The spine (also known as the spinal column, vertebral column, or backbone) is a semi-rigid structure consisting of a block-like series of bones, called vertebrae, connected by discs, ligaments, joints, and muscles. The spine is made up of 33 vertebrae and divided into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. In this guide we will look at all five regions, and how the entire spine works.
The normal adult spine has a graceful set of forward and backward curves—one each in the cervical, thoracic, lumbar, and sacral regions. This design is an adaptation to humankind’s ability to walk upright on two legs. This springlike configuration helps cushion and balance the loads a person places on the spine and prevents jarring forces to the brain that would otherwise happen if the spine was straight.
Some of these curves (thoracic and sacral) are present at birth. These are called primary curves. The open part of these curves faces toward the front of the body.
The cervical and lumbar curves develop after birth as secondary curves. The open part of these curves faces toward the back. Secondary curves form when the infant begins to hold up her own head (the cervical curve) and when she begins to sit up and walk (the lumbar curve). These secondary curves are adaptations in the vertebral column to allow the body’s weight to be properly distributed along the spine so that minimal muscular energy is needed to maintain balance.
Before you learn about features characteristic to a particular region of the vertebral column, you should know about the typical parts of most vertebrae.
The bodies of vertebrae are their principal weight-bearing parts. They become increasingly larger from the cervical region to the lumbar region. Between the bodies are intervertebral discs made of fibrous cartilage.
The bodies attach to the vertebral arch, which consists of a pair of pedicles (Latin for “little feet”) that attach to the body, and a pair of laminae that unite with each other in the middle to complete a bony roof. Between the body and vertebral arch is a hole called the vertebral foramen. Collectively, this series of foramina (the plural form of foramen) forms the vertebral canal, which contains and protects the spinal cord and its coverings.
Several bony parts (called processes) are associated with vertebrae. Most of these act as levers through which muscles move individual vertebrae. These processes are the following:
The bodies of cervical vertebrae are small compared to those in regions lower down the spine because they have a relatively small amount of weight to support. However, the vertebral foramen/canal in cervical vertebrae is large and widest in this part of the spine because the spinal cord has its greatest diameter here.
The spinous processes on cervical vertebrae are typically short and forked, except for one on the vertebra called C7, which is quite long. This is given the special name vertebra prominens. This process is the first bump you can feel when you run your finger down the midline of the spine beginning at the back of the skull.
The transverse processes of cervical vertebrae are unique because they have holes in them, called transverse foramina. Vertebral arteries come up from the neck through these holes before entering the base of the skull to supply blood to part of the brain.
Among the cervical vertebrae, two (C1 and C2) are particularly unique; so much so that they have special names.
The first cervical vertebra is called the atlas. The atlas looks like a ring of bone. It lacks a body. But it has front and rear arches that connect to the sides of bony enlargements called lateral masses. These masses contain the transverse foramina and articular surfaces that attach to the next vertebra and the base of the skull. The synovial joints between the atlas and the skull allow a person to nod his head “yes.”
The axis, or second cervical vertebra, has an upward projection from its body, called the dens. The dens serves as the body of the atlas. It is held in place behind its front arch by a thick, stout ligament called the transverse ligament of the atlas. The dens acts as a pivot to allow rotation between the atlas and axis vertebrae, like when a person shakes her head to say “no.”
The thoracic vertebrae have pretty much the same features as a typical vertebra. There are 12 thoracic vertebrae. The bodies of these vertebrae are heart-shaped and larger than cervical vertebrae. They have a set of small, shallow depressions along each upper and lower edge, called costal (rib) demifacets, where the ribs attach. In addition, most transverse processes also have facets for articulation with ribs. These are sites of costovertebral joints. The spinous processes of thoracic vertebrae are long and slender and directed downward. They overlap each other like shingles on a roof.Thoracic vertebrae.
Because more of a person is supported in the lumbar region, it is understandable that everything about these vertebrae is bigger. They have massive bodies. Their spinous processes are rectangular and directed horizontally backward. These do not overlap each other, a detail that doctors take advantage of during a spinal-tap procedure.
The adult sacrum is a single triangular bone. Its broad base articulates with the body of the fifth lumbar vertebra (called L5). Its narrow apex articulates with the coccyx (tailbone). Along each side of the sacrum are articular surfaces for attachment to the hip bone at the sacroiliac joint. The sacrum contains a bony canal, called the sacral canal, which is connected to the vertebral canal above it. Spinal nerves descend into the sacral canal and contain the dural sac full of nerve roots.
On both front and rear surfaces of the sacrum are four pairs of holes for passage of the sacral nerves to peripheral tissues. The sacral canal ends as an opening called the sacral hiatus. The delicate, filament-like end of the spinal cord passes through this hiatus before attaching to the tailbone.
The coccyx is the “tail end” of the spine. It consists of three to five (usually four) small vertebrae that have lost their resemblance to vertebrae. Rather, they appear as segments of solid bone connected to one another by small amounts of fibrocartilaginous tissue. The coccyx is a triangular structure. It is attached at its base to the sacrum by a fibrocartilaginous joint, which permits a limited amount of movement.
There are two types of joints in the vertebral column: symphyseal and synovial. Synovial joints occur at the articular processes and at costal facets where ribs attach.
The bodies of vertebrae are held tightly to one another by intervertebral discs at symphysis joints. These are shock-absorber devices that “give” in response to compressive forces on the spine. You can think of individual discs as a jelly donut. They consist of a tough outer margin of dense, fibrous connective tissue (the anulus fibrosus) and a soft, jelly-like inner core called the nucleus pulposus. Although the extent of the movement between individual vertebrae is small, considerable movement is allowed over the length of the spine.
From The Complete Idiot’s Guide to Anatomy Illustrated by Mark F. Seifert, Ph.D.