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.

The pictures below present two different views of a typical thoracic vertebra found in the ribcage area of the spine. The one on the right is a lateral view looking at the spine from the side. The one further below is looking at a thoracic vertebra from the top down. The block like structure referred to as the body which is considered the anterior portion of the vertebra, and is connected by pedicles to the posterior portion of the vertebra consisting of the laminae, transverse processes and the spinous process furthest in the rear, which you can feel when you run your hand down someone’s back. The boney portion of the spinal canal (the hole) is created by the body in the front, the pedicles on the sides and the lamina in the rear.

In between each segment is a vertebral disc which consists of strong cartilage and a gel like ball in the middle which acts as a pivotal point. The cartilage separates each segment which provides ease of movement, as well as an opening between each segment to allow spinal nerves an exit from the spinal canal when they branch off from the spinal cord. It is this gel that can migrate toward the outer portion of the disc due to injury or age and cause a herniation. The spinal cord travels down through the spinal canal and attaches to the coccyx at it’s filum terminale. Each vertebra has four facets, two on top and two underneath to articulate with the segment above and below. They are kidney shaped to accommodate the vertebra’s range of motion. The thoracic vertebra have two additional facets to accommodate their attachments bilaterally.

There are strong ligaments that attach vertebra to vertebra and keep them in place during movement. The main ligaments are the anterior longitudinal ligament in the anterior (front) of the spine. and the posterior longitudinal ligament runs along the back of the vertebral bodies within the spinal canal. The ligamentum flavum (yellow ligament) connects the lamina of each vertebra and runs from axis (C-2) to the first sacral segment (S-1). There are interspinous ligaments that attach the spinous processes and intertransverse ligaments that attach transverse process to transverse process.

The muscles of the spine are many due to it’s great need for support, as well as a variety of movements at the different levels. Spinal muscles due to their shear number are discussed at different pages throughout the site.

For now suffice it to say that the muscles that hold up the body during upright posture are sometimes called the antigravity or extensor muscles of the spine. They are the main muscles of the spine and are located on the back of the spine. They are called antigravity muscles because they hold us upright against gravity. They are called extensor muscles because they pull the body back into extension against the forces of gravity that want to pull us down into flexion.

For additional information on the spine see scoliosis, thoracic outlet syndrome, upper cervical strain, spondylosis and whiplash.