As ossification advances in the bones and gelatine is consequently
carried to them in less quantity, it seems that it goes more abundantly
to the articular surfaces; for the cartilages that are met with,
then lose their primitive softness, and have a consistence that is
constantly increasing. Yet much more gelatine leaves the bones, than
is carried to the cartilages; so that we may say that this substance
is continually diminishing, in the organs, in proportion as we advance
in age. We know that it is the parts of young animals particularly,
that are selected to make glue, jelly, &c. The articular cartilages
at this period exhibit a phenomenon that I have frequently noticed in
my experiments; when we macerate them in water for two or three days,
they take a very evident red colour. This colour does not penetrate
deeply; but if we cut the cartilage in many places so as to bring the
fluid in contact with its interior, the whole of it becomes red. The
cartilages of ossification exhibit the same phenomenon, which becomes
less conspicuous as we advance in age; so that in adults generally,
the cartilages do not lose their white colour by maceration. In some
however they take a reddish tinge which is infinitely less bright than
in the fœtus. Whence arises this phenomenon? Does the water give to the
cartilage the cause of its colour, or does it take from it by solution
certain substances which prevented this colour from being developed?
Whatever may be the cause, none of the organs of articulation redden in
this way; all on the contrary, the synovial, the ligaments, &c. become
whiter.
There is usually no sensible demarcation between the cartilage that is
to become bone, and that which is to remain as it is; sometimes however
on the one hand we observe it a more dull colour at the extremity of
the bones, whilst on the other, we never discover the reddish streaks,
which are so frequently seen irregularly scattered on the cartilages of
ossification.
As long as ossification continues, there is between the cartilage and
the osseous portion already formed, a very evident vascular layer, and
it is extremely easy to separate these two portions, which adhere but
slightly to each other. We observe also on the surface of each when
they are separated, several inequalities, projections and depressions
reciprocally adapted to each other. It is the want of adhesion of the
cartilaginous and osseous portions, before complete ossification,
which has no doubt given rise to all that has been written upon the
separation of the epiphyses, a separation which the observations of
modern surgeons have rarely confirmed.
As the calcareous substance arrives at the extremities of the bone, the
vessels gradually disappear, and the adhesions increase. Finally, the
ossification being finished, there is no longer, on the one hand, an
evident vascular net-work between the cartilage and the bone; and on
the other, their union is such, that all rupture between them is almost
impossible. These two characters especially distinguish the relation
of the cartilage of ossification with the bone, from that of the real
cartilage with the same bone. I have observed also that almost always
above its union with the osseous portion, the cartilage of ossification
has less whiteness, a deeper tinge, which extends the distance of two
or three lines, and whose difference is often very considerable; this
is the forerunner of the access of the blood. This arrangement does not
exist in the cartilage of the bones of the adult.
We attribute commonly to the articular motions, the want of
ossification of the cartilages of the moveable articulations; but I
believe that it depends wholly upon the laws of osseous nutrition.
Nature limits there the exhalation of the phosphate of lime, as it
limits at the origin of a tendon the exhalation of the fibrin of the
muscle which corresponds to it; it is because the mode of organic
sensibility changes and the vessels of the cartilages are no longer
in relation with the red part of the blood nor with the calcareous
substance. In fact, by supposing the preceding hypothesis true, why do
the cartilages of the immoveable articulations exist? Why should the
motion which elsewhere favours exhalations and secretions, prevent here
the first of these? Why do preternatural ossifications take place in
the most moveable parts, of which the arteries furnish us an example?
Why, in many anchyloses in which the articular surfaces unite, and in
which the motion is destroyed, do not the cartilages disappear?
The cartilages of the cavities have a mode of origin, development and
nutrition, perfectly analogous to that of the articular cartilages.
I would observe that their texture differs, as well as the texture
of these, from that of the cartilages of ossification, in this, that
these last are crossed by many grey lines, and the others are not.
When we cut the cartilages of ossification in any direction whatever,
their divided surfaces exhibit numerous small points which are the cut
extremities of these lines, which appear to be vessels, that, without
yet circulating blood, contain a fluid of a deeper colour than that of
the cartilaginous texture.