Also known as the spine, this is the rigid pillar which supports the upper parts of the body and protects the SPINAL CORD and nerves arising from it. It is built up of 33 vertebrae placed one upon another, of which five fuse together to form the sacrum, and the lowest four unite in the coccyx.

Each VERTEBRA has a thick, rounded, bony part in front, known as the body, behind which is a ring of bone, the neural ring. These rings are placed one above another forming the bony canal which protects the spinal cord. From each side of the ring a short process of bone known as the transverse process stands out, and from the back of the ring a larger process, the spinous process, projects. These processes give attachment to the strong ligaments and muscles which unite, support, and bend the column. Between the bodies of adjacent vertebrae are thick discs of fibro-cartilage known as an INTERVERTEBRAL DISC. Each consists of an outer portion, known as the annulus fibrosus, and an inner core, known as the nucleus pulposus. These 23 discs provide the upper part of the spine with pliability and resilience.

The first and second cervical vertebrae are specially modified to allow for movements of the head on the spine. From the spinal cord, nerves leave the cord through openings between the vertebrae produced by notches on the upper and lower margins of each ring.