Part 19

From these different considerations we may be easily convinced, I think, that notwithstanding the adhesion of the synovial membrane at different points, it should be considered in a manner precisely analogous to that of the serous membranes, that is to say as a real sac without an opening, everywhere contiguous and spread upon all the organs of the articulation. Besides, do not the fibroserous membranes exhibit similar adhesions, though the separate existence of the two layers which compose them is generally admitted?

From the idea we have formed of the synovial membrane, it is easy to conceive how certain organs pass through the articulation, without the escape of the synovia by the opening which receives or by that which transmits them. The synovial membrane then reflected around these organs, forms for them a sheath which separates them from the fluid and keeps them distinct from the articulation. Thus the tendon of the biceps is no more contained in the articulation of the arm with the scapula, than the umbilical vein, the urachus, &c. are in the peritoneal cavity. With the least care it may be separated from the portion of membrane which forms its sheath.

The preceding considerations lead us also to find a perfect identity in the synovial capsules of the tendons and the articular synovial ones. In the preceding example, these two kinds of membranes are evidently continuous; for the capsule of the groove of the biceps is of the same nature as that of the tendons which have a separate one from it, as the flexors, for example.

Organization.

We have just seen, that the synovial membrane resembles very much in its external conformation the class of serous membranes, it does not less so in its internal organization. This organization is cellular, as is proved by dissection, inflation and especially maceration. The sac which the ganglions form is evidently only a production of the cellular organ; now it is known that this sac exhales and contains a fluid similar to the synovia. Wherever the synovial membrane is free, it is attached externally to this organ and is confounded with it in so direct a manner, that by raising successively its different layers they are seen to be gradually condensed and finally united together to form it. So that in the serous membranes no fibre is visible. It becomes transparent when it is separated accurately on both sides, which is easily done at the knee to a great extent.

I shall not go back to the various proofs which establish the cellular structure of the serous system; almost all these proofs are applicable to the synovial system, which appears to be but a net-work of absorbents and exhalants. Hence it is easy to understand what the red and fatty bunches are that are found around the articulations. They perform in regard to this membrane the functions of the abundant cellular texture which envelops the peritoneum, the pleura, &c. &c. It is there that the blood vessels divide ad infinitum before arriving at the membrane where their ramifications, successively decreasing, finally terminate in the exhalants.

If a remarkable redness sometimes distinguishes these bunches from the cellular texture, it is because the vessels are more concentrated and nearer together in them. For example, in the articulation of the hip, the synovial membrane of which, almost everywhere adherent, only corresponds in the fissure of the cotyloid cavity with the cellular texture, nature has placed there almost all the arterial ramifications that furnish the synovia; hence the reddish tinge of the cellular bunch that is found there. On the contrary, at the knee where much cellular texture surrounds the whole external face of the synovial sac, the vessels more scattered leave to this texture the same colour as that of the external face of the serous membranes, &c. This redness of some pretended synovial glands, the only character that distinguishes them, is then as it were merely accidental; it no more indicates their glandular nature, than it proves it in the pia-mater, in which it is owing to the same cause.

Though the synovial membrane is very analogous to the serous surfaces, it must however exhibit differences of texture, since the fluid it exhales is a little different. In fact, by examining it at the femoro-tibial articulation, where it can be found in considerable pieces, it is seen to be more dense and compact than the serous membranes. Its texture has not the suppleness of theirs; when dried, it is much more brittle; it remains stiff, whilst the serous texture is moved in all directions without the least effort. It resists maceration longer.

Properties.

The properties of texture become evident in articular dropsies, in which the synovial membranes are at first much distended, and in which they contract after the puncture, an operation however that is very rare. Yet it appears that these membranes are only susceptible of a slow and gradual extension. We know that suddenly separated in luxations, their parietes tear instead of stretching; they unite again after the reduction.

