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.