I place this system at the side of the serous, because it has the greatest analogy with it, under the relations, 1st, of its form, which is in each of these organs that of a sac without an opening; 2d, of its texture, which appears to be essentially cellular; 3d, of its functions, which consist in an alternation of exhalation and absorption.
What establishes a real line of demarcation between these two systems is, 1st, that the fluids which lubricate their membranes appear to differ in their composition, though there is much resemblance between them. 2d. In dropsical diatheses which affect at the same time the cellular texture and all the serous surfaces of the peritoneum, the pleura, &c. the affection does not extend to the synovial membranes, which indicates a difference of structure, though we do not know what this difference is. 3d. And reciprocally in dropsies of the articulations, an affection in general very rare, and in those of the tendinous synovial capsules, there is no concomitant affection of the membranes of the great cavities. 4th. The fluid of the articular dropsies does not resemble that which fills the great cavities in the same disease. 5th. The synovial membranes are much more rarely than the serous, the seat of those slow and tubercular inflammations which the serous surfaces so often exhibit. I have however seen two examples of it in the synovial membrane of the knee. I believe that these two systems are the only ones in which this disease is observed; so that it is by its existence a character of resemblance, and by its rarity or frequency, a distinguishing attribute. 6th. The different kinds of adhesions of which I have spoken as taking place on the serous surface, are not met with on the synovial surfaces, where we see only that which identifies these two adhering surfaces, a mode which frequently takes place in anchylosis, which is also as often occasioned by it, as by the stiffness of the parts surrounding the articulation. 7th. The synovial surfaces are not as often as the serous, the seat of those remarkable locomotions of which we have spoken; which depends on this, that the articular organs are not, like the most of those covered with serous surfaces, subject to alternate dilatations and contractions.
The synovial system exhibits evidently two great divisions. To one belongs the articular system, to the other that of the tendinous grooves. Each shall be examined separately.
ARTICLE FIRST.
ARTICULAR SYNOVIAL SYSTEM.
I believe that I first described this essential portion of the synovial system. I shall relate here what I have said of it elsewhere. I shall examine first how it is separated from the blood, afterwards the fluid itself, and then I shall describe the organ which furnishes it.
I. How the Synovia is separated from the mass of Blood.
Every fluid differing from the blood, can be separated from it to be afterwards transmitted to an organ, but in one of the three following ways; 1st, by secretion, a function characterized by the existence of a gland intermediate to the blood vessels that bring the materials to it, and the excretories which carry off the result; 2d, by exhalation, a function distinguished from the first, by the absence of this intermediate gland, and by the immediate continuity of the blood vessel and the exhaling duct; 3d, by transudation, a phenomenon purely physical, almost always happening after death, rarely observed during life, a simple transmission of a fluid by the pores of an organ, towards which it is mechanically determined. Let us examine which of these three modes is that chosen by nature to deposit the synovia upon the articular surfaces.
Is the synovia transmitted by secretion to the articular surfaces?
We are indebted to Clopton Havers for the system which places in the glands the sources of the synovia. Many authors had designated obscurely before him these organs in the articulations; but he made them the particular object of his researches, described them in the different articulations, divided them into two classes, the one principal, the other accessory, and assigned them characters so evident, that according to him, they could not be forgotten. Reddish bunches, spongy, formed by membranes folded upon themselves, situated sometimes without, and sometimes within the articulations, always arranged so as to be protected from too strong a compression, and pouring out through ducts in the form of fringe the fluid they secrete; such are the characters drawn by Havers, which all anatomists since him admire, and the correctness of which the most modern and distinguished authors have acknowledged in their works.