Among the vital properties, the organic sensibility is the only one of this system in the ordinary state, as I have proved by many experiments on living animals in which these surfaces have been laid bare and irritated by various agents. But the increase of life which inflammation produces by raising this sensibility, transforms it into animal sensibility; this is what is observed, 1st, in the wounds in which these membranes are exposed to the contact of the air; 2d, in the long continued irritation they experience from foreign bodies preternaturally developed in the articulation; 3d, in the various affections of the articular surfaces, &c.

This kind of sensibility of the synovial membranes serves to confirm what I have already established above, viz. that most of the articulations, the ginglymoid especially, are destitute of fibrous capsules. In fact, I have observed that these capsules, as well as the lateral ligaments, have a kind of animal sensibility, which is developed by pulling them; so that if all the neighbouring organs of an articulation, except the synovial membrane and the lateral ligaments, are removed and this articulation afterwards twisted, the animal gives signs of the most acute pain. But afterwards cut the ligaments, and leave only the synovial membrane, the twisting no longer gives pain; then there is no fibrous capsule united to the synovial. This experiment, which is easily repeated upon the fore or hind legs, enables us to recognise everywhere the articulations in which the synovial membrane exists alone, and those in which it is found united to a fibrous capsule. This being of the same texture as the lateral ligaments, produces the same pains when it is pulled, as is proved elsewhere by experiments made upon the articulations clothed with these capsules.

The alternate exhalation and absorption which takes place upon the serous surfaces, prove the insensible contractility in them.

I have already observed that the synovial surfaces perform but a small part in the sympathies, that they feel but very slightly the affections of the other organs. Whilst in the acute affections of the important viscera, the skin, the mucous surfaces, the cellular texture, the nerves, &c. &c. have a greater or less sympathetic derangement, all the synovial membranes remain unaffected; they do not become the seat of irregular pains, nor of a more active or slower exhalation. They resemble in this respect the osseous, cartilaginous, and even fibrous systems. Thus it is not necessary that the physician should seek in the synovial system a frequent seat of the accessory symptoms in diseases, of that class of symptoms which does not belong to the injury of the diseased organ itself, but to its relations with other parts.

In the pains of the articulations, there are certainly cases in which the synovial membrane is diseased, and others in which the fibrous organs alone are affected. The distinction of these cases should be sought.

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.

I have already observed in the article on the fibrous capsules, that when the head of a bone remains displaced in a luxation, it is not a membrane analogous to these capsules that is developed around it; it is a real cyst, smooth on its internal surface, moistened with serum, formed at the expense of the cellular texture, and presenting, with a little more thickness, the true appearance of the synovial membranes; it is a preternatural synovial membrane. The motions imparted to the displaced limb appear to increase the serous exhalation in this new membrane; hence no doubt the great advantage of these motions, in order to re-establish in part the mobility of the bones which remain out of their sockets. I have seen a dancer, the head of whose humerus was lodged in the hollow of the axilla, after a luxation that was not reduced, perform very varied motions with it.

ARTICLE SECOND.

SYNOVIAL SYSTEM OF THE TENDONS.

This system noticed by many authors and described by Fourcroy, Soemmering, &c. is precisely of the same nature as the preceding, from which it differs only by its situation; it is often even confounded with it. Thus the synovial membrane of the tendon of the biceps is continuous with that of the scapulo-humeral articulation; thus those of the gemelli are so with the synovial membrane of the femoro-tibial articulation; it is the same membrane which belongs at the same time to the tendon and to the articulation. A remarkable example of it is seen in the extensors of the leg and the ham, to the tendons of which the same articular synovial membrane of the knee serves for a capsule.

But very few tendinous synovial membranes are found in the trunk; almost all are on the extremities where they serve to assist the slipping of the tendons. They are met with, 1st, where a tendon is reflected at an angle upon a bone, as around those of the great lateral peroneus, the peroneus medius, the obturator internus, the great oblique of the eye, &c.; 2d, where a tendon slips upon an osseous surface without being reflected, as at the extremity of the tendo Achillis, as under that of the great glutæus, and those of the psoas and iliacus united; 3d, where a tendon slips in a fibrous capsule, as in those of all the flexors, &c. Their extent is uniformly in proportion to that of the tendons upon which they are spread.