vertebral column

Backbone, giving support to an animal and protecting its spinal cord. It is made up of a series of bones or vertebrae running from the skull to the tail, with a central canal containing the nerve fibres of the spinal cord.

In tetrapods the vertebrae show some specialization with the shape of the bones varying according to position. In the chest region the upper or thoracic vertebrae are shaped to form connections to the ribs. The backbone is only slightly flexible to give adequate rigidity to the animal structure.

There are 33 vertebrae in the human infant, but only 26 in the adult. In the infant there are 7 cervical, 12 dorsal or thoracic, 5 lumbar, 5 sacral, and 4 coccygeal or caudal, vertebrae. In the adult the 5 sacral vertebrae fuse together to form the sacrum, while the 4 coccygeal vertebrae fuse to form the coccyx.

Vertebral structure Each vertebra has a body and an arch. The body is a cylinder connected to the adjacent vertebrae by a disc of fibro-cartilage, which absorbs shock. The arch consists of two halves which join together behind, forming a ring. The whole succession of these rings forms the vertebral canal, or channel, containing the main nerve trunk known as the spinal cord. Viewed from the side, the vertebral column has a series of curves. This curvature maintains the strength of the structure and adapts itself to the various movements of the body. The spinal cord occupies the upper two-thirds of the vertebral canal, and 31 pairs of spinal nerves arising from the spinal cord leave the vertebral canal through spaces at the sides between successive vertebrae.

Injuries and diseases Fracture of the spine occurs when the vertebrae are separated or crushed. This may tear or crush the spinal cord, with the result that parts of the body below the injury are deprived of sensation, and of the power of movement.

The vertebral column develops from many parts, and congenital problems may occur. The vertebral arches may fail to fuse at any or all parts of the column, giving rise to spina bifida. Wedge-shaped vertebrae can cause a sideways curvature (scoliosis) of the spine. Arthritis of the spine (or spondylitis) causes it to become rigid. Spondylosis, or accretion of bone within the vertebral canal, is a common problem of old age, particularly in the cervical region. Symptoms vary but often include a stiff neck and attacks of giddiness. Pressure on the spinal cord can also be due to a slipped disc when the jelly-like centre of an intervertebral disc protrudes posteriorly into the vertebral canal.