Some basic concepts of the development of trunk musculoskeletal tissues have already been covered in Section 2.1 and Boxes 4.1 and 4.2. The axial skeleton is composed of the skull, ribs, sternum, and vertebral column (Plate 6.2). The vertebral column consists of 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3–5 coccygeal vertebrae. This column of bones forms the axis of the body and protects the spinal cord. Cervical, thoracic, and lumbar vertebrae are easily distinguished from one another by several criteria. Generally, vertebrae from more superior regions of the vertebral column have more oval-shaped vertebral bodies and relatively larger vertebral foramina. Each vertebral foramen, through which the spinal cord runs, is bounded on the ventral (anterior) side by the vertebral body and on the dorsal (posterior) side by vertebral arch, formed by the pedicles and lamina. The shapes of the vertebral arches change over a cranio-caudal gradient that reflects their function: The main function of the upper vertebral column is to allow flexibility and movement of the neck and head, while the main function of the lower vertebral column, particularly the sacral region, is to provide support to the upper body and pelvic girdle. In fact, as will be explained below, cervical vertebra 1 (atlas) has no body at all but instead encircles the dens of cervical vertebra 2 (axis), allowing the head to rotate through a large range of motion (Plate 3.31b,c).