222
UNIT TWO
lage of the
sacroiliac
(sa
kro-il
e-ak)
joints.
The pelvic girdle
transmits the body’s weight to the legs at these joints (see
F
g. 7.15).
The sacrum forms the posterior wall of the pelvic cavity.
The upper anterior margin of the sacrum, which represents
the body of the F rst sacral vertebra, is called the
sacral prom-
ontory
(sa
kral prom
on-to
re). A physician performing a vagi-
nal examination can feel this projection and use it as a guide
in determining the size of the pelvis. This measurement is
helpful in estimating how easily an infant may be able to pass
through a woman’s pelvic cavity during childbirth.
The vertebral foramina of the sacral vertebrae form the
sacral canal,
which continues through the sacrum to an
opening of variable size at the tip, called the
sacral hiatus
(hi-a
tus). This foramen exists because the laminae of the
last sacral vertebra are not fused. On the ventral surface of
the sacrum, four pairs of
anterior sacral foramina
provide
passageways for nerves and blood vessels.
Coccyx
The
coccyx
(kok
siks), or tailbone, is the lowest part of the ver-
tebral column and is usually composed of four vertebrae that
fuse between the ages of twenty-F ve and thirty (F g. 7.37).
Variations in individuals include three to F ve coccygeal
vertebrae with typically the last three fused. In the elderly, the
coccyx may fuse to the sacrum. Ligaments attach the coccyx
to the margins of the sacral hiatus. Sitting presses on the coc-
cyx, and it moves forward, acting like a shock absorber. Sitting
down with great force, as when slipping and falling on ice,
can fracture or dislocate the coccyx. The coccyx also serves
as an attachment for the muscles of the pelvic fl oor.
Table 7.8
summarizes the bones of the vertebral column, and Clinical
Application 7.3 discusses disorders of the vertebral column.
PRACTICE
21
Describe the structure of the vertebral column.
22
Explain the diF
erence between the vertebral column of an adult
and that of an infant.
23
Describe a typical vertebra.
24
How do the structures of cervical, thoracic, and lumbar vertebrae
diF
er?
7.8
THORACIC CAGE
The
thoracic cage
includes the ribs, the thoracic vertebrae,
the sternum, and the costal cartilages that attach the ribs to the
sternum. These bones support the pectoral girdle and upper
limbs, protect the viscera in the thoracic and upper abdominal
cavities, and play a role in breathing
(f
g. 7.38)
.
Ribs
The usual number of
ribs
is twenty-four—one pair attached to
each of the twelve thoracic vertebrae. Some individuals have
extra ribs associated with their cervical or lumbar vertebrae.
Sacrum
The
sacrum
(sa
krum) is a triangular structure at the base of
the vertebral column. It is composed of F
ve vertebrae that
develop separately but gradually fuse between ages eighteen
and thirty
(f g. 7.37)
. Sometimes only four vertebrae fuse to
form the sacrum and the F fth vertebra becomes a sixth lum-
bar vertebra. The spinous processes of these fused bones form
a ridge of tubercles, the
median sacral crest.
Nerves and blood
vessels pass through rows of openings, called the
posterior
sacral foramina,
located to the sides of the tubercles.
The sacrum is wedged between the hip bones of the
pelvis and joins them at its
auricular surfaces
by F brocarti-
(b)
Thoracic vertebra
(c)
Lumbar vertebra
(a)
Cervical vertebra
Bifid spinous process
Vertebral foramen
Lamina
Body
Lamina
Pedicle
Body
Lamina
Pedicle
Body
Superior articular
facet
Transverse foramen
Transverse process
Spinous process
Vertebral foramen
Vertebral foramen
Facet that articulates
with rib head
Facet that articulates
with rib tubercule
Superior articular
facet
Spinous process
Superior articular
process
Transverse process
Transverse process
FIGURE 7.36
Superior view of (
a
) a cervical vertebra, (
b
) a thoracic
vertebra, and (
c
) a lumbar vertebra.
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