spine

The human spine extends every night during sleep. During the day, the cartilage discs between the vertebra are squeezed when the body is in a vertical position, standing or sitting, but at night, with pressure released, the discs swell and the spine lengthens by about 8 mm/0.3 in.

Backbone of vertebrates. In most mammals, it contains 26 small bones called vertebrae, which enclose and protect the spinal cord (which links the peripheral nervous system to the brain). The spine articulates with the skull, ribs, and hip bones, and provides attachment for the back muscles.

In humans it is made up of individual vertebrae, separated by intervertebral discs. In the adult there are seven cervical vertebrae in the neck; twelve thoracic in the upper trunk; five lumbar in the lower back; the sacrum (consisting of five rudimentary vertebrae fused together, joined to the hipbones); and the coccyx (four vertebrae, fused into a tailbone). The human spine has four curves (front to rear), which allow for the increased size of the chest and pelvic cavities, and for a degree of spring so as to minimize jolting of the internal organs.