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The axial skeleton is divided into three sections: the skull, the vertebral_column (spinal_column), and the thorax.

The vertebral column extends from the skull to the pelvis, and creates a central support system for the body. The bones of the vertebral column are called the vertebrae (sing: vertebra). There are 24 single vertebral bones (7 cervical, 12 thoracic, and 5 lumbar) and 9 fused bones (sacrum has 5; coccyx has 4).

The vertebral column is flexible because of pads of fibrocartilage that lie between each bone. These pads are called intervertebral_discs. Each disc has a fluid center called the nucleus_pulposus and an outer ring of fibrous tissue called the annulus_fibrosus.

The basic structure of a vertebra consists of:

centrum (body),





superior and inferior_articular_processes, and the


There are twelve thoracic vertebrae (T-1 through T-12), and they are distinguished by their distinctive heart-shaped centrum, which has costal_demifacets (one superior and one inferior) for the attachment of ribs.

They also have long spinous processes and an oval or round vertebral foramen. Their transverse processes have articular_facets that connect with the tubercles of the ribs.

Spinous process Transverse costal facet for tubercle of a rib Transverse process Transverse process Costal demifacet for head of a rib Superior articular process Superior articular process Vertebral foramen Centrum or body Superior articular process Superior articular process Centrum or body Vertebral foramen Spinous process Transverse process Transverse process Centrum or body Superior articular process Superior articular process Spinous process Transverse costal facet for rib tubercle Transverse process Spinous process Intervertebral foramen Transverse process Superior articular process Superior demifacet for head of supior rib Transverse costal facet Centrum or body Inferior demifacet for head of inferior rib