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.