Axial Skeleton

Structure and Function of the Vertebral Column

The vertebral column consists of 33 vertebrae in five different regions of the spine: cervical, thoracic, lumbar, sacrum, and coccyx.

The vertebral column, also known as the spine or spinal column, is the portion of the axial skeleton that supports both the skull and the trunk of the body. It extends from the base of the skull downward past its connection with the pelvis. In addition to providing support, the vertebral column makes all body movement possible, it accommodates the stress associated with that motion, and, probably most important, it protects the spinal cord. The vertebral column consists of a continuous chain of 33 vertebrae, as well as 23 intervertebral discs. An intervertebral disc is a flattened structure that sits between and cushions vertebrae. Discs are composed of a fibrocartilage ring, the annulus fibrosus, surrounding a gelatinous center, the nucleus pulposus. The length of an adult vertebral column, including vertebrae and intervertebral discs, is typically about 71 cm (28 in.).

The spinal column is grouped into five different regions, each with a specific number of vertebrae. Vertebrae in each region are numbered sequentially from the top to the bottom of that region. The cervical region at the top of the spine includes the seven vertebrae in the neck; these are numbered C1–C7. Beneath the cervical region is the thoracic region which is the region of the spine in the chest that consists of 12 vertebrae, numbered T1–T12. The spine's lumbar region consists of five vertebrae in the lower back; these are numbered L1–L5. The five sacral vertebrae at the base of the spine, numbered S1–S5, follow the lumbar vertebrae. Finally, four tiny, unnumbered coccygeal vertebrae make up the coccyx, or vestigial tail of the spine. The total number of vertebrae is 33: 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused vertebrae making up the sacrum, and 4 fused vertebrae making up the coccyx. The sacrum is the region of the vertebral column that lies between the hip bones and is made up of fused vertebrae.

Five Regions of the Vertebral Column

The 33 vertebrae that make up the vertebral, or spinal, column are grouped into five regions.

Anatomy of the Vertebral Column

At birth, the vertebral column is C-shaped and later in life takes on an S-shape, where scoliosis, lordosis, and kyphosis could result in abnormal curvatures of the spine.

The term column as used to describe the spine is misleading, because the vertebral column is actually curved rather than straight. In adults the spine has four major curves that make up an S-shape. Two of these curves originated with the developing embryo's continuous C-shaped curve in the uterus. A newborn's vertebral column retains this C-shape until the infant can raise its head. At that point a curve develops in the cervical region. Later, when the infant can sit up and then begins walking, a curve forms in the lumbar region. Altogether the curves in an adult spine take on this alternating form, with curves described in the anterior direction: a convex curve in the cervical region, a concave curve in the thoracic region, a convex lumbar curve, and a concave curve in the combined sacral and coccygeal (pelvic) region. This S-shape in the human spine enables walking upright on two legs (bipedal locomotion).

Abnormal curvatures of the spine disrupt this S-shape. Scoliosis, is a lateral, or sideways, bend in the vertebral column, usually in the thoracic region. Causes of scoliosis are unclear but are believed to include inherited factors. Lordosis is an abnormal, pronounced curvature of the vertebral column's lumbar region. Known as swayback, lordosis can be caused by osteoporosis or other spinal ailments, excessive abdominal weight, or participation in activities that load weight on the spine. Kyphosis is an abnormal spinal curvature that takes the form of an exaggerated concave bend in the vertebral column's thoracic region. This causes a humpback shape that is most often attributed to osteoporosis, as well as to some of the other causes of lordosis.

Abnormal Curvatures of the Vertebral Column

The normal S-shaped curve of the vertebral column is sometimes disrupted to produce abnormal spinal curvatures. The most common of these are scoliosis, lordosis, and kyphosis, all of which affect posture and mobility.

