221
CHAPTER SEVEN
Skeletal System
Two of the cervical vertebrae, shown in
figure 7.34
,
are of special interest. The F
rst vertebra, or
atlas
(at
las),
supports the head. It has practically no body or spine and
appears as a bony ring with two transverse processes. On
its superior surface, the atlas has two kidney-shaped
facets,
which articulate with the occipital condyles.
The second cervical vertebra, or
axis
(ak
sis), bears a
toothlike
dens
(odontoid process) on its body. This process
projects upward and lies in the ring of the atlas. As the head
is turned from side to side, the atlas pivots around the dens
(
F gs.
7.34 and
7.35
).
Thoracic Vertebrae
The twelve
thoracic vertebrae
are larger than those in the
cervical region. Their transverse processes project posteriorly
at sharp angles. Each vertebra has a long, pointed spinous
process, which slopes downward, and a facet on each side of
its body, which articulates with a rib.
Beginning with the third thoracic vertebra and moving
inferiorly, the bodies of these bones increase in size. Thus,
they are adapted to bear increasing loads of body weight.
Lumbar Vertebrae
The F ve
lumbar vertebrae
in the small of the back (loin) sup-
port more weight than the superior vertebrae and have larger
and stronger bodies. Compared to other types of vertebrae,
the thinner transverse processes of these vertebrae project
laterally, whereas their short, thick spinous processes project
posteriorly nearly horizontal.
±igure 7.36
compares the struc-
tures of the cervical, thoracic, and lumbar vertebrae.
Anterior
Posterior
FIGURE 7.35
Radiograph of the cervical vertebrae.
(b)
(c)
(a)
Anterior
Anterior articular
facet for atlas
Inferior articular
process
Posterior
Atlas
Axis
Transverse
foramen
Vertebral
foramen
Transverse
foramen
Body
Transverse
process
Transverse
process
Facet that articulates
with dens (odontoid process)
of axis
Dens (odontoid
process)
Facet that articulates
with occipital condyle
Superior
articular facet
Spinous process
Spinous
process
Dens
FIGURE 7.34
Atlas and axis. (
a
) Superior view of the atlas. (
b
) Right lateral view and (
c
) superior view of the axis.
The painful condition of spondylolisthesis occurs when a vertebra
slips out of place over the vertebra below. Most commonly the F
fth
lumbar vertebra slides forward over the body of the sacrum. Persons
with spondylolysis (see previous box) may be more likely to develop
spondylolisthesis, as are gymnasts, football players, and others who
±
ex or extend their vertebral columns excessively and forcefully.
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