendon, they mount up to the fleshy fibres, as on the obturator internus. 3d. By reflecting from the tendon upon the neighbouring organs, they answer in general instead of much cellular texture; but in the grooves of the flexors, it is the fibrous sheaths which they clothe.
In all the great motions, the tendinous synovial membranes, stretched more or less, undergo various locomotions, always less however than those of the serous surfaces.
The very various forms, which the sac without opening of the tendinous synovial membranes exhibits, can be reduced to two general modifications. 1st. Some are rounded sacs, species of bladders; such are those upon the supra-spinatus, the psoas, iliacus, obturator internus, &c. All these membranes are remarkable for this, that they never cover the tendon entirely, but only on one side; that they never form internal folds and that they are never surrounded by fibrous sheaths. 2d. The others, belonging especially to the flexors, and to the different tendons which traverse the sole of the foot, form at first a kind of cylindrical sac which lines the canal half fibrous, half cartilaginous in which the tendon slips; then they are reflected around it, cover it wholly and form for it a true sheath which prevents it from being moistened by the synovia. This kind of tendinous synovial membrane represents then truly two canals, at the superior and inferior extremities of which are found two cul-de-sacs which unite them and complete the sac without an opening. Internal folds are here frequently found going from one canal to the other. All the synovial membranes of the flexors have one of them under the tendon.
Organization; Properties; Development.
The organization of the tendinous synovial membranes is precisely analogous to that of the articular ones. Principally cellular, the texture of these membranes is without any apparent fibre; its softness is very evident; very few blood vessels are distributed to it, though the contrary has been said; absorbents and exhalants especially predominate in it. These, filled with blood in inflammation, give to the membrane, a reddish tinge, more or less deep. In this state the synovia is not exhaled; sometimes even adhesions are formed, as I have observed in a subject in whom the fibrous sheaths and their tendons of the index and the middle finger seemed to be united. The inflammatory phenomena of the tendinous synovial membranes are especially remarkable in whitlows, a disease, one species of which has evidently its seat in the synovial membrane of the fingers, is analogous to the inflammation of the pleura, the peritoneum, and to that of the articulations. It is more dangerous than the inflammation of the synovial membranes in the form of bladders or bags, because the fibrous sheath which surrounds the inflamed membrane, not being able to stretch and yield to the swelling, like the cellular texture which surrounds the synovial bags, produces real strangulations, which it is often necessary to remove. I do not know whether the synovial texture of the tendons is exposed to the slow and tubercular inflammations, common to the articular serous and synovial systems. Its vital properties and those of texture appear to be precisely the same as those of this last. Like it, it receives with difficulty the sympathetic influence of the other organs; it is unaffected during the derangement of the other systems in acute diseases; it remains sound in their alterations arising from chronic affections. I would observe also that all its affections are almost local. For example, there is not, as in the serous system, species of dropsical diathesis, that is to say of cases in which all the synovial sacs are filled at the same time.
The tendinous synovial membranes, fine and delicate in the fœtus and in infancy, readily yield to the numerous motions which constantly succeed that age. More dense and compact in the adult, they become rigid in old age, exhale less fluid, are dry, and do not contribute a little, by the state in which they are, to the general slowness of the motions which that age brings with it.
There are many synovial membranes the existence of which is variable; such as, for example, that of the great glutæus, in the place of which there is often found only a cellular mass. These membranes are in general very dry when they exist. Synovia can scarcely be discovered in them. They resemble in this respect the articular synovial membranes of the vertebræ, the clavicle, &c.