All the articulations can be referred to two general classes. Mobility
is the character of the first, immobility that of the second.
One belongs to all the bones which serve for locomotion, to some of
those destined to internal functions, as the ribs, the lower jaw, &c.
The other is especially met with in the bones, the union of which forms
the cavities designed to defend the organs; this we see in the head,
the pelvis, &c.
_Moveable Articulations. Observations upon their Motions._
I divide moveable articulations into four kinds, the characters
of which are borrowed from the different motions they execute. To
understand this division, it is necessary previously to know the
motions of the articulations in general. These motions can be included
under four species, which are, 1st, opposition; 2d, circumduction; 3d,
rotation; 4th, sliding.
1st. The motion of opposition is that which is made in two opposite
directions, for example, from flexion to extension, from adduction
to abduction, and vice versa. This motion is extensive or limited;
extensive when it is made in all directions, first in the four named
above, then in all the intermediate ones; limited, when it only takes
place from flexion to extension, from adduction to abduction, &c. The
thigh in its articulation with the pelvis enjoys an extensive motion of
opposition. The tibia in its articulation with the femur has a limited
motion of opposition.
2d. Circumduction is the motion in which the bone describes a kind of
cone, the summit of which is in its superior articulation, and the
base in the inferior; so that it is found successively in flexion,
adduction, extension and abduction, or in abduction, extension,
adduction and flexion, according to the motion by which it begins, and
that moreover it goes through all the intermediate directions. Hence we
see that circumduction is a motion composed of all those of opposition,
and in which the bone, instead of moving from one direction to an
opposite one, as in the preceding case, moves from one direction to
another nearest to it, describing thus by its extremity a circle which
is the base of a cone of which I have spoken, and which is so much the
greater as the bone is so much the longer. We easily understand that
among the bones, those only whose motion of opposition is loose, enjoy
that of circumduction.
3d. Rotation is wholly different from the preceding motion. In this
there was locomotion, a moving of the bone from one place to another;
here it remains always in the same place; it only turns upon its axis.
The humerus and the femur enjoy this motion which is simple.
4th. Sliding belongs to all the articulations. It is an obscure motion,
by which two surfaces go in an opposite direction, by sliding as it
were upon each other. In all the other motions, this is met with; but
it often exists without them.
It is easy to understand, from these views upon the articular motions,
the division into genera of the class of moveable articulations. In
fact, there are articulations in which all the motions are united; in
others, there is no rotation; in many rotation and circumduction are
wanting, and opposition exists only in one direction; some have only
rotation. Finally, there are those in which rotation, circumduction and
opposition are nothing, and sliding alone remains.
Hence we see that nature moves here as elsewhere by gradation, that
from the most moveable articulations to those that are the least so,
there are different degrees of decrease, that she descends gradually to
the immoveable articulations, that she is finally reduced to the motion
of sliding alone, like that which exists at the carpus, the tarsus,
&c. There is even an intermediate one between sliding and immobility;
it is the articulation of the symphysis pubis; which can be considered
with that of the humerus as forming the two extremes of the series of
moveable articulations.
All the articulations of which I have spoken are with contiguous
surfaces; this is the general character of those which are moveable.
There is however an exception to this rule; it is the articulation of
the body of the vertebræ, in which there is continuity and mobility.
The symphysis pubis is also in part continuous in its surfaces, and yet
has sometimes obscure motions. Hence arises a division of the moveable
articulations, into those with continuous surfaces, and into those with
contiguous ones.
_Immoveable Articulations._
The immoveable articulations are sometimes with surfaces inserted into
each other, as the bones of the cranium, in which many projections and
depressions reciprocally receive each other; sometimes with surfaces
in juxta-position, as in the articulation of the temporal with the
parietal, the two superior maxillary bones with each other; sometimes
with implanted surfaces, as in the teeth.
All the different divisions that I have mentioned will be easily
understood by the following table; it is not the same as that which
I have given in my treatise on the membranes; I think it presents
a classification a little more useful in this, that it offers for
a characteristic two things essential to be known in all kinds of
moveable articulations, viz. 1st, the relation of the articular
surfaces which characterizes the orders; 2d, the number of motions
of each which distinguishes the genera. There are no orders in the
immoveable articulations because, except the relation of surfaces, the
articulations have not differences sufficient to occasion them to be
sub-divided.
_Table of the Articulations._
┌ CLASSES. ┌ ORDERS. ┌ GENERA.
│ │ │1st. Extensive Opposition,
│ │ │ Circumduction and
│ │ │ Rotation.
│ │1st. With │2d. Extensive Opposition
│ │ Contiguous │ and Circumduction.
│1st. Moveable. │ Surfaces. │3d. Limited Opposition.
│ │ │4th. Rotation.
│ │ └5th. Sliding.
ARTICULATIONS. │ │2d. With Continuous
│ └ Surfaces.
│
│ ┌1st. With Surfaces
│ │ in juxta-position.
│ │2d. With Surfaces
│ │ inserted
└2d. Immoveable. │ into each other.
│3d. With Surfaces
└ implanted.
After having thus divided the articulations, let us offer upon each
class some general observations. But let us first remark that the
preceding table, considered in respect to the moveable articulations
with contiguous surfaces, indicates perfectly the disposition of these
articulations as to luxations, which are so much the more frequent as
the motions are more extensive. The first genus is the most exposed to
it, the last the least so; and the others are more or less so according
to their distance from the first.