Part 15

1st. However strong the adhesions of the synovial membrane may be, they can be destroyed without a solution of continuity, by a slow, careful dissection begun at the place where the membrane is reflected from the cartilage upon the capsule. It can be taken away in parts after long continued maceration. 2d. In consequence of certain inflammations, this membrane acquires a thickness and opacity which enable us to distinguish it from all the neighbouring organs, from those even to which it adheres the most. 3d. The synovial bags are all as adherent as the articular synovial one, to the cartilages of their sheath and to this sheath itself; yet every one acknowledges their distinct existence. 4th. There are articulations with a fibrous capsule, in which the fibres are separated so as to leave a space between them through which the synovia would escape, if the synovial membrane did not line them. When air is forced into the articulation, this membrane rises up in these spaces and exhibits a texture wholly different from that of the capsule. Bertin observed this, but thought that these pellicles were insulated, and did not see that they depended upon the continuity of the membrane which is extended over the whole articulation. 5th. We have observed in the article upon the serous system, that the smooth and polished appearance which the surface of the organs and the cavities exhibit, is always given to them by these membranes, and that they never derive it from their peculiar structure; now we shall see that the synovial membrane has almost the same texture as the serous; then it appears that in the places in which the articular organs exhibit this character, it is from it that they receive it, though it cannot be distinguished as well upon these organs, as where it is free. Besides, the articulations that are evidently destitute of this membrane, have not this smooth and polished appearance. Such are the surfaces of the symphysis pubis, and of the sacro-iliac symphysis which are found, though contiguous, unequal, rough, &c. We have also proved that this organic form is never owing to compression.

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.

_Forms; Relations; Synovial Fluid._

The tendinous synovial membranes, are, like the articular ones, sacs without an opening, spread on the one hand on the tendon and on the other upon the neighbouring organs. These sacs are differently shaped according to the arrangement of the tendon, but their general conformation is uniform. We see from this that every tendinous synovial membrane has two faces, one which forms the interior of the sac, which is everywhere free and contiguous to itself, the other which lines the adjacent organs.

The free surface is constantly moistened by a fluid precisely similar to that of the articulations, furnished like it by exhalation, and not as authors have said by red bodies situated in the neighbourhood, bodies of which oftentimes there is no trace visible, and which, when they exist, have nothing glandular in them. This fluid is in general much less abundant than in the articulations, at least in the dead body. But there are varieties in the different synovial bags; those of the tendo Achillis, of the tendons of the psoas and iliacus united, of that of the obturator internus, &c. are always more moist than those of the flexor tendons, &c.

Is it to the absence of synovia that must be attributed the species of crepitation which the tendons sometimes make in their motions? I know not. I would only observe that this crepitation has some analogy with the crackling noise of the joints of the fingers when they are bent quickly, a noise, which does not depend, as might be supposed, on the friction of the osseous surfaces; in fact, when it has been once produced, it cannot be again, though there may be friction again. Besides it is known that this crackling noise arises from the forced elongation of the phalanges, and consequently from the separation of their articular surfaces, as well as from the flexion.

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.

GLANDULAR SYSTEM.