At birth, the human spine comprises 33 individual bones, called vertebrae, that are stacked atop each other. These interlocking bones are connected through elastic ligaments and spinal disks, which provide flexibility to the spine, explains the United States National Institutes of Health. However, some of these bones fuse together during adulthood and only the 24 topmost vertebrae are movable.
The adult human spine aids the body to maintain an upright position and enables range-of-motion by serving as the body's primary structural support. It also functions by protecting neural spinal fibers and the spinal cord.
The adult spine forms natural curvatures that stabilize the vertebral column. It is divided into five major regions: cervical, thoracic, lumbar, sacral and coccyx. The cervical spine, or neck vertebrae, are composed of seven bones, while the thoracic spine, or upper back vertebrae, are made up of 12 bones. The lumbar spine, or the lower back vertebrae, consists of five bones. The sacral region, or sacrum, comprises five merged bones. The coccyx region, or tailbone, is composed of four fused coccygeal bones. It is the bottommost region that supports the bulk of the body's weight.
Certain factors may affect the natural alignment of the spine, including obesity and lack of muscular strength, notes Mayfield Clinic. These may further result in abnormalities in the spine's natural curvatures. Lordosis, also known as "sway back," refers to an irregular curve of the lumbar region. Kyphosis, commonly referred to as "hunchback," impacts the thoracic spine.