Page 271 - Fundamentals of anatomy physiology

258 Unit 2 Support and Movement
Important muscles controlling the position of the head and
neck attach to the especially robust spinous process of the axis.
In children, the fusion between the dens and axis is in-
complete. Impacts or even severe shaking can cause dislocation
of the dens and severe damage to the spinal cord. In adults, a
hit to the base of the skull can be equally dangerous, because
a dislocation of the atlas–axis joint can force the dens into the
base of the brain, with fatal results.
7 &T i p s T r i c k s
To remember the difference between the atlas and the axis
and their respective movements, consider Greek mythology
and the Earth. In this case, the head (Earth) is held up by Atlas
and is capable of nodding “yes” movements. The Earth rotates
on its axis, and the axis allows for us to shake our heads in a
“no” movement.
The Vertebra Prominens (C7)
The transition from one vertebral region to another is not
abrupt. The last vertebra of one region generally resembles
the first vertebra of the next. The vertebra prominens, or sev-
enth cervical vertebra (C7), has a long, slender spinous process
(Figure 7–19a) that ends in a broad tubercle that you can feel
through the skin at the base of the neck. This vertebra is the
interface between the cervical curve, which arches anteriorly,
and the thoracic curve, which arches posteriorly (Figure 7–17).
The transverse processes of C7 are large, providing additional
surface area for muscle attachment.
The ligamentum nuchae (lig-uh-MEN-tum-Nu. -ke. ; nucha,
nape), a stout elastic ligament, begins at the vertebra promi-
nens and extends to an insertion (attachment site) along the
occipital crest of the skull. Along the way, it attaches to the spi- massive spinous processes provide surface area for the at-
nous processes of the other cervical vertebrae. When your head
is upright, this ligament acts like the string on a bow, main-
taining the cervical curvature without muscular effort. If you
have bent your neck forward, the elasticity in the ligamentum
nuchae helps return your head to an upright position.
Thoracic Vertebrae
There are 12 thoracic vertebrae (Figure 7–20a). A typical tho-
racic vertebra has a distinctive heart-shaped body that is more
massive than that of a cervical vertebra (Figure 7–20b). The
vertebral foramen is relatively smaller. The long, slender spi-
nous process projects posteriorly and inferiorly. The spinous
processes of T10, T11, and T12 increasingly resemble those of the
lumbar region as the transition between the thoracic and lum-
bar curves approaches. Because the inferior thoracic and lum-
bar vertebrae carry so much weight, the transition between the
thoracic and lumbar curves is difficult to stabilize. As a result,
compression fractures or compression–dislocation fractures
from a hard fall tend to involve the last thoracic and first two
lumbar vertebrae.
Each thoracic vertebra articulates with ribs along the dor-
solateral surfaces of the body. The costal facets on the verte-
bral bodies articulate with the heads of the ribs. The location
and structure of the articulations vary somewhat among tho-
racic vertebrae (Figure 7–20a). Vertebrae T1–T8 each articulate
with two pairs of ribs, so their vertebral bodies have two costal
facets (superior and inferior) on each side. Vertebrae T9–T11 have
a single costal facet on each side, and each vertebra articulates
with a single pair of ribs.
The transverse processes of vertebrae T1–T10 are relatively
thick and contain transverse costal facets for rib articulation
(Figure 7–20b,c). Thus, rib pairs 1 through 10 contact their ver-
tebrae at two points: a costal facet and a transverse costal facet.
Table 7–1 summarizes the features of thoracic vertebrae.
Lumbar Vertebrae
The five lumbar vertebrae are the largest vertebrae
(Figure 7–21a). The body of a typical lumbar vertebra
(Figure 7–21b,c) is thicker than that of a thoracic vertebra, and
the superior and inferior surfaces are oval rather than heart
shaped. Also note that (1) lumbar vertebrae do not have costal
facets; (2) the slender transverse processes, which lack trans-
verse costal facets, project dorsolaterally; (3) the vertebral fo-
ramen is triangular; (4) the stumpy spinous processes project
dorsally; (5) the superior articular processes face medially (“up
and in”); and (6) the inferior articular processes face laterally
(“down and out”).
The lumbar vertebrae withstand the most weight. Their
tachment of lower back muscles that reinforce or adjust the
lumbar curve. Table 7–1 summarizes the features of lumbar
vertebrae.
&T i p s T r i c k s
To remember the number of bones in the first three spinal
curves, think about mealtimes. You eat breakfast at 7 a.m.
(7 cervical vertebrae), lunch at 12 p.m. (12 thoracic vertebrae),
and dinner at 5 p.m. (5 lumbar vertebrae).
The Sacrum
The sacrum consists of the fused components of five sacral
vertebrae. These vertebrae begin fusing shortly after puberty.
In general, they are completely fused at age 25–30. The sa-
crum protects the reproductive, digestive, and urinary organs.
Through paired articulations, it attaches the axial skeleton to
the paired hip bones, or pelvic girdle, of the appendicular skel-
eton (Figure 7–1a).