Spinal Column

All vertebrae conform to a basic plan, but individual variations occur in the different regions. A typical vertebra consists of an anterior, more-or-less cylindric body and a posterior arch composed of two pedicles and two laminae, the latter united posteriorly to form a spinous process. These processes vary in shape, size, and direction in the various regions of the spine. On each side, the arch also supports a transverse process and superior and inferior articular processes; the latter form synovial joints with corresponding processes on adjacent vertebrae, and the spinous and transverse processes provide levers for the many muscles attached to them. The increasing size of the vertebral bodies from above downward is related to the increasing weights and stresses borne by successive segments, and the sacral vertebrae are fused to form a solid wedge-shaped base—the keystone in a bridge whose arches curve down toward the hip joints. The intervertebral disks act as elastic buffers to absorb the numerous mechanical shocks sustained by the spinal column.

Only limited movements are possible between adjacent vertebrae, but the sum of these movements confers a considerable range of mobility on the vertebral column as a whole. Flexion, extension, lateral bending, rotation, and circumduction are all possible, and these actions are freer in the cervical and lumbar regions than in the thoracic. Such differences exist because the disks are thicker in the cervical and lumbar areas, the splinting effect produced by the thoracic cage is lacking, the cervical and lumbar spinous processes are shorter and less closely apposed, and the articular processes are shaped and arranged differently.

At birth, the spinal column presents a general posterior convexity, but later, the cervical and lumbar regions become curved in the opposite directions—when the infant reaches the stages of holding up its head (3 to 4 months) and sitting upright (6 to 9 months). The posterior convexities are primary curves associated with the fetal uterine position, whereas the cervical and lumbar anterior secondary curves are compensatory to permit the assumption of the upright position. There may be additional slight lateral deviations due to unequal muscular traction in right-handed and left-handed persons.

Human evolution from a quadrupedal to a bipedal posture was mainly effected by the tilting of the sacrum between the hip bones, by an increase in lumbosacral angulation, and by minor adjustments of the anterior and posterior depths of various vertebrae and disks. An erect posture greatly increases the load borne by the lower spinal joints, and, good as these ancestral adaptations were, some static and dynamic imperfections remain and predispose to strain and backache. The length of the vertebral column averages 72 cm in the adult male and 7 to 10 cm less in the female. The vertebral canal extends through the entire length of the column and provides an excellent protection for the spinal cord, the cauda equina, and their coverings. The spinal vessels and nerves pass through intervertebral foramina formed by notches on the superior and inferior borders of the pedicles of adjacent vertebrae, bounded anteriorly by the corresponding intervertebral disks, and posteriorly, by the joints between the articular processes of adjoining vertebrae. Pathologic or traumatic conditions affecting any of these structures may produce pressure on the nerves or vessels they transmit.

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