Part 29

The word cartilage is employed too vaguely. It designates, in the common acceptation, bodies whose organization differ essentially. The cartilages of the nose and those of the articular surfaces have certainly but a very remote analogy between them; it is necessary then to establish a line of demarcation. I have endeavoured to do it by making two systems of them; one comprehends the cartilages properly so called, the other, the fibro-cartilaginous substances, such as those that are between the vertebræ, those in the middle of some articulations, &c. As this last is a compound of the fibrous and cartilaginous system, I shall not treat of it until I have spoken of the fibrous system.

By thus limiting the sense of the word cartilage, it gives us the idea of a hard, elastic, white substance, having an inorganic appearance, though it has a real organization. We find this animal substance in different parts of the body; it is met with especially, 1st, at the articular extremities of the moveable bones; 2d, on the articular surfaces of the immoveable bones; 3d, on the parietes of certain cavities, which it contributes in great measure to form; such are the cartilages of the nasal partition, of the ribs, the larynx, &c. Hence three different classes, which exhibit varieties in their forms, in their organization, &c.

ARTICLE FIRST.

OF THE FORMS OF THE CARTILAGINOUS SYSTEM.

The forms of the cartilages vary according to the class to which the cartilage belongs.

I. Forms of the Cartilages of the Moveable Articulations.

In every moveable articulation, there is at each osseous extremity, a cartilage which covers this extremity, which facilitates by its suppleness the motion of the two bones, the very hard substance of which would occasion by friction too great a shock; which reflects by its elasticity a considerable part of the motion, thus made more extensive; which breaks, by yielding a little, the violence of the shocks the limbs experience, and which thus render these shocks less dangerous.

The general character of the internal conformation peculiar to these cartilages is that of being always less thick than broad, of having a thickness which is in the ratio of their breadth: so that the cartilages of the great articulations exceed in this respect two, three, or four times even, those of the smaller articulations; and these cartilages are moulded upon the articular forms and exhibit two faces and a circumference.

One of these faces adheres to the bone; it unites with it so intimately, that fractures take place any where else rather than here. The means of union are not exactly known; this is certain, that the cartilage is not an elongation, a continuation of the cartilaginous parenchyma of the bone; the fibres of this parenchyma are not continuous with those of the cartilages. If it really was so, by removing from a long bone, fresh and clothed with its cartilage, the phosphate of lime which penetrates it, we should see this continuity, the bone and the cartilage would not differ; now I have constantly observed that in this experiment the action of the acid detaches the cartilage from the bone, either in fragments or as a whole. We see the cartilaginous fibres of the bone deprived of its saline base, terminating evidently on the convex surface which the cartilage embraces; no solution of continuity has taken place. In general, the appearance of the cartilaginous parenchyma, separate from its calcareous portion, is wholly different from that of a true cartilage., I presume that this difference is owing to the quantity of gelatine, which is greater in the second than in the first. The action of the acids, especially the nitric, is the best means of separating a cartilage from its osseous head; maceration does not produce this phenomenon under a great length of time; in ebullition, as the gelatine melts, it is less apparent.

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.

On the head these cartilages are very numerous; those of the cranium have more thickness on the convex than on the concave side, hence the more rapid disappearance of sutures in the last than in the first direction.

Wherever they are found, they contribute to unite the bones which form the cavities; hence, as we have said, there is less danger for these from external bodies, since the motion lost then in part in their soft texture, produces a less effect than if the cavity was only one osseous piece.

It appears that these cartilages have much more affinity to the phosphate of lime, than those of the moveable articulations, which rarely ossify, whilst at an advanced age these always become osseous, of which the cranium especially furnishes us examples. I would observe however that the cartilages of the indented surfaces have much more tendency to ossification than those of the surfaces in juxta-position. At least it is more common to see the parietal bones united with the occipital and frontal, than to see the union of the ossa maxillaria, ossa palati, &c.

III. Forms of the Cartilages of the Cavities.

The cartilages of the cavities have two different forms according to the parts which they contribute to form. They are, 1st, long, as in the ribs; 2d, flat, as in the larynx, the nasal partition, &c.

All are covered on the exterior with a fibrous membrane like the periosteum, and in which different muscles are inserted. In order to see this membrane it is necessary to macerate the cartilage for a day or two; it then becomes white and very evident in its thickness, and the direction of its fibres. The cartilages of the cavities do not exhibit the numerous foramina which are seen in the bones, because the blood vessels do not penetrate them. But few eminences and depressions are observed in them. We can hardly consider their forms in a general manner, because destined to very different uses, they have but little resemblance in their conformation.

ARTICLE SECOND.

ORGANIZATION OF THE CARTILAGINOUS SYSTEM.

In examining a cartilage in its interior, it is difficult to recognize in it an organic texture; there is one however, which is composed of a peculiar texture and of common textures.