Just as the Earth rotates around its axis, the axial skeleton lies along the midline, or center, of the body. Think of your spinal column and the bones that connect directly to it — the rib (thoracic) cage and the skull. The axial skeleton also consists of 33 bones in the vertebral column, laid out in four distinct curvatures, or areas.
The cervical, or neck, curvature has seven vertebrae, with the atlas and axis bones positioned in the first and second spots, respectively, forming a joint connecting the skull and vertebral column. The axis, or second cervical vertebra, contains the dens, or odontoid process.
The atlas rotates around the dens, turning the head. (In a sense, the atlas bone holds the world of the head on its shoulders, as the Greek god Atlas held the Earth.)
The thoracic, or chest, curvature has 12 vertebrae that articulate with the ribs, most of which attach to the sternum anteriorly by costal cartilage, forming the rib cage that protects the heart and lungs.
The lumbar, or small of the back, curvature contains five vertebrae and carries most of the weight of the body, which means that it generally suffers the most stress.
The pelvic curvature includes the five fused vertebrae of the sacrum anchoring the pelvic girdle by the sacroiliac articular joint and four fused vertebrae of the coccyx, or tailbone.
The spinal cord extends down the center of the vertebrae only from the base of the brain to the uppermost lumbar vertebrae.
Each vertebra, except the atlas, consists of a body and a vertebral arch, which features a long dorsal projection called a spinous process that provides a point of attachment for muscles and ligaments. On either side of this are the laminae, broad plates of bone on the posterior surface that form a bony covering over the spinal canal. The laminae attach to the two transverse processes, which in turn are attached to the body of the vertebra by regions called the pedicles.
The vertebrae align to form a large opening, called the vertebral foramen, allowing the passage of the spinal cord. Laterally, between the vertebrae, are openings called the intervertebral foramina that allow the spinal nerves to exit the vertebral column. The vertebra have superior articulating facets that articulate with inferior articulating facets of the adjacent vertebra, increasing the rigidity of the column, making the backbone more stable.
Fibrocartilage discs located between the vertebrae act as shock absorbers. Openings in the transverse process, called the transverse foramina, allow large vessels and nerves ascending the neck to reach the brain.
Connecting to the vertebral column are the 12 pairs of ribs that make up the thoracic cage. All 12 pairs attach to the thoracic vertebrae, but the first 7 pairs attach to the sternum, or breastbone, by costal cartilage; they’re called true ribs.
Pairs 8, 9, and 10 attach to the cartilage of the seventh pair, which is why they’re called false ribs. The last two pairs aren’t attached in front at all, so they’re called floating ribs.
The sternum has three parts:
Manubrium: The superior region that articulates with the clavicle, at the clavicular notch, and the first two pairs of ribs located up top, where you can feel a jugular notch in your chest in line with your clavicles, or collarbones.
Body: The middle part of the sternum forms the bulk of the breastbone and has notches on the sides where it articulates with the third through seventh pairs of ribs.
Xiphoid process: The lowest part of the sternum is an attachment point for the diaphragm and some abdominal muscles. (Interesting fact: Emergency medical technicians learn to administer CPR at least three finger widths above the xiphoid.)