Chapter 5 The Skeletal System 127 Copyright Goodheart-Willcox Co., Inc. vertebra supports the weight of the head, neck, arms, and the entire trunk positioned above that vertebra. The size and angulation of the vertebral processes also vary throughout the spinal column. This changes the orientation of the facet joints, which interconnect the vertebrae and serve to limit range of motion in the different spinal regions. The Spinal Curves The characteristic shapes of the vertebrae in the different spinal regions also form the normal spinal curves. As Figure 5.13 shows, the cervical and lumbar curves are posteriorly concave, while the thoracic, sacral, and coccyx curvatures are anteriorly concave. These alternating curves make the spine stronger and better able to resist potentially injurious forces than if it were straight. The thoracic and sacral curves are known as primary spinal curves because they are present at birth. The lumbar and cervical curves are referred to as secondary spinal curves. They develop after the baby begins to raise the head, sit, and stand, as increased muscular strength enables the young child to shift body weight to the spine. Abnormal spinal curvatures can develop due to genetic or congenital abnormalities, or as a result of the spine being habitually subjected to asymmetrical forces (Figure 5.18). Exaggeration of the lumbar curve is called lordosis, accentuation of the thoracic curve is called kyphosis, and any lateral deviation of the spine is known as scoliosis. Intervertebral Discs Intervertebral discs composed of fibrocartilage provide cushioning between all articulating vertebral bodies that are not fused. These discs serve as shock absorbers and allow the spine to bend. The differences in the anterior and posterior thicknesses of these discs produce the normal cervical, thoracic, and lumbar curves. In a normal adult, the discs account for approxi- mately one-quarter of the height of the spine. When a person is lying in bed during overnight sleep, the discs absorb water and expand slightly. During periods of upright standing and sitting, when the discs are bearing weight, they lose a small amount of fluid and compression occurs. For this reason, people may be as much as three-fourths of an inch taller when they first arise in the morning. Injury and progressive position, each vertebra must support the weight of all of the body parts positioned above it. Think about what this means. While a cervical vertebra supports only the weight of the head and neck, a lumbar Vertebral arch Articular processes Superior articular process Superior articular process Inferior articular facet Inferior articular facet Transverse process Transverse process The major components of a typical vertebra (superior view) Lateral and slightly inferior view Inferior view Vertebral body Vertebral body Spinous process Spinous process Inferior articular process Pedicle Pedicle Arrow passing through vertebral foramen Vertebral body Vertebral foramen © Body Scientifi c International Figure 5.16 Three views of a typical vertebra.