General Anatomy, Applied to Physiology and Medicine, Vol. 3 (of 3)

by Xavier Bichat

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.