The spine extends from the head to the sacrum, and in most people consists of 24 vertebrae. The vertebral column can be divided into three segments:
Cervical: The first seven vertebrae, all in the neck. These are smaller and lightly built.
Thoracic: Twelve vertebrae, each articulating with the twelve pairs of ribs.
Lumbar: The lower five vertebrae, between the ribs and the sacrum. These have the largest vertebral bodies.
Each of the three segments of the spine has a curve. The cervical spine and the lumbar curve both are convex anteriorly, while the thoracic spine curves the opposite way, convex posteriorly. Each vertebra is shaped a bit like a wedge to support these curves, especially noticeable with the five lumbar vertebrae. The lumbar curve is unique to humans, allowing us to maintain a vertical posture above our pelvis.