spinal column The terms ‘spine’, ‘spinal column’, and ‘vertebral column’ all refer to the series of bones — 24 separate vertebrae plus the sacrum and the coccyx — that extend from neck to tail. They represent the segmental architecture of our phylogenetic ancestors. The topmost vertebra is known as the ‘atlas’ because it supports the skull at joints with the occipital bone, as the Titan of Greek mythology supported the earth. For descriptive convenience the bones are described as 7 cervical, 12 thoracic, and 5 lumbar vertebrae. The lowest lumbar vertebra articulates with the sacrum, and this in turn with the coccyx. The sacrum itself represents 5 fused bones, and the coccyx another vestigial 2–3 or more — the remains of the ancestral tail. Each of the vertebrae is a bony ring, and they are aligned to form a continuous tunnel, the vertebral canal, down the whole column to the end of the sacrum. This encloses the spinal cord and the spinal nerve roots that enter and leave it, together with their surrounding membranes. The thick ‘bodies’ of the vertebrae are in the front of the canal, and their arches form the sides and back. The flat upper and lower surfaces of the vertebral bodies are linked, each to the next, by an adherent, cartilaginous intervertebral disc; each disc is fibrous and strong where it adheres to bone and around its circumference, but encloses a softer centre that allows some angulation of one vertebral surface relative to the next; thus there is a degree of flexibility — greater at some levels of the spine than others. The vertebral arches each have seven bony projections, including the midline ‘spines’ that can be felt through the skin of the back. The other pairs form joints with those of adjoining vertebrae and provide attachment for muscles and ligaments. The arrangement of the projections and joints is such as to allow some rotation in parts of the spine above the lumbar region. Nerve roots pass to and from the spinal cord through gaps between the arches of adjacent vertebrae, linking the cord with the peripheral nerves. The thinnest, flat parts of the arches — the laminae — provide surgical access to the spinal cord within the canal: hence the term laminectomy for making an opening into the spinal canal. The gaps between the arches of adjacent lumbar vertebrae, when the spine is flexed, allow the insertion of a needle for sampling cerebrospinal fluid in a spinal tap or lumbar puncture.

Sheila Jennett

See also bipedalism; evolution, human; skeleton; slipped disc.