252
UNIT 2
Covering, Support, and Movement of the Body
8
Examples include the intervertebral joints and the pubic symphy-
sis of the pelvis (Figure 8.2b, and see
Table 8.2
on p. 254).
Check Your Understanding
1.
What term is a synonym for “joint”?
2.
What functional joint class contains the least-mobile joints?
3.
Of sutures, symphyses, and synchondroses, which are
cartilaginous joints?
4.
How are joint mobility and stability related?
For answers, see Appendix H.
Synovial Joints
Describe the structural characteristics of synovial joints.
Compare the structures and functions of bursae and
tendon sheaths.
List three natural factors that stabilize synovial joints.
Synovial joints
(si-no
9
ve-al; “joint eggs”) are those in which the
articulating bones are separated by a fluid-containing joint cavity.
Tis arrangement permits substantial freedom of movement, and
all synovial joints are freely movable diarthroses. Nearly all joints
of the limbs—indeed, most joints of the body—fall into this class.
General Structure
Synovial joints have six distinguishing features
(Figure 8.3)
:
Articular cartilage.
Glassy-smooth hyaline cartilage covers
the opposing bone surfaces as
articular cartilage
. Tese thin
(1 mm or less) but spongy cushions absorb compression placed
on the joint and thereby keep the bone ends from being crushed.
Joint (articular) cavity.
A feature unique to synovial joints,
the joint cavity is really just a potential space that contains a
small amount of synovial fluid.
Articular capsule.
Te joint cavity is enclosed by a two-layered
articular capsule
, or
joint capsule
. Te tough external
fibrous
layer
is composed of dense irregular connective tissue that
is continuous with the periostea of the articulating bones. It
strengthens the joint so that the bones are not pulled apart.
Te inner layer of the joint capsule is a
synovial membrane
composed of loose connective tissue. Besides lining the fi-
brous layer internally, it covers all internal joint surfaces that
are not hyaline cartilage. Te synovial membrane’s function is
to make synovial fluid.
Synovial fluid.
A small amount of slippery
synovial fluid
occupies all free spaces within the joint capsule. Tis fluid
is derived largely by filtration from blood flowing through
the capillaries in the synovial membrane. Synovial fluid has
a viscous, egg-white consistency (
ovum
5
egg) due to hy-
aluronic acid secreted by cells in the synovial membrane, but
it thins and becomes less viscous during joint activity.
Synovial fluid, which is also found
within
the articular
cartilages, provides a slippery, weight-bearing film that re-
duces friction between the cartilages. Without this lubricant,
rubbing would wear away joint surfaces and excessive fric-
tion could overheat and destroy the joint tissues, essentially
“cooking” them. Te synovial fluid is forced from the car-
tilages when a joint is compressed; then as pressure on the
joint is relieved, synovial fluid seeps back into the articular
cartilages like water into a sponge, ready to be squeezed out
again the next time the joint is loaded (put under pressure).
Tis process, called
weeping lubrication
, lubricates the free
surfaces of the cartilages and nourishes their cells. (Re-
member, cartilage is avascular.) Synovial fluid also contains
phagocytic cells that rid the joint cavity of microbes and cel-
lular debris.
Reinforcing ligaments.
Synovial joints are reinforced and
strengthened by a number of bandlike
ligaments
. Most of-
ten, these are
capsular ligaments
, which are thickened parts
of the fibrous layer. In other cases, they remain distinct and
are found outside the capsule (as
extracapsular ligaments
)
or deep to it (as
intracapsular ligaments
). Since intracapsu-
lar ligaments are covered with synovial membrane, they do
not actually lie
within
the joint cavity.
People said to be double-jointed amaze the rest of us by
placing both heels behind their neck. However, they have the
normal number of joints. It’s just that their joint capsules and
ligaments are more stretchy and loose than average.
Nerves and blood vessels.
Synovial joints are richly supplied
with sensory nerve fibers that innervate the capsule. Some of
Periosteum
Ligament
Fibrous
layer
Synovial
membrane
(secretes
synovial
fluid)
Joint cavity
(contains
synovial fluid
)
Articular
(hyaline)
cartilage
Articular
capsule
Figure 8.3
General structure of a synovial joint.
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