Some anatomists of this age have however thrown doubts upon these glandular bodies. Lieutaud confounds them with the fatty cellular texture. Desault did not distinguish them from it. Every thing confirms me in the same opinion, which many considerations appear to establish in an undoubted manner. The following are these considerations; 1st, these reddish bunches are met with only in certain articulations. There are many of them in which their existence cannot be established but by supposition. 2d. The greatest number of the synovial membranes of the tendons certainly do not exhibit any of them, though Havers, Albinus, Juncke and Fourcroy admit them in all, founded no doubt upon analogy and not upon inspection. Yet the synovia is separated equally in both cases, and lubricates the surfaces of the articulations and of the tendinous sheaths; this separation is then independent of glandular action. 3d. If the best marked synovial glands are examined, such as that of the cotyloid cavity, no trace can be discovered there of this parenchyma of an unknown nature, but remarkable for its structure, which composes in general the glands, and which distinguishes them from every other part and forms their true organic character. 4th. No excretory duct can be demonstrated in these organs. Those in the form of fringes, admitted by Havers, are imaginary. Bertin himself has acknowledged this truth, though he attributed to these bodies a glandular structure. The transudation of the fluids injected by the arteries in the neighbourhood of the articulation, proves the existence of these ducts no better than it establishes them in the cavities of the serous membranes in which it also takes place, and yet in which it is well proved that no gland pours out the albuminous fluid that constantly lubricates these cavities. 5th. Inflation resolves completely these fatty bunches into cellular texture. Maceration produces the same effect. When gradual and long continued ebullition has removed all the fat from them, there remains only a mass of cells pressed together, and similar to those of the common cellular texture. 6th. The glandular character is manifested in certain morbid cases, by a peculiar swelling and hardening, of which the other organs except the glands, such as the muscles, the tendons, &c. never offer an example. The liver, the kidneys, the salivary organs, all the considerable glands are remarkable for this. So true is this character, that it serves to indicate glands, the delicacy of which conceals them in the natural state. For example, the existence of the cryptÆ of the stomach, the urethra, and several other mucous membranes, is founded first upon the analogy of the other membranes of this class, but principally upon the preternatural development which these cryptÆ acquire in certain diseases. Never, on the contrary, do the pretended synovial glands present to the observer a similar development. Always in the diseases of the articulations, a common swelling seems to identify them with the neighbouring cellular texture. They have not like the other glands, affections distinct from those of this texture, no doubt because they have not a peculiar vitality, because being mere elongations of the neighbouring cellular texture, they partake of its nature and properties, and ought consequently to partake of all its conditions, as it in its turn ought immediately to receive the influence of their affections.
The considerations which I have just offered successively form, I think, sufficient data to resolve the problem proposed above, by establishing as an incontestable proposition, that the synovia is not transmitted by secretion to the articular surfaces.
Let us examine the second mode of transmission stated by authors.
Is the synovia transmitted by transudation to the articular surfaces?
It was an opinion anciently received, that the marrow of the long bones oozes through the pores of their extremities and through those of the cartilages which terminate them, to lubricate the articular surfaces. Havers renewed this idea forgotten at the time he wrote, united this source of the synovia to that which had placed it in the glands, and thus formed of this fluid a mixture composed of two fluids differently transmitted to the articulation. The most of those who succeeded him partook of his opinion upon this point. Those even, such as Desault, who rejected the existence of the articular glands and the secretion of synovia, admitted the transudation of it founded upon the following observations. 1st. A long bone, stripped of its soft parts and exposed to the air, allows a fatty oozing to pass through the pores of its cartilages which does not cease till the medullary juice is completely exhausted. 2d. The mechanical compression of the cartilaginous extremity of a long bone produces for a moment the same effect. Are these facts, which are evident in the dead bone, also real in the living one? Different considerations, which I will now state, induce one to believe the contrary.
1st. The vital forces, the effect of which is to impart to all the organs which they animate a degree of tone sufficient to resist the entrance of the fluids, leave, when they are extinct, the fibres of these same organs in a state of laxity that renders them everywhere permeable. Thus transudation is now considered as hardly any thing else than a phenomenon that takes place only after death, which, if transformed into a vital one, would offer an evident exception to the laws of nature that are especially characterized by simplicity and uniformity. 2d. The fatty oozing takes place in the experiment noticed above, not only through the pores of the cartilages, but also through those of the whole surface of the bone; so that by reasoning from what has been here observed upon the dead body, it is evident that during life the whole bone would be, if you may so say, immersed in an atmosphere of synovia, a consequence, which being proved false by the most simple inspection, demonstrates the falsity of the principle deduced from it. 3d. The articulations of the cartilages of the larynx are lubricated like those of the bones, by the synovial fluid; and yet here all transudation of marrow is impossible, as it does not exist in the substances of the cartilages. 4th. The marrow is almost always sound in diseases which, affecting the articulations alter the fluid that lubricates them. And reciprocally the synovia does not take a different character in the affections of the interior of the bones, which have an influence especially upon the medullary organ. 5th. Finally, the experiment that I have made, and which has been stated in the article upon the marrow, evidently proves the non-transudation of this fluid.
