Chapter 7
The Skeleton
219
7
the spine (bending too far backward). Te posterior ligament,
which resists hyperflexion of the spine (bending too sharply
forward), is narrow and relatively weak. It attaches only to the
discs. However, the
ligamentum flavum
, which connects ad-
jacent vertebrae, contains elastic connective tissue and is espe-
cially strong. It stretches as we bend forward and then recoils
when we resume an erect posture. Short ligaments connect each
vertebra to those immediately above and below.
Intervertebral Discs
Each
intervertebral disc
is a cushionlike pad composed of
two parts. Te inner gelatinous
nucleus pulposus
(pul-po
9
sus;
“pulp”) acts like a rubber ball, giving the disc its elasticity and
compressibility. Surrounding the nucleus pulposus is a strong
collar composed of collagen fibers superficially and fibrocarti-
lage internally, the
anulus fibrosus
(an
9
u-lus fi-bro
9
sus; “ring of
fibers”) (Figure 7.18a, c). Te anulus fibrosus limits the expan-
sion of the nucleus pulposus when the spine is compressed. It
also acts like a woven strap to bind successive vertebrae together,
withstands twisting forces, and resists tension in the spine.
Sandwiched between the bodies of neighboring vertebrae,
the intervertebral discs act as shock absorbers during walking,
jumping, and running. Tey allow the spine to flex and extend,
and to a lesser extent to bend laterally. At points of compression,
the discs flatten and bulge out a bit between the vertebrae. Te
discs are thickest in the lumbar and cervical regions, which en-
hances the flexibility of these regions.
Collectively the discs account for about 25% of the height of
the vertebral column. Tey flatten somewhat during the course
of the day, so we are always a few millimeters shorter at night
than when we awake in the morning.
Homeostatic Imbalance
7.3
Severe or sudden physical trauma to the spine—for example,
from bending forward while li±ing a heavy object—may result
in herniation of one or more discs. A
herniated
(
prolapsed
)
Lordosis
, or swayback, is an accentuated
lumbar
curvature. It,
too, can result from spinal tuberculosis or osteomalacia. ²empo-
rary lordosis is common in those carrying a large load up front,
such as men with “potbellies” and pregnant women. In an attempt
to preserve their center of gravity, these individuals automatically
throw back their shoulders, accentuating their lumbar curvature.
Ligaments
Like a tall, tremulous ²V transmitting tower, the vertebral col-
umn cannot possibly stand upright by itself. It must be held in
place by an elaborate system of cable-like supports. In the case
of the vertebral column, straplike ligaments and the trunk mus-
cles assume this role.
Te major supporting ligaments are the
anterior
and
pos-
terior longitudinal ligaments
(Figure 7.18)
. Tese run as
continuous bands down the front and back surfaces of the verte-
brae from the neck to the sacrum. Te broad anterior ligament
is strongly attached to both the bony vertebrae and the discs.
Along with its supporting role, it prevents hyperextension of
(a) Scoliosis
(b) Kyphosis
(c) Lordosis
Figure 7.17
Abnormal spinal curvatures.
Supraspinous ligament
Intervertebral
disc
Anterior
longitudinal
ligament
Intervertebral foramen
Posterior longitudinal
ligament
Anulus fibrosus
Nucleus pulposus
Sectioned body
of vertebra
Transverse process
Sectioned
spinous process
Ligamentum flavum
Interspinous
ligament
Inferior articular process
(a) Median section of three vertebrae, illustrating the composition
of the discs and the ligaments
Posterior longitudinal
ligament
Anterior longitudinal
ligament
Body of a vertebra
Intervertebral disc
(b) Anterior view of part of the spinal column,
showing the anterior longitudinal ligament
Figure 7.18
Ligaments and fibrocartilage discs uniting the vertebrae.
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