We commonly distinguish three states in the development of the bones,
viz. the mucous, the cartilaginous and the osseous states.
_Mucous State._
The mucous state may be considered as existing at two periods: 1st. In
the first days of the development of the embryo, a period in which
the whole of its organs forms only a homogeneous and mucous mass, in
which it is not possible to distinguish any line of demarcation, and in
which the parenchymas of nutrition alone exist. All the organs are then
of the same nature; the bones are in fact mucous like all the other
organs, if by this word we understand a state in which the cellular
texture existing alone with the vessels and the nerves, is penetrated
by so large a quantity of juices, that it has the form of a mucilage,
and gives the appearance of it to the embryo. 2d. We may understand by
the words mucous state, that more advanced period of osseous nutrition,
in which the bones can be already distinguished, seen through the
transparency that the other parts of the limb still have, and in which
they have a consistence much greater than that of the parts which
surround them; now this state is only the commencement of that of
cartilage; for the parenchyma of nutrition takes the cartilaginous
character when it begins to be penetrated by gelatine, and it is in
fact penetrated by this substance when it has more consistence, since
it is that which gives it this consistence, and hence an existence
distinct from the surrounding parts. If in the early periods, this
cartilage is softer, if it flattens under the finger when pressed, if
it even has an appearance partly mucous, it is because the gelatine is
not yet in sufficiently large proportion, and because the nutritive
parenchyma still predominates; gradually its quantity increases, and
then the cartilaginous nature is more evidently developed.
It follows hence that the bones have three periods in their
development; one is common to them with all the other organs; it is the
mucous period; the two others especially characterize them; these are
the cartilaginous and osseous periods. Let us examine their phenomena.
_Cartilaginous State._
All the bones are cartilaginous before taking their last form.
This state of cartilage begins at a period that is difficult to be
determined; it is when on the one hand the circulating system begins to
carry gelatine and present it to the organs, and when on the other the
organic sensibility of the parenchyma of nutrition of the bones is put
in relation with this substance. Then the consistence of the bone is
constantly increasing, because the gelatine is constantly accumulating;
now it accumulates in the same direction that the phosphate of lime
afterwards takes; that is to say, in the long bones it is in the
middle of the body, in the flat bones it is in the centre, and in the
short ones it is in the centre also, that this substance is at first
exhaled, which afterwards extends gradually to the extremities of the
first, the circumference of the second and the surface of the third.
I would observe however that we do not see, during the formation of
the cartilaginous bone, those longitudinal striæ in the long bones,
radiated ones in the flat, irregularly crossed ones in the short, which
distinguish the osseous state in its formation, and which seem to
indicate to the eye the course of the phosphate of lime.
The cartilaginous state exhibits a peculiarity that distinguishes
it from the osseous state; it is that all the bones that are to be
afterwards united by means of cartilage, such as those of the cranium,
the face, the vertebral column and the pelvis make only one piece;
whilst all those that are to be held together by ligaments, whose
articulations are consequently moveable, are found very distinct, as
the femur, the tibia, the clavicle, &c.
The broad bones, those of the cranium especially, do not exhibit in
so distinct a manner the cartilaginous state. Their appearance, at
this period of ossification, is even rather membranous. It arises from
this; as they are found interposed between the periosteum and the
dura mater, and as their tenuity is very great, we can with difficulty
distinguish them on the interior of these two membranes. But when we
dissect the parts with care, we can distinguish the bone yet soft, from
this double covering.
The cartilaginous state appears in the clavicle, the scapula, the
ribs, before being discoverable in the other bones in which it is
afterwards seen. When we examine the bones in this state, we find them
of different consistence and solidity; where the exhalation of gelatine
has commenced, they are incompletely cartilaginous; as we go from this
point, they partake more or less of the mucous state. The cartilaginous
bone has no internal cavity, no medullary system, &c.
_Osseous State._
When the whole bone is cartilaginous, and even when some points appear
to be still mucous, the exhalation of calcareous substance begins, and
then the osseous state manifests itself; the following is the manner;
the bone becomes more dense, then of a deeper colour, and finally
of a very evident yellow in its middle, that is to say where the
ossification should begin; gradually a red point appears; these are the
vessels that begin to receive the red portion of the blood, and not to
be developed as some anatomists pretend, to be hollowed out according
to their expression, by the force of the impulse of the heart. They
always existed; the white fluids alone penetrated them before, then the
red globules are admitted into them. At the same time the neighbouring
parts are encrusted with calcareous substance. This period is then
remarkable in two particulars, viz. in respect to the entrance of the
blood into the cartilaginous bone, and in regard to the exhalation of
the phosphate of lime. These two phenomena are always inseparable; when
there is redness in one part of the cartilages, there are also osseous
points; this is observed not only in common ossification, but also in
those which are not so, such as the ossifications of the cartilages
of the larynx, of the ribs, &c. When we examine the progress of the
exhalation of the earthy substance, we see always in the bones, whether
long, flat or short, a very red vascular layer, between the cartilage
and the portion of ossified bone. This layer seems to serve as a
precursor to the osseous state. Why do the vessels of the bones which
before had admitted only white fluids, receive then red globules? It
is not, as Boerhaave would have said, had he treated of ossification,
because their caliber increases, but because the sum of their organic
sensibility increasing, they are then found in relation with the red
portion, which until then was foreign to them. Their caliber might be
treble or quadruple the diameter of the red globules, but these would
not enter if the organic sensibility repelled them, as the larynx
rises against a body which attempts to enter it, though this body may
be infinitely less than the glottis. It is by an increase of organic
sensibility, that must also be explained how the bone, until then a
stranger to calcareous substance, being in relation only with the
gelatine, appropriates also to itself the first of these substances,
and is penetrated with ease.
