The anatomy and design of the human spine is unique in the animal world and it is due mostly to upright posture. For one, humans have more longitudinal curves. For another we have retained tail segments and those tail segments curve under not up. The most unique difference about us, though, is our ability to carry our skull with the greatest of ease balanced atop an almost perfectly upright structure.
It is because of this uniqueness in design to accommodate upright posture that we have paid a price. In certain ways we have become more vulnerable, more fragile. In other ways we are more complex and prone to extremely involved diseases. This section will discuss some of the unique differences in the centerpiece of our frames relative to our vertebrate relatives, as well as diseases that humans can develop as a result of it.
The human spinal column consists of 24 movable segments called vertebra, and the lower end is the sacrum which has 5 additional vertebra that are most typically fused. Fusion of the sacrum’s segments usually occurs in the late teens and is totally fused by age 25. On occasion the first segment of the sacrum remains unfused or the last vertebra, the 24th more commonly known as L5, is fused to the sacrum. Both of these conditions can cause problems. The last section known as the coccyx or tailbone generally has three to five fused segments, although the jury is still out on how many fuse and at what age.
The first 24 segments are divided into three sections according to their differing characteristics. The cervical section, which has 7 vertebrae, the thoracic section, which has 12 vertebrae and the lumbar section, which has 5 vertebrae. All three sections are shaped differently according to their specific function, movement, muscle and boney attachments. Basically speaking, however, all the vertebrae have similar components.
The cervical spine has two of the most unique vertebra in the entire spinal column. The first is C1, called atlas, which connects the spine to the base of the skull. The second cervical vertebra, C2, which is beneath the atlas is called the axis vertebra.
The thoracic spine is unique in that it attaches to the ribs bilaterally to support the ribcage or chest cavity. The ribcage stiffens the thoracic section of the spinal column and limits its range of motion compared to the neck and low back.
The lumbar spine attaches to and articulates with the last segment of the thoracic section and the first segment of the sacrum. Because the lumbar vertebra bear the weight of the entire spinal column above it, they have the largest vertebral bodies. Although the segments have slightly greater ranges of motion than the thoracic section the lumbar section has less than half the segments of the thoracic section, which limits its range or motion.
The sacrum and coccyx are part of the lowest portion of the spinal column but are also an integral part of the pelvis. The pelvis has the most limited range of motion in the spinal column. In some anatomical textbooks the sacroiliac joints of the pelvis are considered to be syndesmodial joints, which means almost fused and non-moveable. Despite it nearly non-movable joints, chiropractors have long recognized that pelvic misalignments, even slight ones, can cause major musculoskeletal and other health problems. The human pelvis is a complex topic and will be covered separately on this website.
The spinal column has anterior to posterior curves (front to back) the purpose of which is to reduce the load due to the force of gravity on upright posture. Upright posture and the serpentine curves of the spinal column makes humans unique in the mammal world. The cervical section of the spinal column has a lordotic curve meaning it curves toward the front of the body. The thoracic section has a kyphotic curve, curving toward the back of the body, the lumbar curve is like the cervical curve, that is lordotic and the sacrum and coccyx curve toward the back like the thoracic section.