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.