About fifty thousand species share a backbone plan
The spinal column—vertebral column, spine, or backbone—is the axial skeleton's core and the trait that names vertebrates. Roughly fifty thousand animal species have one. Humans usually count thirty-three vertebrae: seven neck, twelve chest, five lumbar, then fused sacral and coccygeal pieces.
The backbone is a stack of segments encasing and protecting the spinal cord, cushioned by intervertebral discs. Human vertebrae are fairly typical of mammals, reptiles and birds, which is one reason the human spine is so thoroughly studied. Doctors use bony landmarks such as the spinous processes to guide lumbar punctures and spinal anaesthesia.
Counts vary a little between people. The total usually falls between 32 and 35, and in about one person in ten the number or regional split differs from the textbook pattern. The neck almost never changes, while the tailbone varies most, holding three to five fused pieces. The upper 24 vertebrae move independently, separated by discs; in adults the lower nine are fused into the sacrum and coccyx.
Each vertebra has a weight-bearing body at the front and an arch at the back; together they enclose the canal the cord runs through. The arch, built from two pedicles and two laminae, carries seven projections: four articular processes that form facet joints with the neighbours above and below, two transverse processes and one spinous process, which anchor back muscles and ligaments. The angle of the facet joints limits how far each segment can twist or bend. Long ligaments run down the front and back of the bodies, and others link the laminae and processes.
Seen from the side, the spine is an S of alternating curves that add strength, springiness and shock absorption for upright walking, deepening under load and rebounding afterwards. The chest and sacral curves bow backward and exist before birth, so they are called primary. The neck and lower-back curves bow forward and form later: the neck curve once an infant can support its own head, around three to four months, and sit, around nine, and the lumbar curve with walking. The lumbar curve is more pronounced in women than in men.
Source: Spinal column
