264
UNIT TWO
ence of each bone of the joint near its articular end. Thus,
the outer F
brous layer of the capsule completely encloses the
other parts of the joint. It is, however, fl
exible enough to per-
mit movement and strong enough to help prevent the articu-
lar surfaces from being pulled apart.
Bundles of strong, tough collagenous F
bers called
liga-
ments
(lig
ah-mentz) reinforce the joint capsule and help
bind the articular ends of the bones. Some ligaments appear
as thickenings in the F brous layer of the capsule, whereas
others are
accessory structures
located outside the capsule.
In either case, these structures help prevent excessive move-
ment at the joint. That is, the ligament is relatively inelastic,
and it tightens when the joint is stressed.
The inner layer of the joint capsule consists of a shiny,
vascular lining of loose connective tissue called the
syn-
ovial membrane.
This membrane, only a few cells thick,
covers all of the surfaces within the joint capsule, except
the areas the articular cartilage covers. The synovial mem-
brane surrounds a closed sac called the
synovial cavity,
into which the membrane secretes a clear, viscous fl
uid
called
synovial fl
uid.
In some regions, the surface of the
synovial membrane has villi as well as larger folds and
projections that extend into the cavity. Besides filling
spaces and irregularities of the joint cavity, these exten-
sions increase the surface area of the synovial mem-
brane. The membrane may also store adipose tissue and
form movable fatty pads in the joint. This multifunctional
membrane also reabsorbs fl
uid, which is important when
A tubular
joint capsule
(articular capsule) that has two
distinct layers holds together the bones of a synovial joint.
The outer layer largely consists of dense connective tissue,
whose F
bers attach to the periosteum around the circumfer-
Gelatinous
core
Spinous process
Band of
fibrocartilage
Body of
vertebra
Pubis
Fibrocartilage disc of symphysis pubis
Intervertebral
discs
(a)
(b)
Spongy
bone
Joint cavity
filled with
synovial
fluid
Synovial
membrane
Articular
cartilage
Joint
capsule
FIGURE 8.6
Fibrocartilage composes (
a
) the symphysis pubis that separates the pubic bones and (
b
) the intervertebral discs that separate
vertebrae.
FIGURE 8.7
The generalized structure of a synovial joint.
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