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.