Humans usually have 33 vertebrae, yet about 10 percent of people break the pattern
A typical human spine stacks 33 vertebrae: seven in the neck, twelve in the chest, five in the lower back, five fused into the sacrum and four forming the tailbone. Totals can range from 32 to 35, with extra or missing thoracic, lumbar and coccygeal bones the usual variations.
Each vertebra is an irregular bone made of bone and some hyaline cartilage, and vertebrates take their very name from these building blocks. Across species the basic design is remarkably consistent, with the sharpest contrast between aquatic animals and everything else. Within one human spine, though, the bones change shape and size along the column to meet different demands of load and mobility, and posture and disease affect their size as well.
The front portion, the vertebral body or centrum, carries the weight. Inside it is spongy cancellous bone wrapped in a thin shell of dense cortical bone; the arch and processes behind get thicker cortical coverings. The body's upper and lower surfaces, the endplates, grip the intervertebral discs, spread loads evenly, anchor the discs' collagen fibres and act as a semi-permeable barrier for water and dissolved substances.
Behind the body sits the vertebral arch, assembled from two pedicles, two laminae and seven processes, eleven parts in all. Body and arch together enclose the vertebral foramen, the opening that houses the spinal canal and shields the spinal cord. Notches above and below each pedicle line up with those of neighbouring vertebrae, creating gaps, the intervertebral foramina, through which spinal nerves and blood vessels pass in and out.
The processes do the mechanical work. A central spinous process points backward and downward to anchor muscles and ligaments, while two transverse processes jut sideways; in the chest they carry facets where the ribs attach. In the lower back the transverse process is sometimes called the costal process, because it corresponds to a rib that never develops there. Paired articular facets limit how far the spine can twist and bend, linked by a thin bridge of bone called the pars interarticularis.
Source: Vertebra
