The spine or vertebral column is a curved skeletal structure composed of vertically aligned small bones called vertebrae. Most commonly, it consists of 33 vertebrae. However, the number of sacral and coccygeal vertebrae can vary among people.

The vertebrae are interconnected by 23 cartilaginous intervertebral discs. Together with the bones of the skull, auditory ossicles, hyoid bone, ribs and sternum, the spine forms the axial skeleton. It extends from the occipital bone of the skull to the tip of the coccyx.

All vertebrae are organized into five regions identified as the following:

• Cervical spine - formed by 7 cervical vertebrae;
• Thoracic spine - composed of 12 thoracic vertebrae;
• Lumbar spine - made of 5 lumbar vertebrae;
• Sacral spine - corresponds to a single sacral bone called the sacrum that is formed by the fusion of 5 sacral vertebrae;
• Coccygeal spine - corresponds to a single bone called the coccyx that is formed by the fusion of 4 coccygeal vertebrae.

The spine holds the body in an upright position, assists in balance, serves as a shock absorbent and allows to perform such movements as flexion or extension of the trunk. Besides the mentioned functions, the spine also protects the vertebral canal that goes through it and houses the spinal cord. Moreover, it serves as a conduit for major nerves passing from the brain to the rest of the body. And finally, the spine supports the weight of the upper body.

The alignment of all vertebrae produces several curves of the spine in the sagittal plane. The cervical and lumbar portions curve forward, while the thoracic and sacral parts curve backward. This arrangement gives the spine great strength and shock-absorbing properties.