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Spine Info
Spinal Column, common name applied to the structure of bone or cartilage surrounding and
protecting the spinal cord in vertebrate animals. It is also called a vertebral
column, spine, or backbone.
Anatomy and Physiology
The spinal column forms the major part of the skeleton. To it are attached the skull, shoulder
bones, ribs, and pelvis. In very primitive animals having a vertebral column, the spine
consists of a solid cartilaginous rod known as the notochord. Although remnants of the
notochord persist in the cartilages that form part of the apparatus connecting adjoining
vertebrae, in higher animals the notochord is replaced by a series of separate bones called
vertebrae.
The shape and number of vertebrae vary among different animals. In general, the vertebrae are
stacked like a column of poker chips and are held together by ligaments, the connective tissue
that holds bones together at a joint. In humans the spinal column contains 33 vertebrae: 7
cervical vertebrae in the neck; 12 thoracic, or dorsal, vertebrae in the region of the chest,
or thorax, providing attachment for 12 pairs of ribs; 5 lumbar vertebrae in the small of the
back; 5 fused sacral vertebrae forming a solid bone, the sacrum (see SACROILIAC JOINT), which
fits like a wedge between the bones of the hip; and a variable number of vertebrae fused
together to form the coccyx at the bottom of the sacrum.
Before birth, the human spinal column forms a single curve with the convex surface toward the
back; at birth, two primary curvatures are present, both of which are concave forward. The
upper one is located in the thoracic and the lower one in the sacral region. If the child
develops normally, two compensatory forward curvatures develop in the cervical and lumbar
regions, just above the primary curvatures. These normal curvatures provide a degree of
resilience that would not be possible in a series of rigid, straightly stacked bones.
Most of the individual vertebrae are shaped somewhat like rings; the body, or thick portion
of the ring, is located toward the front portion of the body. Between each of the separate
vertebrae is a thick, fibrous disk of cartilage called an intervertebral disk that forms the
principal joint between the bodies of adjoining vertebrae; however, the vertebrae also move
with each other at several other joints.
Most vertebrae consist of a body, a large mass of solid bone that is the weight-bearing part of
the vertebra. Extending backward on each side of the body is a thick pillar of bone, or
pedicle. The pedicles and back of the body help to form a circular opening, the vertebral
foramen, through which the spinal cord passes. Two plates of bone, known as the laminae,
meet the pedicles and join with each other in an angle at the back of the vertebra to complete
the circular opening. The canal formed by the juxtaposition of the intervertebral foramens of
all the vertebrae is called the neural canal.
On each side, at the junction of the pedicle and
lamina, is a projection known as the transverse process. At the angle formed by the junction
of the two laminae is another projection, the spinous process. At the base of each transverse
process is a smooth, movable structure that forms joints with the adjacent vertebrae. In erect
animals, one pair of these processes is located on the top surface and another pair on the
bottom surface of each vertebra.The vertebrae of each region of the mammalian spinal cord have
definite characteristics. In the upper cervical vertebrae, each transverse process is pierced
by a hole through which the vertebral artery passes.
The spinous processes of these vertebrae are very short. The first two cervical vertebrae are
unlike any of the others. The first cervical vertebra, known as the atlas, has no body; the
body is replaced by an arch of bone enclosing a depression. The superior articular processes of
the axis are jointed to the occipital condyles, or rounded projection of bone, of the skull.
The second cervical vertebra, known as the axis or epistropheus, has a projection on the top
of its body that fits like a pivot into the special depression in the atlas. On the transverse
processes of the thoracic vertebrae are special articulating surfaces for the ribs; the spinous
processes, which are long, project downward and overlap each other. The lumbar vertebrae have
large, heavy bodies and reduced transverse and spinous processes. The fused sacrum and coccyx
are described above.
Contributed by:
Frederick A. Mettler
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