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The spine: a flexible stack of 33 vertebrae that supports the trunk, transmits weight, and protects the spinal cord.
Medically reviewed & updated
The vertebral column, or spine, is the central pillar of the axial skeleton. In humans it is composed of 33 vertebrae arranged in five regions: 7 cervical (neck), 12 thoracic (upper/mid back), 5 lumbar (lower back), 5 sacral (fused into the sacrum), and 4 coccygeal (fused into the coccyx, or tailbone). The cervical, thoracic, and lumbar vertebrae remain mobile, while the sacral and coccygeal segments fuse in adulthood.
A typical vertebra consists of a weight-bearing vertebral body in front and a vertebral arch behind; together they enclose the vertebral foramen. The stacked foramina form the vertebral (spinal) canal, which houses and protects the spinal cord. Seven bony processes project from each vertebra—two superior and two inferior articular processes, two transverse processes, and one spinous process—serving as attachment points for muscles and ligaments and as articulation points between adjacent vertebrae.
Viewed from the side, the adult spine shows alternating curves: a forward-convex cervical lordosis, a backward-convex thoracic kyphosis, a lumbar lordosis, and a sacral kyphosis. These curves act like a spring, distributing load and improving balance and shock absorption.
Between adjacent vertebral bodies sit intervertebral discs, cartilaginous cushions made of a tough outer annulus fibrosus and a gel-like inner nucleus pulposus. The discs make up roughly a quarter of the column's length and allow flexible movement while cushioning compressive forces. Overall, the vertebral column supports the head and trunk, transmits body weight to the pelvis and legs, anchors the ribs and back muscles, permits a wide range of movement, and protects the delicate spinal cord and exiting nerve roots.
Disc and curvature problems are common. When the annulus fibrosus weakens or tears, the nucleus pulposus can bulge out—a herniated (slipped) disc—and press on nearby nerves, causing back pain, sciatica, or numbness. Abnormal sideways curvature is called scoliosis, while exaggerated front-to-back curves are termed hyperkyphosis or hyperlordosis. This content is educational, not medical advice.