I will observe only that there is this difference between the
exhalation of the two, that the first comes immediately from the red
portion of the blood, since wherever it is deposited, there is, as I
have said, blood vessels; whilst the second appears to come immediately
from the white fluids, since the vessels of the tendons, the cartilages
and the other parts that they nourish, do not evidently receive in
their natural state any red globules, and all that circulates in them
appears to be white.
The osseous state commences with the end of the first month in the
clavicle, the ribs, &c.; it is a little more slow in the other bones;
we know not its precise period. The following is its progress in the
three kinds of bones.
_Progress of the Osseous State in the Long Bones._
We distinguish at first in the middle of these bones, a small
osseous cylinder, very slender in its centre, enlarging towards the
extremities, hollow in the interior for the rudiments of the medullary
system, perforated by a nourishing foramen whose size is then in
proportion very great, receiving also a very large vessel. This osseous
cylinder, at first very slender in comparison with the cartilaginous
extremities of the bone, is in a very evident disproportion to them in
this respect; it is formed of very delicate fibres, and is gradually
enlarged and extended, until it reaches near the extremities where
it is found at birth; the most of these extremities are not then
bony. Some time after, and at a period which varies in the different
bones, there is developed in these extremities an osseous point which
begins at the centre, and which is always preceded by the passage of
the blood in the vessels. These new germs increase at the expense of
the cartilage which is gradually lessened between the body and the
extremity of the bone; at the end of some time there remains only a
slight partition which ossification also seizes upon; so that the bone
is then wholly osseous from one extremity to the other. The secondary
points which are developed in the different apophyses, also unite; so
that its substance is everywhere homogeneous. It is not until the age
of sixteen or eighteen years, that nature has completely finished this
work.
_Progress of the Osseous State in the Broad Bones._
The mode of origin of ossification varies in this kind of bones. Those
which are symmetrical, have always two points or more, which correspond
upon each side of the median line; sometimes one of them is found upon
this line. When these points of ossification are in equal number, they
are always upon the sides; if they are in unequal number, one of them
is upon the line.
The irregular bones sometimes have but one of them, as in the parietal;
at other times many appear in them, as in the temporal; but there is
never a similar arrangement among them; only they correspond with those
of the opposite bone.
At the first point of ossification in a broad bone, we perceive at
first reddish spots, then we observe the phosphate of lime spreading in
rays from the centre to the circumference of the bone. The osseous rays
are very evident upon the bones of the cranium. Portions not ossified
at first fill their interstices, which new rays afterwards occupy. All
terminate in an unequal manner, without touching, so that by separating
an ossified portion of a broad bone from the membranous portion to
which it belongs, its circumference looks cut like the extremity of a
comb; hence, as we shall see, the origin of sutures.
The delicacy of these bones is extreme in the early periods; they have
not then any of the texture of the cells. At birth but few of the
osseous centres are yet united; cartilaginous and membranous spaces
separate them; these spaces are greater towards the angles than towards
the edges, and generally at points the most distant from the primitive
osseous centres. The bones with many points of ossification are formed
of separate pieces, more or less distant from each other. Those with
one point only, have but one piece.
After birth these bones extend more and more, their thickness and
hardness increase; they are divided into two compact layers, the space
between which is filled by the texture of the cells; gradually they
touch at their edges, and then the sutures are formed in the cranium;
for there is this difference between their ossification and that of
the long bones, that it takes place always from the centre to the
circumference, and that new osseous points are not developed at the
circumference to meet those of the centre. But when this happens, the
union does not then take place as in the long bones, but sutures are
formed; and it is this which occasions the ossa wormiana, which are so
much the larger as the osseous point is the sooner developed, because
it has had time to extend itself more, before meeting the general
ossification of the bone.
When a broad bone is developed by many points and there exists upon its
surface an articular surface, it is usually the centre in which all the
points unite at the period in which ossification terminates; we see
this in the cotyloid cavity, in the condyle of the occipital bone, &c.
There is often in the broad bones two well marked periods for their
ossification; it is so in those which, like the sacrum and sternum, are
developed by a great number of points. These points begin at first to
unite into three or four principal pieces which divide the bone; this
is the first period; then much later, the union of the pieces takes
place; this is the second period.
_Progress of the Osseous State in the Short Bones._
The short bones remain in general longer cartilaginous than the others.
Often at birth many are still so, those of the tarsus and carpus in
particular. The body of the vertebræ is ossified sooner; a point is
developed at the centre, and extends over the whole surface.
These phenomena are nearly analogous to those of the ossification of
the extremities of the long bones, which the short bones resemble so
much. After birth, the whole cartilaginous portion, is, if we may so
say, invaded by the calcareous substance, which mixes with it, and
there is finally only the articular cartilages left.
There are bones, as the occipital and sphenoid, which partake of the
character of the broad and short bones; their ossification is mixed,
and follows the mode of one or the other, according to the part of the
bone that we examine.