Part 16

IV. _Of the Means of Union between the Articular Surfaces._

The articular surfaces would soon separate, if different organs did not retain them in place. These organs are the cartilages and the membranes for the immoveable articulations, the ligaments and the muscles for the moveable.

_Union of the Immoveable Articulations._

The two first orders of immoveable articulations, those with inserted surfaces and those with surfaces in juxta-position, have cartilages between the osseous surfaces, the breadth and thickness of which are found so much the greater in proportion as they are examined in subjects nearest infancy. Almost all the bones of the head are held together in this manner, which allows them to yield a little, and consequently prevents their fracture.

In the articulations of the pelvis, there are besides the cartilages, ligaments; but as these articulations perform in certain cases small sliding motions, we can consider them as intermediate between the moveable and immoveable articulations; it is on this account that they have the two kinds of organs especially destined to strengthen the articular surfaces of each of the classes, viz. the cartilages and the ligaments.

The immoveable articulations with implanted surfaces, an order which comprehends only the teeth, have nothing between the surfaces as a means of union, but a mucous membrane, the palatine. Hence why in the swellings of this membrane, in scorbutic affections, after the use of mercury, &c. the teeth become loose.

_Union of the Moveable Articulations._

The moveable articulations with contiguous surfaces have the ligaments especially as a means of union, which are found in the five genera, but under different forms which will be examined hereafter. This kind of organ unites much suppleness with great resistance, a double attribute which it derives from its peculiar texture, and which renders it very proper for this function. Let us observe however that these two properties are in an inverse ratio in the two extreme ages of life, that suppleness is the companion of infancy, that stiffness and resistance are the character of the ligaments in old age. Hence in part the multiplicity of motions in one age, and their slowness and difficulty in the other.

The cartilages are not in this articular order, as in the preceding, means of union, but means of motion, by their smooth and polished surfaces.

As to the synovial membrane that is found exclusively in this order, such is its extreme tenuity, that it can hardly be considered as uniting the surfaces, and its use appears to be confined to the exhalation of synovia.

It is not the same with the muscles; they can be considered as forming at the same time around the moveable articulations, a power for the whole of the bone, and a resistance for its extremities, which they prevent from being displaced, by forming around them supports, the efficiency of which is in proportion to the efforts that are made to displace these extremities. In fact, it is in the great motions that these efforts are the most considerable; now then the neighbouring muscles of articulation strongly contracted, hard during their contractions, have a powerful tendency to prevent the osseous extremity from abandoning that which corresponds with it. In rest when the relaxed muscles offer but little resistance, the effort for support is nothing. A paralyzed limb can be luxated much more easily than another, by external violence.

The order of moveable articulations with contiguous surfaces, has as a means of union, a substance, the nature of which is between that of the ligaments and that of the cartilages.

ARTICLE FIFTH.

DEVELOPMENT OF THE OSSEOUS SYSTEM.

There is no system in which anatomists have traced in a more accurate manner than in this, the different states in the different periods of life. The remarkable difference of a bone examined in the first months when gelatine almost alone composes it, compared with a bone of an adult in which the calcareous substance predominates, has especially arrested their attention upon this point. Let us examine the phenomena of ossification in all the ages; these phenomena should be considered during and after growth. In general, while this continues, there are some portions of the osseous system not ossified, as the neck of the femur, for example; ossification is not complete, the bones are not perfectly developed until towards the sixteenth or eighteenth year, and sometimes even later.

I. _State of the Osseous System during Growth._

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.

II. _State of the Osseous System after its Growth._

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.