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.