Desault, in order to explain the manner in which the synovia is separated from the blood, added to this pretended transudation of the marrow, an oozing furnished by all the parts contained in the articulation, such as the capsular and inter-articular ligaments, the internal fat, the cartilages, &c. A comparison will suffice to show the value of this hypothesis. What should we say of a system in which to explain the production of the serous fluid of the abdomen, the source of it should be placed in the liver, the spleen, the intestines, and in general in all the organs of this cavity? No doubt we should answer, that a fluid of the same nature, could not be furnished by parts of such different structure, that it is much more simple to search for a single source in the single membrane that covers all the gastric viscera. The application is exact and the analogy perfect between it and the articular cavity.
We can, I believe, without fear of error, conclude from all that has been said above, that the synovia is not transmitted by transudation to the articular surfaces.
I will now examine the last mode pointed out for the separation of the synovia.
Is the synovia transmitted by exhalation to the articular surfaces?
The solution of the two preceding problems seem naturally to lead to that of the question which we here propose. The certainty of the two following data may, I think, be relied on; 1st. Secretion, exhalation and transudation are the only means by which a fluid different from the blood can be transmitted to an organ. 2d. Secretion and transudation are foreign to the transmission of the synovia. Now from these two certain data, can we not draw this conclusion as certain, that exhalation is the mode by which the synovia is carried to the articulations? But let us add to these negative proofs some considerations which establish this proposition positively.
The most striking relations are observed between the synovia and the fluid that lubricates the parietes of the serous membranes. 1st. The relation of composition. These two fluids are essentially albuminous. Albumen predominates in both, though a little different in each, as Marguerron has demonstrated. Havers had previously pointed out this analogy; he knew that these two fluids are coagulable by alkohol, the acids, and caloric, without knowing the principle to which this property is owing. 2d. The relation of functions. Both are destined to lubricate surfaces on which much motion takes place, to diminish the friction which is the inevitable consequence of it and to prevent fatal adhesions. Both are in the same state on their respective surfaces; it is merely a dew spread upon these surfaces, and soon taken up from them. 3d. The relation of affections. Inflammation dries up the source of both, and produces adhesions, more common in the serous membranes and more rare in the articulations in which they produce anchylosis. Both are subject to preternatural enlargement, which is designated by a common word, viz. dropsy. 4th. The relation of absorption. The lymphatic system is for both the way by which they re-enter the circulation, after having remained sufficiently long upon their respective surfaces.
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.
III. Of the Synovial Membranes.
We have seen all the great cavities lined by the serous membranes, which form by their folds species of sacs without an opening, and which embrace both the organs and the parietes of these cavities. There exists in all the moveable articulations membranes precisely analogous, the uses of which are the same, the nature of which is not different and which I call synovial, because their parietes constantly exhale and absorb synovia.
Forms.
Every synovial membrane then, should be considered as a sac without an opening, spread upon the organs of articulation, upon the diarthrodial cartilages, upon the internal face of the lateral and capsular ligaments, upon the whole of the inter-articular ligaments when they exist, upon the prominent fatty bunches in some articular cavities, &c. It is from it that these different organs borrow the smooth, polished and shining appearance which characterizes them in these cavities, and which they have not elsewhere. Just as by dissecting carefully the gastric organs, the peritoneum can be removed, its sac remaining whole, so we can conceive of the possibility of separating this membrane, notwithstanding the intimate adhesions it forms in some places. All the parts that it embraces are out of the articular cavity, though projecting into this cavity, as the lungs are found on the exterior of the sac formed by the pleura, the liver on the exterior of the peritoneal sac, &c. &c.
