Nothing is easier after what has been said upon the osseous nutrition,
than to understand the formation of the callus. We know that it has
three periods, 1st, the development of the fleshy granulations; 2d,
their change into cartilage; 3d, the change of this cartilage into
bone. This triple phenomenon takes place in a space of time that varies
according to the age, the fracture, the kind of bone, &c. but which is
in general longer than that of the other cicatrices.
The development of fleshy granulations is a phenomenon common to
every species of organ which has experienced a solution of continuity
and whose divided edges are not in immediate contact. Here these
granulations arise from every part of the divided surface, from the
periosteum, the compact texture and that of the cells, the last
especially. Those of one side unite to those of the opposite. Thus far
the osseous cicatrix does not differ from that of the other parts. This
state corresponds with the mucous state of natural ossification. As the
fleshy fibres are but the extension of the nutritive parenchyma, they
have its vital forces; their organic sensibility follows the same laws
as in ordinary nutrition; at first it is in relation with gelatine;
this is then exhaled; then commences the cartilaginous state; then the
osseous cicatrix takes a peculiar character, which distinguishes it
from that of the other organs.
At the end of a longer time, the organic sensibility increases in the
parenchyma of cicatrization which the fleshy granulations form; then
these become in relation with the calcareous substance which comes to
the bone, and which they had until then repelled; they admit it then,
as well as the red portion of the blood which always precedes it in
every species of ossification.
Hence we see that the callus is cellular and vascular in the first
period; that in the second it contains cellular texture and vessels,
with gelatine; that in the third, it has cellular texture, vessels, and
gelatine, with calcareous substance.
It has not the regular forms of the sound bone, because the parenchyma
of cicatrization arising irregularly upon the osseous surfaces, the
exhalation and absorption of gelatine cannot be made in a precise and
regular manner. The callus is so much the larger in proportion to the
separation of the ends of the bone, because the fleshy granulations
having had more space to go over in order to meet, are more extensive,
and consequently have absorbed more nutritive substance.
If the constant motion of the fractured ends prevents on each side
the granulations, or what is the same thing, the two parenchymas of
cicatrization from uniting, then, notwithstanding the exhalation of
the nutritive substances in each of them, the bone does not unite, and
hence the preternatural articulations.
Callus is formed with difficulty when the ends divided and laid
bare, suppurate with the neighbouring parts, as happens in compound
fractures, because the formation of pus expends the nutritive
substances destined to repair the fracture. The further considerations
upon this singular production belong to pathology.
I have not exposed in this chapter the ideas of the ancients, who
thought that the bones were formed by the hardening of an osseous
juice, the existence of which there is nothing to demonstrate; nor
those of Haller, who imagined that the heart hollowed out arterial
channels in the osseous substance by its own impulse, and hardened this
substance by the pulsation of the arteries; nor those of Duhamel, who
made every thing depend upon the periosteum; I refer to various works
that have a thousand times refuted these opinions.
Without refuting any one in particular, I would remark that they have
one fundamental error, viz. that of considering osseous nutrition in
an insulated manner, of not presenting it as a division of general
nutrition, of admitting for its explanation reasonings only applicable
to the bones, and which are not derived as consequences from those
which serve to establish the nutrition of all the organs. Let us
never lose sight of this essential principle, upon which rest all the
phenomena of the economy, viz. that over a multitude of effects, a very
small number of causes only presides. Let us mistrust every explanation
which is partial and mutilated, which circumscribes the resources of
nature according to the limits of our weak understandings.