The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate, in which the notochord (a flexible rod of uniform composition) found in all chordates has been replaced by a segmented series of bones (vertebrae) separated by intervertebral discs. The vertebral column houses the spinal canal, a cavity that encloses and protects the spinal cord.
The number of vertebrae differ between species. Here are different vertebral formulae (note that all mammals have seven cervical vertebrae):