The surface of the cartilage opposite to the bone, is intimately united to the synovial membrane of the articulation. It borrows from it the polish which it has; for wherever these substances do not correspond to this membrane, they lose this character, as we see in the larynx, in the cartilages of the ribs, &c. Here the means of adhesion is an extremely compact cellular texture, which neither maceration nor dissection can remove in layers. As the synovial membrane is wholly formed from this texture, it appears that upon the cartilage it is only an elongation of that which, after having contributed to the structure of this organ, is condensed upon its surface and organized as a membrane.
The circumference of the cartilages of which we are treating, terminates insensibly upon the osseous surface; it ceases at the place, where the synovial membrane quits the bone to be reflected.
The two corresponding cartilages of a moveable articulation are so arranged and adapted to each other, that in the medium position of the articulation, they touch exactly at all their points; now we know that the medium position of an articulation is that in which the bone inclines in neither direction, in which all the muscles contracting uniformly and making an equal resistance, keep it equally from extension and flexion, from adduction and abduction, &c. &c. and hold it in the exact medium. It is this position which the limbs take, when the will does not direct the muscular effort, as for example, in the fœtus, in sleep, in rest, &c.; this is what some convulsions produce, in which all the muscles of a limb are agitated with an equal effort, by an extraordinary influx of the nervous power, &c. In no other position does the contact of the two articular cartilages take place at all their points; one portion of the surface of each always pushes with more or less force the parts surrounding the articulation, and distends them. The softness of the cartilaginous texture renders less painful this pressure, which would be distressing in the great motions, if the bones preserved their hardness at the extremity of the lever which they represent.
The cartilaginous forms of which we are treating, besides these common characters, have others peculiar to each kind of moveable articulations.
1st. In the first kind, the convex crust which covers the osseous head, is thick in the centre, but not at the circumference. An opposite arrangement takes place in the concave crust which receives this head; often even as in the humerus, and the femur, the thickness of this crust is increased at its circumference by a fibro-cartilaginous band. In this way, the effort is borne unequally by the two cartilages; but the uniformity of contact is established.
2d. In the second kind, which differs from the first by the absence of the motion of rotation, though in general a convexity is also received into a concavity, the arrangement for both the cartilages is nearly the same. Yet if two convex surfaces slide upon each other, an example of which we have in the condyle of the jaw and the transverse apophysis, then the cartilage is constantly becoming thinner towards the circumference of each; but then an inter-articular cartilage, thick at the circumference, thin in the middle, supplies the place of this arrangement, and establishes at all points a contact, which, without it, would only take place at the centre.
3d. In the third kind, the cartilaginous crust which covers the prominences and depressions, which reciprocally receive each other upon the extremities of the two bones, exhibits nearly an uniform thickness, as we see at the elbow, the knee, &c.; so that the pressure comes equally upon the whole articular surface.
4th. In the fourth and fifth kinds, the cartilaginous crusts moulded on the form of the osseous surfaces, have also a thickness nearly uniform at all points; I have found upon the bones of an adult, that this thickness is a line and a half in the radio-cubital articulation and that of the atlas, and a line in the carpal and metacarpal articulations.
II. Forms of the Cartilages of the Immoveable Articulations.
The cartilages are found only in two kinds of immoveable articulations, viz. in those with surfaces in juxta-position and those with indented surfaces. They form in all a very thin layer, continued upon both bones which articulate together, arising from their osseous portion, like those described before, being of the same nature as it, and forming a band so much the more close as we advance in age.