By Elizabeth A. Murray, Mount St. Joseph University
The axial skeleton consists of the skull, the hyoid bone, the vertebrae, the ribs, and the sternum. But of all those elements, the most movement occurs within the spine. The spinal column is the axis of our major body movements of the trunk. It is a flexible strut off of which the limbs extend, and it is a major supportive element for our posture, whether sitting or standing.
The Five Regions of the Spine
The segmental nature of the spine can be seen in the repeating series of 24 vertebrae, along with the sacrum and coccyx. The stacking of these elements provides both the support and the flexibility needed for posture and movement.
The vertebral column is broken down into five main regions: a series of seven vertebrae in the neck called cervical vertebrae, 12 vertebrae in the rib cage known as our thoracic vertebrae, five spinal elements in the low back called lumbar vertebrae, plus the sacrum and the coccyx.
The sacrum is usually formed from five sacral vertebrae that unite during development into a single triangular bone, typically when you’re in your teens.
The coccyx fuses from three or four small elements in humans. In four-legged animals that have a tail, a series of caudal vertebrae extends from the sacrum into the tail. Humans only have a tail when we’re embryos, and those few coccygeal vertebrae are the vestigial remnant.
The Vertebral Foramen
Let’s first look at a single typical vertebra for its major bony landmarks, and then compare the similarities and differences among the major types of vertebrae. Nearly all vertebrae have a central region known as the vertebral body. Connected to that is a neural arch composed of two main parts: a pair of pedicles, which are stalklike extensions that attach the neural arch to the vertebral body, and a pair of laminae, which complete the neural arch by meeting each other posteriorly.
This arch surrounds an open region called the vertebral foramen, in which the spinal cord and its nerve roots reside. Above each pedicle is a superior articular process, and below each pedicle is an inferior articular process.
Remember that an articulation is a joint, so we will see these facets are part of the anatomy that links the bones of the spine together. Laterally off the neural arch are a pair of transverse processes, since they go in a transverse orientation, and posteriorly there’s a spinous process that extends off of the posterior aspect of the laminae. These processes—like many projections off of bones—serve as attachment sites for ligaments and muscles.
Different Types of Vertebrae
Let’s first understand the shorthand that is used when discussing the vertebrae. For each, the numbering starts at the top of each section and proceeds inferiorly. So, the seven cervical vertebrae are typically referred to as C1, C2, C3, C4, C5, C6, and C7, with C1 closest to the skull. Below C7 begin the 12 thoracic vertebrae, called T1, T2, and so on, down to T12. Inferior to this are the five lumbar vertebrae—referred to as L1, L2, L3, L4, and L5—followed by the sacrum and coccyx.
Fulfilling the convention that bone names are often the basis for naming structures around them—such as nerves and vessels—the spinal nerves that exit between adjacent vertebrae through the intervertebral foramina are also named and numbered in a similar way as their corresponding vertebrae.
Cervical vertebrae are the smallest of the three types of vertebrae; they have a more petite vertebral body, and those bodies have a concave superior surface. Cervical vertebrae also typically have two prongs to their spinous processes, called a bifid spinous process.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Thoracic and Lumbar Vertebrae
But, what about the thoracic vertebrae—what puts them into their own group? Each thoracic vertebra has several articulation sites for a pair of ribs. These are sometimes called costal facets because in medical terminology, costo means “rib”. There are 12 thoracic vertebrae, and each articulates with a pair of ribs.
So, we have a total of 24 ribs, assuming we’re in the 70% club that follow the anatomical rules for a given feature. In addition, the bodies of thoracic vertebrae go from a relatively small size following the cervical spine to a relatively large size as they approach the lumbar region. In fact, the size of the bodies of all vertebrae increase as you go from superior to inferior in the spine, at least above the sacrum, to help support the increasing amount of weight placed on each one.
The five lumbar vertebrae have large and boxy bodies, smaller transverse processes, but larger spinous processes. Their superior and inferior articular processes are also larger. They lack transverse foramina and articulation sites for ribs, since those are only found on cervical and thoracic vertebrae, respectively. The lumbar vertebrae really don’t have a specific feature of their own, other than their large size.
Sacrum and Coccyx
The sacrum is quite different than the rest of the spine. It is a triangular bone, typically made up of five former vertebrae. The sacrum has a body that is topped by an intervertebral disc that forms a cartilaginous amphiarthrotic joint with the inferior surface of the body of the fifth lumbar vertebra, L5. The sacrum also has a set of superior articular facets that join with the inferior articular facets of L5. So, in that sense, the sacrum is like other vertebrae.
The sacral canal terminates in an opening called the sacral hiatus that can be palpated on the surface and used as a bony landmark for the administration of an epidural anesthetic. On the superior and anterior aspect of the sacrum is a forward projection called the sacral promontory—and it is an important landmark in obstetrics. How far it projects anteriorly into the pelvis can determine whether a woman will likely be capable of a vaginal delivery or may need a cesarean section.
Lastly, the coccyx is the small tailbone that articulates with the sacrum; it is usually composed of three to four fused coccygeal vertebrae. So, while the spinal column starts with 33 vertebrae that ultimately fuse into 26 bones—seven cervical, 12 thoracic, and five lumbar vertebrae, plus a single sacrum and single coccyx—there are only about 23 or 24 total intervertebral discs.
Common Questions about the Different Sections of the Spinal Column
The spine consists of five regions: seven cervical vertebrae, 12 thoracic vertebrae, five lumbar vertebrae, sacrum, and coccyx.
It’s a central region in nearly all vertebrae on the spine. There is also a neural arch composed of a pair of pedicles connected to the vertebral body, as well as a pair of laminae
There are seven cervical vertebrae within the spine, which are referred to as C1, C2, C3, C4, C5, C6, and C7. Below the C7 come the 12 thoracic and then the five lumbar vertebrae.
