Spinal Column.

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Spinal Column.
I

INTRODUCTION

Spinal Column, common name applied to the structure of bone or cartilage surrounding and protecting the spinal cord in vertebrate animals (see Animal). It is also
called a vertebral column, spine, or backbone.

II

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 (see Joints). 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.

III

ABNORMALITY AND DISEASE

Pathologic curvatures of the human spinal column are common. These conditions may be caused by weak ligaments, by poor posture habits, by disease or congenital
abnormalities of the spinal column, by injury, or by spasm of the back muscles.
One well-known spinal curvature is hunchback, a humped condition of the dorsal spine resulting from an extreme curvature of the spine. This curvature may be either a
kyphosis, which is an accentuation of the normal posterior curvature, or a combination of kyphosis and extreme lateral curvature (scoliosis), which is known as
kyphoscoliosis. Scoliosis alone rarely creates a hunchback appearance. Kyphosis may be mild or severe. Although most cases of kyphosis are congenital, the condition
may result from spinal injury. (Before the development of effective antitubercular drugs, a hunchback condition was sometimes caused by Pott's disease, a form of
tuberculosis affecting the vertebrae.) Collapse may occur in elderly people, particularly women, whose bones may become soft and brittle, causing a dorsal kyphosis.
When curvature results from collapsed vertebrae, the person loses height along with developing the curvature. As a result of the spinal deformity in hunchback, the ribs
become contorted, compressing or displacing the lungs and other structures within the chest cavity and thrusting the collarbone and shoulder blades into distorted
positions. Deformations take place in the hips and other parts of the body in its effort to maintain balance.
Treatment for a hunchback condition is varied. A mild kyphosis can often be corrected with plaster casts and braces if diagnosed before the skeletal frame has
completed its growth. Congenital deformities cannot be cured. The malformation of the vertebrae are embryological in origin; only a certain amount of the deformity can
be minimized by surgery and local manipulation. Hunchback caused by spinal disease is also only slightly amenable to surgery and local treatment. Traction, pads, and
plaster-jacket supports are used in manipulative treatments, and electrical stimulation is also being studied. Scoliosis is sometimes addressed surgically by insertion of a
metal rod along the spine.
The spine may also be affected by herniated or ruptured nucleus pulposus, a condition in which the posterior longitudinal ligament, lying on the forward side of the
neural canal, gives way or allows passage of some of the substance of the cartilage between the vertebrae into the canal. This ruptured disk material presses on the
nerve roots or the spinal cord, causing great pain, a condition known as slipped disk.

Contributed By:
Frederick A. Mettler
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