The Human Body

Spinal Column

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Spinal Column, common name applied to the structure of bone or cartilage surrounding and protecting the spinal cord in vertebrate animals (see Animal). It is also called a vertebral column, spine, or backbone,

ANATOMY AND PHYSIOLOGY

The spinal column forms the major part of the skeleton. To it are attached the skull, shoulder bones, ribs, and pelvis. In very primitive animals having a vertebral column, the spine consists of a solid cartilaginous rod known as the notochord. Although remnants of the notochord persist in the cartilages that form part of the apparatus connecting adjoining vertebrae, in higher animals the notochord is replaced by a series of separate bones called vertebrae.

The shape and number of vertebrae vary among different animals. In general, the vertebrae are stacked like a column of poker chips and are held together by ligaments, the connective tissue that holds bones together at a joint. In humans the spinal column contains 33 vertebrae: 7 cervical vertebrae in the neck; 12 thoracic, or dorsal, vertebrae in the region of the chest, or thorax, providing attachment for 12 pairs of ribs; 5 lumbar vertebrae in the small of the back; 5 fused sacral vertebrae forming a solid bone, the sacrum (see Sacroiliac Joint), which fits like a wedge between the bones of the hip; and a variable number of vertebrae fused together to form the coccyx at the bottom of the sacrum.

Before birth, the human spinal column forms a single curve with the convex surface toward the back; at birth, two primary curvatures are present, both of which are concave forward. The upper one is located in the thoracic and the lower one in the sacral region. If the child develops normally, two compensatory forward curvatures develop in the cervical and lumbar regions, just above the primary curvatures. These normal curvatures provide a degree of resilience that would not be possible in a series of rigid, straightly stacked bones.

Most of the individual vertebrae are shaped somewhat like rings; the body, or thick portion of the ring, is located toward the front portion of the body. Between each of the separate vertebrae is a thick, fibrous disk of cartilage—called an intervertebral disk—that forms the principal joint between the bodies of adjoining vertebrae; however, the vertebrae also move with each other at several other joints.