222
UNIT 2
Covering, Support, and Movement of the Body
7
lumbar vertebral structure. Te thoracic vertebrae increase in
size from the first to the last. Unique characteristics of these
vertebrae include the following:
Te body is roughly heart shaped. It typically bears two small
facets, commonly called
demifacets
(half-facets), on each
side, one at the superior edge (the
superior costal facet
) and
the other at the inferior edge (the
inferior costal facet
). Te
demifacets receive the heads of the ribs. (Te bodies of ±
10
±
12
vary from this pattern by having only a single facet to
receive their respective ribs.)
Te vertebral foramen is circular.
Te spinous process is long and points sharply downward.
With the exception of ±
11
and ±
12
, the transverse processes
have facets, the
transverse costal facets
, that articulate with
the tubercles of the ribs.
Te superior and inferior articular facets lie mainly in the
frontal plane, a situation that prevents flexion and extension,
but which allows this region of the spine to rotate. Lateral
flexion, though possible, is restricted by the ribs.
Lumbar Vertebrae
Te lumbar region of the vertebral column, commonly referred
to as the small of the back, receives the most stress. Te en-
hanced weight-bearing function of the five lumbar vertebrae
(L
1
–L
5
) is reflected in their sturdier structure. Teir bodies are
massive and kidney shaped in a superior view (see ±able 7.2,
Figure 7.16, and Figure 7.21). Other characteristics typical of
these vertebrae:
Te pedicles and laminae are shorter and thicker than those
of other vertebrae.
Te spinous processes are short, flat, and hatchet shaped and
are easily seen when a person bends forward. Tese pro-
cesses are robust and project directly backward, adaptations
for the attachment of the large back muscles.
Te vertebral foramen is triangular.
Te orientation of the facets of the articular processes of the
lumbar vertebrae differs substantially from that of the other
vertebra types (see ±able 7.2). Tese modifications lock the
lumbar vertebrae together and provide stability by prevent-
ing rotation of the lumbar spine. Flexion and extension are
possible (as when you do sit-ups), as is lateral flexion.
Sacrum
Te triangular sacrum, which shapes the posterior wall of
the pelvis, is formed by five fused vertebrae (S
1
–S
5
) in adults
(
Figure 7.22
, and see Figure 7.16). It articulates superiorly (via
its
superior articular processes
) with L
5
and inferiorly with
the coccyx. Laterally, the sacrum articulates, via its
auricular
surfaces
, with the two hip bones to form the
sacroiliac joints
(sa
0
kro-il
9
e-ak) of the pelvis.
Te
sacral promontory
(prom
9
on-tor
0
e; “high point of
land projecting into the sea”), the anterosuperior margin of
the first sacral vertebra, bulges anteriorly into the pelvic cavity.
Te body’s center of gravity lies about 1 cm posterior to this
Transverse
process
Spinous
process
Superior articular
process
Superior
articular
process
Transverse
process
Spinous
process
Transverse
costal facet (for
tubercle of rib)
Body
Intervertebral
disc
Intervertebral
disc
Body
Dens of axis
Transverse ligament
of atlas
C
1
(atlas)
C
2
(axis)
C
3
Inferior articular
process
Bifid spinous
process
Transverse processes
C
7
(vertebra
prominens)
Inferior costal
facet (for head
of rib)
Inferior articular
process
Inferior
articular
process
(a) Cervical vertebrae
(c) Lumbar vertebrae
(b) Thoracic vertebrae
Figure 7.21
Posterolateral views of articulated vertebrae.
Notice the bulbous tip on the spinous process of C
7
, the vertebra
prominens. (For a related image, see
A Brief Atlas of the Human
Body
, Figures 19, 20, and 21.)
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