225
CHAPTER SEVEN
Skeletal System
A surgical procedure called a
laminectomy
may relieve the pain of a herniated disc by remov-
ing a portion of the posterior arch of a vertebra.
This reduces the pressure on the aF
ected nerve
tissues. Alternatively, a protein-digesting enzyme
(chymopapain) may be injected into the injured
disc to shrink it.
Sometimes problems develop in the curva-
tures of the vertebral column because of poor
posture, injury, or disease. An exaggerated tho-
racic curvature causes rounded shoulders and a
hunchback. This condition, called kyphosis, occa-
sionally develops in adolescents who undertake
strenuous athletic activities. Unless corrected
before bone growth completes, the condition can
permanently deform the vertebral column.
C
hanges in the intervertebral discs may
cause various problems. Each disc is
composed of a tough, outer layer of
±
brocartilage (annulus ±
brosus) and an elastic
central mass (nucleus pulposus). With age, these
discs degenerate—the central masses lose
±
rmness, and the outer layers thin and weaken,
developing cracks. Extra pressure, as when a
person falls or lifts a heavy object, can break the
outer layers of the discs, squeezing out the cen-
tral masses. Such a rupture may press on the spi-
nal cord or on spinal nerves that branch from it.
This condition, called a
ruptured,
or
herniated disc,
may cause back pain and numbness or loss of
muscular function in the parts innervated by the
aF
ected spinal nerves.
Sometimes the vertebral column develops
an abnormal lateral curvature, so that one hip
or shoulder is lower than the other. This may
displace or compress the thoracic and abdomi-
nal organs. With unknown cause, this condition,
called scoliosis, is most common in adolescent
females. It also may accompany such diseases as
poliomyelitis, rickets, or tuberculosis. An accen-
tuated lumbar curvature is called
lordosis,
or
swayback.
As a person ages, the intervertebral discs
shrink and become more rigid, and compression
is more likely to fracture the vertebral bodies.
Consequently, height may decrease, and the tho-
racic curvature of the vertebral column may be
accentuated, bowing the back.
7.3
CLINICAL APPLICATION
Disorders of the Vertebral Column
Clavicles
The
clavicles
(klav
˘
ı-k’lz) are slender, rodlike bones with
elongated S-shapes (fig. 7.40). Located at the base of the
neck, they run horizontally between the manubrium and the
scapulae. The sternal (or medial) ends of the clavicles articu-
late with the manubrium, and the acromial (or lateral) ends
join processes of the scapulae.
The clavicles brace the freely movable scapulae, help-
ing to hold the shoulders in place. They also provide attach-
ments for muscles of the upper limbs, chest, and back. The
clavicle is structurally weak because of its elongated double
curve. If compressed lengthwise due to abnormal pressure
on the shoulder, it is likely to fracture.
In the epic poem the
Iliad,
Homer describes a man whose “shoulders
were bent and met over his chest.” The man probably had a rare inher-
ited condition, called cleidocranial dysplasia, in which certain bones do
not grow. In the condition, the skull consists of small fragments joined
by ±
brous connective tissue, rather than the normal large, interlocking
hard bony plates. The clavicles are stunted or missing.
Cleidocranial dysplasia was ±
rst reported in a child in the huge
Arnold family, founded by a Chinese immigrant to South Africa. The
child had been kicked by a horse, and X rays revealed that the fonta-
nels atop the head had never closed. The condition became known as
“Arnold head.” It is caused not by a horse’s kick, but by a malfunction-
ing gene that normally instructs diF
erentiation of bone cells.
The xiphoid process begins as a piece of cartilage. It
slowly ossifies, and by middle age it usually fuses to the
body of the sternum.
Red marrow within the spongy bone of the sternum produces blood
cells into adulthood. The sternum has a thin covering of compact
bone and is easy to reach, so samples of its marrow may be removed
to diagnose diseases. This procedure, a sternal puncture, suctions
(aspirates) some marrow through a hollow needle. (Marrow may also
be removed from the iliac crest of a hip bone.)
PRACTICE
25
Which bones comprise the thoracic cage?
26
Describe a typical rib.
27
What are the diF
erences among true, false, and ²
oating ribs?
28
Name the three parts of the sternum.
7.9
PECTORAL GIRDLE
The
pectoral
(pek
tor-al)
girdle,
or shoulder girdle, is com-
posed of four parts—two clavicles (collarbones) and two
scapulae (shoulder blades). Although the word
girdle
sug-
gests a ring-shaped structure, the pectoral girdle is an incom-
plete ring. It is open in the back between the scapulae, and
the sternum separates its bones in front. The pectoral girdle
supports the upper limbs and is an attachment for several
muscles that move them
(f
g. 7.40)
.
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