263
CHAPTER EIGHT
Joints of the Skeletal System
turally than F
brous or cartilaginous joints. They consist of
articular cartilage; a joint capsule; and a synovial membrane,
which secretes synovial fl
uid.
Virtuoso violinist Niccolò Paganini (1782–1840) astounded concert-
goers with his ability to reach three octaves across the bridge of his
instrument. So lax were his joints that he could bend his thumb back-
ward until the nail touched the back of his hand.
Paganini had “benign joint hypermobility syndrome,“ deF
ned as
a range of motion much greater than normal. Today the condition is
studied in people whose professions make lax joints either a beneF
t
or a liability. In athletes and dancers, for example, loose joints increase
the risk of injury. Musicians are especially interesting. The nimble F
n-
gers, hands, and wrists of hypermobility syndrome help woodwind
and string players, but lax joints also tend to cause back and knee
problems. Rather than gaining strength from repetitive movements
of playing instruments, these joints must bear weight from long
hours of sitting in one position.
Perhaps rock guitarists make the best use of hypermobile joints.
They stretch their F
ngers like Paganini while jumping about onstage
to better distribute their weight on the other joints!
8.3
GENERAL STRUCTURE OF
A SYNOVIAL JOINT
The articular ends of the bones in a synovial joint are cov-
ered with a thin layer of hyaline cartilage. This layer, the
articular cartilage,
resists wear and minimizes friction when
it is compressed as the joint moves
(f g. 8.7)
.
The joints between the costal cartilages and the sternum
of ribs 2 through 7 are usually synovial joints.
2.
Symphysis
(sim
fı˘-sis). The articular surfaces of the
bones at a symphysis are covered by a thin layer of
hyaline cartilage, and the cartilage, in turn, is attached
to a pad of springy F
brocartilage. Limited movement
occurs at such a joint whenever forces compress or
deform the cartilaginous pad. An example of this type
of joint is the symphysis pubis in the pelvis, which
allows maternal pelvic bones to shift as an infant passes
through the birth canal
(f
g. 8.6
a
)
.
The joint formed by the bodies of two adjacent
vertebrae separated by an intervertebral disc is also a
symphysis (F g. 8.6
b
and reference plate 11, p. 40). Each
intervertebral disc is composed of a band of F brocartilage
(annulus F brosus) that surrounds a gelatinous core
(nucleus pulposus). The disc absorbs shocks and helps
equalize pressure between the vertebrae when the body
moves. Each disc is slightly fl exible, so the combined
movement of many of the joints in the vertebral column
allows the back to bend forward or to the side or to twist.
They are amphiarthrotic joints because these joints allow
slight movements.
Synovial Joints
Most joints of the skeletal system are
synovial
(sı˘-no
ve-al)
joints,
and because they allow free movement, they are diar-
throtic (di
ar-thro
tik). These joints are more complex struc-
Periodontal
ligament
Alveolar
process of
mandible
Root of
tooth
Crown of
tooth
Thoracic
vertebra
Costal cartilage
Manubrium
First rib
FIGURE 8.4
The articulation between the root of a tooth and the
mandible is a gomphosis.
FIGURE 8.5
The articulation between the F
rst rib and the
manubrium is a synchondrosis.
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