Your back supports the weight of your thorax, arms, and head. But the construction of your spine allows you to bend and twist your body. The spine forms the centerpiece of your back. The spine can bend, flex, and twist because it includes 24 vertebrae. If the spine had a single rigid bone, you would have limited movement in your back. But since your spine has many smaller bones connected through joints, they can move relative to each other. The top seven vertebrae, the cervical vertebrae, sit in the neck. Doctors call the next 12 vertebrae the thoracic vertebrae because these bones form a joint with the 12 ribs of your thorax. The next five vertebrae form the lumbar spine in your lower back. The sacrum and coccyx sit below your lumbar spine. These fused vertebrae connect your spine to your pelvis and form your tailbone.
Each vertebra has a body and wing-shaped processes. When the bodies align, they form a column that supports your body weight. Ligaments and tendons attach to the processes. The ligaments hold the vertebrae together, and the tendons anchor back muscles to the spine. The spinal canal sits between the bodies and the processes. The spinal cord runs through the spinal canal and includes all of the nerves connecting your brain to your body. Discs sit between the vertebrae. Each disc has a fibrous outer annulus that surrounds a gel-like nucleus. The discs cushion the spine and give the vertebrae smooth, hard surfaces to move on. Some of the largest muscles in your body sit in your back. Tendons anchor these muscles to your shoulder blades, spine, skull, ribs, and hips.
