_f.l._, Fibrous layer, _s.l._, synovial layer of joint capsule. The
articular cartilages are white, bones dotted, and the joint cavity
black in the figure.
]
3. The =joint capsule= (Capsula articularis) is, in its simplest form, a
sac, the margins of which are attached around the articulating surfaces.
It consists of two layers—an external one, composed of fibrous tissue
(Stratum fibrosum), and an internal one, the synovial layer or membrane
(Stratum synoviale). The =fibrous layer=, sometimes termed the capsular
ligament, is attached either close to the margins of the articular
surfaces or at a variable distance from them. Its thickness varies
greatly in different situations: in certain places it is extremely
thick, and sometimes cartilage or bone develops in it; in other places
it is practically absent, the capsule then consisting only of the
synovial membrane. Parts of the capsule may undergo thickening and so
form ligaments, which are not separable, except artificially, from the
rest of the capsule. The =synovial layer= lines the joint cavity except
where this is bounded by the articular cartilages; it stops normally at
the margin of the latter. It is a thin membrane, and is richly supplied
by close networks of vessels and nerves. It frequently forms =folds=
(Plicæ synoviales) and =villi= (Villi synoviales), which project into
the cavity of the joint. The folds commonly contain pads of fat which
fill up interstices and vary in form and position in various phases of
movement. The synovial membrane secretes a fluid, the =synovia=, which
resembles white-of-egg and lubricates the joint.[28] In many places the
membrane forms extra-articular pouches, which facilitate the play of
muscles and tendons.
The =joint cavity= (Cavum articulare) is inclosed by the synovial
membrane and the articular cartilages. Normally, it is, strictly
speaking, only a potential cavity, which contains nothing but a small
amount of synovia.
The student must guard against a false conception of the joint cavity
which may result from dissections and diagrams, in which an actual
cavity of considerable extent appears to exist. A correct idea of the
intimate apposition of the parts is best obtained from the study of
frozen sections. It is also instructive to examine joints which have
been injected so as to distend the capsule fully. It is then seen that
the cavity is often of much greater potential extent than one might
suppose, and that the capsule is often very irregular in form, _i.
e._, forms a variety of sacculations.
The foregoing are constant and necessary features in all diarthroses.
Other structures which enter into the formation of these joints are
ligaments, articular discs or menisci, and marginal cartilages.
4. =Ligaments.=—These are strong bands or membranes, usually composed of
white, fibrous tissue, which bind the bones together. They are pliable,
but practically inelastic. In a few cases, however, _e. g._, ligamenta
flava, ligamentum nuchæ, they are composed of elastic tissue. They may
be subdivided, according to position, into =periarticular= and
=intraarticular=. Periarticular ligaments are frequently blended with or
form part of the fibrous capsule; in other cases they are quite
distinct. Strictly speaking, intraarticular ligaments, though within the
fibrous capsule, are not in the joint cavity; the synovial membrane is
reflected over them. The term seems justifiable, however, on practical
grounds. Ligaments which connect directly opposed surfaces of bones are
termed interosseous. The special names are based usually on their
position, form, and attachments, _e. g._, lateral or collateral,
cruciate, sacro-iliac, etc. In many places muscles, tendons, and
thickenings of the fasciæ function as ligaments and increase the
security of the joint. Atmospheric pressure and cohesion play a
considerable part in keeping the joint surfaces in apposition.
5. =Articular discs= or =menisci= are plates of fibro-cartilage or dense
fibrous tissue placed between the articular cartilages, and divide the
joint cavity partially or completely into two compartments. They render
certain surfaces congruent, _e. g._, femoro-tibial joint, allow greater
range or variety of movement, and diminish concussion.
6. A =marginal cartilage= (Labrum glenoidale) is a ring of
fibro-cartilage which encircles the rim of an articular cavity. It
enlarges the cavity and tends to prevent fracture of the margin.
=Vessels and Nerves.=—The =arteries= form anastomoses around the larger
joints, and give off branches to the extremities of the bones and to the
joint capsule. The synovial membrane has a close-meshed network of
capillaries; the latter form loops around the margins of the articular
cartilages, but do not usually enter them. The =veins= form plexuses.
The synovial membrane is also well supplied with lymphatics.
Nerve-fibers are especially numerous in and around the synovial membrane
and there are special nerve-endings, _e. g._, Pacinian bodies and the
articular end-bulbs described by Krause.
=Movements.=—The movements of a joint are determined chiefly by the form
and extent of the joint surfaces and the arrangement of the ligaments.
They are usually classified as follows:
1. =Gliding.=—This refers to the sliding of one practically plane
surface on another, as in the joints between the articular processes of
the cervical vertebræ.
2. =Angular Movements.=—In these cases there is movement around one or
more axes. Motion which diminishes the angle included by the segments
forming the joint is termed =flexion=, while that which tends to bring
the segments into line with each other is called =extension=.
With reference to the joints of the lower parts of the limbs, it seems
advisable to employ the terms =dorsal= and volar or =plantar flexion=,
since these joints can be “overextended.” Similarly, the terms =dorsal=
and =ventral flexion= should be applied to the corresponding movements
of the spinal column. The meaning of the term =lateral flexion= is
evident. These movements are all rotations around axes which are
approximately either transverse or vertical. Depression, elevation, and
lateral movement of the lower jaw fall in this category.
3. =Circumduction.=—This designates movements in which the distal part
of the limb describes a circle or a segment of one. In man such movement
is easily performed, but in quadrupeds it is possible to a limited
degree only, and is to be regarded usually as an indication of disease.
4. =Rotation.=—As a matter of convenience, this term is reserved to
indicate rotation of one segment around the longitudinal axis of the
other segment forming the joint. It is seen typically in the
atlanto-axial joint.
5. =Adduction= and =abduction= designate respectively movement of a limb
toward and away from the median plane, or of a digit toward and away
from the axis of the limb.
=Classification.=—This is based on the form of the joint surfaces and
the movements which occur. The following chief classes may be
recognized:
1. =Arthrodia=, or gliding joint. In these the surfaces are practically
flat, admitting of gliding movement. Examples: carpo-metacarpal joints;
joints between the articular processes of the cervical and thoracic
vertebræ.
2. =Ginglymus=, or hinge-joint. In this class the joint surfaces consist
usually of two condyles, or of a segment of a cylinder or cone, which
are received by corresponding cavities. In typical cases the movements
are flexion and extension, _i. e._, around a single transverse axis.
Examples: occipito-atlantal and elbow joints.
3. =Trochoid=, or pivot joint. In these the movement is limited to
rotation of one segment around the longitudinal axis of the other.
Example: atlanto-axial joint.
4. =Enarthrosis=, or ball-and-socket joint. These are formed by a
surface of approximately spherical curvature, received into a
corresponding cavity. They are multiaxial, and allow of the greatest
variety of movement, _e. g._, flexion, extension, rotation, abduction,
adduction, circumduction. Examples: hip and shoulder joints.[29]