The bones, having become completely ossified, continue to undergo
different phenomena which anatomists have too much neglected. The
general growth in height is terminated when ossification is complete;
and it even appears that the term of these two is nearly the same; but
that in breadth continues for a long time after; compare the small and
slender body of a young man of eighteen years, with the stout and well
proportioned one of a man of forty, and you will see the difference.
The bones follow the general law; their nutrition is prolonged for
their thickness, when that for their length ceases. It appears that
then the vessels which penetrate by the foramina of the first and
second order, do not contribute to this nutrition, which draws its
materials from those of the third; now, as we know that these very
superficial vessels are arrested in the external fibres of the bone,
and do not penetrate within, we understand, 1st, how, the growth going
on without, the bone increases in thickness; 2d, how this increase
takes place especially on the compact texture, the proportional
thickness of which is in the direct ratio of the age, as we may see
by inspecting the different bones of the child, the adult and the old
person.
This external growth has made it believed that the periosteum
contributes to it especially by the ossification of its layers; but we
shall see in the article upon this membrane, what opinion should be
entertained upon this subject.
It is principally at this period in which the work of nutrition seems
spread upon the osseous surface, that the different eminences, which
are scattered over this surface, become more evident; then especially
all the prominences of insertion become more prominent; there is
in respect to these eminences a remarkable difference between the
skeleton of a child and that of a full grown man. In the fœtus they
hardly exist, as we see particularly by the different apophyses of the
vertebræ, the spinous especially. As these eminences are generally the
most distant parts from the primitive osseous centres, it appears that
it is to this circumstance that must be attributed the slowness of
their formation, since ossification always goes from points where it
begins, to the most distant ones.
When the bone has all its dimensions it still continues to be the
seat of a very active nutrition; exhalation constantly brings to it
gelatinous and calcareous substances which absorption afterwards takes
up; so that it is continually composed and recomposed. The experiment
with madder evidently proves this; if we feed an animal for some time
upon this, the bones become red much more easily, in proportion as
the animal is younger; so that by amputating a limb after some time,
the bones have an appearance wholly different from what is natural
to them; if, after this amputation, we discontinue the use of the
madder for some time, and then amputate another limb, the bones will
be found to have entirely resumed their natural colour; now we know
that the calcareous substance is the vehicle of the colouring matter,
since while the bones are only cartilaginous the madder has no effect
upon them. The calcareous substance is then alternately furnished and
taken away from the bones. Besides, the formation and resolution of
exostoses, the softening and brittleness of the bones, the phenomena
of the production of callus, &c. are they not an evident proof of
the exhalation and absorption of this principle? It appears clearly
that the urinary system is the way by which nature gets rid of the
calcareous and even of the gelatinous substance. It would be curious
to analyze accurately the urine of ricketty patients, and that of
those affected with cancer; it is probable that the first of these
substances predominates in the urine of the first, and the second in
that of the others; I know of no positive experiments upon the subject.
Can we, by giving to patients gelatine or the phosphate of lime,
restore to their bones the suppleness or solidity which they have lost?
No, because it is necessary not only to introduce these substances into
the economy but also to restore to the bones their peculiar organic
sensibility which they no longer have, and which, by placing these
substances in relation to them, would enable them to appropriate these
to their own nourishment. The blood might be loaded with earthy and
gelatinous principles, and the bones would repel these principles, so
long as their sensibility was not in relation with them.
The double motion of nutrition continues always in the bones, as we
advance in age; but its proportions change. The gelatine is constantly
diminishing and the calcareous substance constantly increasing.
Finally, in extreme old age this last predominates so much, that it
would destroy their life, if general death did not take place before
that of the bones.
It is to this that must be attributed the greyish colour that these
organs then take; hence also their constantly increasing weight; hence
consequently the difficulty of the motions of the limbs, since at the
same time that the force of the muscular powers is diminished by age,
the osseous resistance which they have to overcome increases.
At this period of life, the calcareous substance predominates so
much in the economy, that it is thrown upon different organs, such
as the arteries, the cartilages, the tendons, which then take the
osseous character. We might say that by accumulating in our parts this
substance foreign to life, nature wishes to prepare them insensibly for
death.
In general, it is those organs whose nutritive substance is gelatine,
which have the greatest tendency to be placed in relation with the
calcareous substance, and consequently to be encrusted with it. Hence
why the cartilages especially are ossified; why those of the sutures
disappearing, the bones of the cranium become continuous; why the
larynx is finally almost all osseous; why the cartilages of the ribs
are often as solid as the ribs themselves; why oftentimes many vertebræ
united form a more or less considerable continuous mass. I would
observe however that the arteries, which have so great a tendency to
ossification, are not so evidently gelatinous as many other substances
which ossify much less easily, as the tendons for example.