The human spine is made up of 33 bones called vertebrae. However, not all of these bones are the same. The vertebral column is divided into five regions based on the general structure of the spine – cervical, thoracic, lumbar, sacral, and the coccyx. However, the sacrum and coccyx are actually considered part of the pelvis, so the spine has only three curves: cervical, thoracic, and lumbar. The other two regions form somewhat of a tail. The spine forms an S-like shape to help support the upright stance of humans, with cartilage between the vertebrae to absorb the shock from movement. The spine acts like a coiled spring, with the bones in each region containing features that accommodate their location in the body.
Denoted as C1 through C7, with the numbers increasing from the base of the skull downward, the cervical spine contains seven vertebrae that form the neck. C1 is a ring-shaped bone also known as the atlas, and C2 is a peg-like bone also known as the axis. These two bones interlock to hold the skull on top of the spine and allow the head to turn side to side and nod up and down. See a diagram at SpineUniverse. The occipital bone at the back of the cranium is sometimes referred to as C0, or the occiput, because it is the bone that C1 connects with.
Vertebrae farther down the column become larger to support more weight. Following C7, the thoracic curve has 12 interlocking bones, T1 through T12, that act as attachments for the rib cage protecting the internal organs. At T11 and T12 the ribs actually do not attach to the spine but are referred to as floating ribs. The thoracic spine itself is constructed with four facet joints between each vertebrae. The Southern California Orthopedic Institute has information about how the vertebrae are structured and interact.
Constructed similarly to the thoracic spine, the five vertebrae of the lumbar curve are much larger because they carry most of the upper body’s weight. Starting after T12 they are numbered L1 through L5. L5 is connected to the sacrum and moves with it.
Five bones make up the sacrum (S1 through S5 and are fused into one bony mass. This part of the spine is part of the pelvis because it provides a point of attachment for the hips, forming the pelvic girdle.
At the tip of the sacrum is the tailbone, or coccyx, which is made of four fused bones. It is a vestigial part of the anatomy that no longer functions but was once the tail in human’s primitive ancestors and is considered a mammalian trait. The tail is still seen in developing humans in utero during the first trimester, but very few people are born with one.