ARTICLE SIXTH.
OF THE FIBROUS CAPSULES.
The fibrous capsules are infinitely more rare in the economy, than they have heretofore been thought to be. The scapulo-humeral and the ilio-femoral articulations are almost the only ones furnished with them. Elsewhere there is nothing scarcely but synovial membranes.
I. Forms of the Fibrous Capsules.
These capsules form a kind of cylindrical sac open at the two extremities, attached by the circumference of its openings, around the superior and inferior articular surfaces, intermixing at its insertion with the periosteum. They are so much the more loose, as the motions of the articulations are the more extended; that of the humerus, for example, allows a much greater separation of the osseous articular surfaces, than that of the femur; their length in fact is almost the same. Now, as on the one hand, the neck of the first bone is much less than that of the second, and as on the other, both these capsules are inserted at the base of this neck, it follows that the extent of the separation is in the inverse ratio of the length of the articular necks.
Much cellular texture surrounds these capsules externally, which the tendinous fibres and even the tendons coming from the neighbouring muscles, strengthen remarkably. They are sometimes open to allow tendons to pass which are inserted in the bone between them and the synovial membrane, an example of this is seen in the scapulo-humeral articulation for the sub-scapularis. Anatomists who have observed the insertion of the tendons in the capsules, have concluded from it, that the muscles of these tendons were destined to prevent the capsule from being pinched by the articular surfaces in motion. This appears to me improbable; but at least the muscles are destined to prevent the looseness of the capsule during the great motions, which would have been weakened by this looseness; thus there are many of this kind of muscles at the humeral capsule, whilst we see none of them at the femoral, which is, as I have said, much less loose. Within, the capsules are very closely united to the synovial membrane, especially in adults; for in infants, this adhesion is less. Near their extremity however this relation does not exist, because the synovial membrane being reflected upon the cartilage, a triangular space is left between it and the capsule which is attached to the bone, and as this arrangement continues all round the articulation, there results from it a kind of circular canal, filled with cellular texture, and covered with vessels, which I have sometimes distended with an injection pushed into a small opening made for the purpose.
The intimate union of the capsule with the synovial membrane prevents its folds and also its contusion in the great articular motions.
II. Functions of the Fibrous Capsules.
Why are the fibrous capsules found only around the first kind of articulations? The reason of it is plain; as these articulations have in all directions motions nearly equal, they should have on all sides an equal resistance, whilst the others moving in one or two directions only, the ligaments were unnecessary except at certain places, to limit these motions. Hence why for example, the fibrous system is spread out like a membrane around the ilio-femoral articulation, and collected into distinct fasciæ around the femoro-tibial, where the synovial membrane is almost everywhere bare.
We understand from all that has been said, that the only use of the fibrous capsules is to strengthen the articular relations, and that this use has no connexion with synovial exhalation.
When in luxations not reduced, the head of the bone has left the articular cavity, a new membrane is formed around it in the cellular texture which serves for a capsule; but this membrane has not the texture of the former one. I have observed in two subjects, that no fibre could be distinguished in it, that its texture was very analogous to that of the different cysts that are often found in many parts of the economy, of those especially that form round foreign bodies, the presence of which is not a cause of suppuration, and that consequently these preternatural capsules belong rather to the class of serous than to that of the fibrous membranes.