Part 23

As the separate motion of each of the articulations of the fifth genus is almost nothing, nature usually unites several at the same place, for the purpose of producing a sensible, general motion, as we see in the carpus, the tarsus, the vertebræ, &c.; this is also a reason of the difficulty there is in luxating this genus of articulations. In fact, how great the general motions may be, two bones, taken separately, move but little upon each other; now it is only the extent of the motion of the two separate bones from each other, that can produce the displacement.

III. Observations upon the Immoveable Articulations.

We have only pointed out orders in this class, because its varieties are not sufficiently great to assign genera for them.

1st. The order of immoveable articulations with surfaces in juxta-position, is met with where the mechanism of the part alone is almost sufficient to insure the solidity of the bones which are found only placed at the side of each other, without holding by any insertion, and having only between them a slight cartilaginous layer; the superior maxillary bones, wedged in between the malar bones, the ossa ungues, the ethmoid, the ossa palati, the vomer, and the frontal bone, are supported more by the general mechanism of the face, than by any articular attachments that unite them to each other; thus the squamous portion of the temporal bone supports the parietal, more by the abutting arches, than by the manner of the union of their respective surfaces. Remove this general mechanism of the part, you will soon see all the articulations separate.

2d. The order of immoveable articulations with inserted surfaces, owes also in some measure its solidity to the general mechanism of the part; but this mechanism would be insufficient to insure this solidity; thus the bones, instead of having almost plain surfaces, exhibit very evident prominences and depressions which are inserted into each other, as we see in the articulations of the parietal bones with each other, with the sphenoid, the occipital, the frontal, &c.; these are called sutures. This order sometimes approximates the preceding, as in the union of the parietal and frontal bones, which, reciprocally aiding each other, are supported by this mechanism, more than by their insertions; sometimes it resembles the following order, as in the articulation of the occipital and parietal bones, in which the very deep insertions almost alone insure the solidity of the union. This order is never seen except upon the edges of the flat bones; the insertion of these edges compensates for their want of size, by multiplying the points of contact. The eminences and depressions forming the insertion are always of an irregular form and size. They are exactly fitted to each other, they are not alike in two bones of the same species, taken from two different subjects; so that we cannot unite to a detached left parietal bone, the right parietal bone of another individual. There has been much dispute upon the formation of the sutures; they are an effect of the laws of ossification, an effect which we can account for no more than we can for all the others, and all the general phenomena of growth; we shall see the progress they follow in this formation. This articular order is gradually effaced with age, and the bones unite together by the ossification of the thin intermediate cartilage. It is more rare that the preceding order disappears. I have seen, however, in extreme old age, different articulations of this order cease to be evident, those of the maxillary bones between themselves especially.

3d. The order of articulations with implanted surfaces borrows none of its solidity from the mechanism of the part; it owes it entirely to the relation of the surfaces, which are so united and embraced by each other, that displacement is impossible. There is but one example of this articular order, it is the teeth with the jaws.

Age does not here efface the articulation, and thus confound the two bones as in the preceding orders, because the medium of union is the palatine membrane, which belongs to the mucous system, and which by its organization has no tendency to ossification; whereas in the preceding cases, the intermediate cartilage has a natural disposition to become encrusted with the phosphate of lime.

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.