Vertebra (version 2)
The vertebrae (singular: vertebra) are those bones which compose the spine in vertebrates.
There are 33 vertebrae in humans, including the five that are fused to form the sacrum and the four coccygeal bones.
The 24 remaining are divided into:
- Seven cervical vertebrae.
- Twelve thoracic vertebrae.
- Five lumbar vertebrae.
General Structure
The anterior (front) of a vertebra is the vertebral body. In the spine, intervertebral discs separate adjacent vertebral bodies. Behind the vertebral body, a vertebral foramen (hole) is formed by two pedicles and two laminae. The spinal column runs through this foramen. Two transverse processess and one spinous process are posterior to (behind) the vertebral body. The spinous process comes out the back, one transverse process comes out the left, and one on the right. The spinous processes of the cervical and lumbar regions can be felt through the skin. Superior and inferior articular facets on each vertebra act to restrict the range of movement possible.
Cervical vertebrae
These are generally small and delicate. Their spinous processes are short (with the exception of C7 which has the first palpable spinous process), and often split. Numbered top-to-bottom from C1-C7, atlas (C1) and axis (C2), are the vertebrae that allow the neck so much rotation. Specifically, the atlas allows the skull to move up and down, while the axis allows the upper neck to twist left and right. The axis also houses the first intervertebral disk of the spinal, which ends in the sacrum.
Thoracic vertebrae
Their spinous processes point downwards, and are quite long. They have surfaces that articulate with the ribs. Some rotation can occur at the thoracic vertebrae, but the ribs prevent much flexion.
Lumbar vertebrae
These vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant flexion and extension, moderate lateral flexion (sidebending), and a small degree of rotation. The discs between these vertebrae create a [lumbar] lordosis (curvature that is concave posteriorly) in the human spine.