The synovial membrane is found in all the moveable articulations, the greatest number of which has only it and the lateral ligaments. What is commonly called the fibrous capsule is only met with around some of the articular surfaces. The connexions of the humerus, the femur, and one or two other bones, the extremities of which are joined by enarthrosis, present the only examples of it. There are seen in these articulations two very distinct coverings. One which is fibrous is external, and is found arranged in the form of a sac open above and below, embracing by its two great openings the surfaces of the two bones, and being intermixed around them with the periosteum, the fibres of both interlace with each other. The other cellular, which is the synovial membrane, lines the first on the interior, separates afterwards from it when it arrives towards the two diarthrodial cartilages, and is reflected upon them, instead of being united to the periosteum. Boyer has pointed out this arrangement in regard to the femur.
In all the ginglymoid articulations, as in those of the elbow, the knee, the phalanges, the hand, the foot, &c. &c. the fibrous capsule is wholly wanting. The fibres, instead of extending and interlacing in the form of a membrane, are collected into fasciculi more or less thick, which form the lateral ligaments. There is only found in them the internal layer of the enarthrodial articulations, that is to say the synovial membrane, which does not contract here any adhesion with the periosteum, but is reflected upon the cartilages. By taking it at the place of this reflection, it can be detached far enough to prove that it has an external organization wholly different from that which the idea of a capsular ligament at first presents to the mind. This arrangement is very easily perceived by the least dissection, at the knee behind the tendon of the crurÆus and the inferior ligament of the patella, at the elbow under the tendon of the triceps, at the phalanges under that of the extensor, &c. All the arthrodial articulations have also an analogous organization, as will be seen in the Descriptive Anatomy; so that it is ascertained that the fibrous capsules exist but in a very small number of articulations, that almost all have only synovial sacs which are spread out and reflected upon the osseous surfaces, without being attached around them, as all authors have said.
I have proved this remarkable difference of the articulations by many dissections. Some anatomists were in the way to discover it, when they observed that the different capsules appeared to be wholly formed of cellular texture. It is in fact the texture of the synovial membrane, which differs essentially in this from the fibrous capsules. The mode of capsule for all articulations may be preserved, if it is wished, but different ideas must then necessarily be attributed to it. Compare, for example, the fibrous capsule of the femur with the synovial capsule of the knee; you will find on the one hand, 1st, a cylindrical sac with two great openings for the osseous extremities, and with many small ones for the vessels; 2d, a fibrous interlacing, similar to that of the tendons, the aponeuroses, &c.; 3d, a mode of sensibility analogous to that of these organs; 4th, the use of retaining strongly in place the articulated bones, which have only this bond to strengthen their union. On the other hand you will observe, 1st, a sac without an opening; 2d, a cellular structure, the same as that of the serous membranes; 3d, a sensibility of the same nature as theirs; 4th, the simple function of containing the synovia and separating it, the bones being tied by strong ligaments. Besides, the different reagents have upon the fibrous capsules an influence wholly different from that which they exert upon the synovial. Ebullition yellows them, renders them semi-transparent, softens them like tendons and gradually melts them like gelatine. The synovial ones boiled remain whitish, and furnish but little of this substance. I would observe that the yellowish tinge and semi-transparency of the boiled fibrous capsules are a certain means of distinguishing the articulations in which they exist and those which are destitute of them.
The existence of the synovial capsule in the greatest number of articulations in which it is found alone, is placed beyond a doubt by the slightest inspection. In those in which it is united to a fibrous capsule, it is very clearly distinguished in several places. Thus in the thigh, it is found upon the interarticular ligament, upon the fatty bunch in the cotyloid cavity, and upon the neck of the bone at the places where it leaves the fibrous capsule, to be reflected upon the cartilages, &c.; but its adhesion to these cartilages and to the internal face of the capsule, may excite some doubts as to its arrangement in the form of a sac everywhere closed, which we have attributed to it; it is then essential to offer some considerations that may dissipate these doubts.
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 foetus 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 foetus 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.