Anatomy of the Vertebrae, Sacrum, and Coccyx

Most vertebrae have a body, vertebral arch, and superior and inferior articular processes and some fuse together to form the sacrum and coccyx.
Vertebrae from different spinal regions exhibit both similarities and differences. These differences reflect the functions of vertebrae in each region. Vertebrae typically contain a body, a prominent bony disc that bears weight. Intervertebral discs attach to the body. A vertebral arch projects to the rear of each body. It consists of two parts, a thick pedicle, which is a structure that connects one bone to another, and a flatter lamina, a thin plate on each side of the pedicle. Together the structures form the vertebral foramen, a triangular opening in each vertebra that provides the passageway for the spinal cord in the vertebral column. Spine muscles attach to processes protruding from each vertebral arch. Transverse processes project sideways. Spinous processes project backward and downward; these make up the row of bumps that can be felt and seen down the back. Two other sets of processes on each vertebra allow articulation between vertebrae. Paired superior articular processes project upward, meeting and interlocking with paired inferior articular processes that point downward.

Vertebral Foramen

Each vertebra includes a vertebral foramen, an opening that the vertebra surrounds. The spinal cord passes through each vertebral foramen so that it is protected within the connected vertebral column.
Cervical vertebrae are the smallest vertebrae outside of the coccyx. The first cervical vertebra is called the atlas, and the second is the axis. These two vertebrae are highly modified to support and provide rotation for the head. The atlas includes a wide lateral mass on each side containing concave regions that articulate with the skull's occipital condyles, which are each of two large, convex processes that allow attachment of the spine to the skull by articulating with the first cervical vertebra of the spine. The axis carries a unique structure, a knob called the dens, that contributes to complex articulation with the atlas. The atlas allows nodding "yes," and the axis allows rotating the head to indicate "no." Cervical vertebrae are the only ones to have a transverse foramen in each transverse process. Together these openings permit passage of vertebral arteries. Thoracic vertebrae possess long, thin, downward-pointing spinous processes, as well as long transverse processes. On the T1–T10 vertebrae, these processes include facets, or hinge-like joints, and the superior articular process of the subsequent vertebrae, which are known as costal facets and articulate with the ribs. Lumbar vertebrae are large, with thick bodies and heavy processes. They possess facets facing in two directions, providing articulation that strengthens the lower vertebral column.

Comparing Types of Vertebrae

Vertebrae along the spine share many structures, but they are shaped differently depending on the function of each spinal region. Vertebrae also vary in size across regions and possess structures unique to each region.
The five sacral vertebrae that are fused in adults make up the sacrum, a single bony plate. Fused transverse processes form ear-shaped lateral surfaces on the sacrum, the auricular surfaces, which connect the sacrum to pelvic bones. The sacral spinous processes of vertebrae S1–S4 are partially fused, creating the median sacral crest. No spinous process forms on the fifth sacral vertebra, forming an opening called the sacral hiatus, a common site for anesthetic injections. The sacral promontory, a bulging structure that separates the abdominal and pelvic cavities, projects from the first sacral vertebra.

The triangular coccyx, or tailbone, consists of three to five tiny fused vertebrae. Coccygeal vertebrae lack vertebral foramina and developed processes.

Anatomy of the Thoracic Cavity

The thoracic cavity protects the heart and lungs and is made up of the thoracic vertebrae, 12 pairs of ribs, and the sternum.

The thoracic cavity, is the area of a human's body from the neck to the abdomen, including the area within the ribs, breastbone, and dorsal vertebrae and containing the heart, lungs, and associated blood vessels. The name is descriptive because the bones involved create a cage or chamber that surrounds the vulnerable organs. The structure is semirigid, meaning that respiratory muscles cause its volume to regularly expand and contract as air is inhaled and exhaled.

The thoracic cavity is made up of the thoracic vertebrae, 12 pairs of ribs, and the sternum. The sternum, also called the breastbone, is a sword-shaped bony plate located in front of the heart. It divides into three regions from top to bottom: the broad manubrium, the long body, and the narrow xiphoid process. The notch at the top of the manubrium, the jugular notch, can be felt just between the clavicles. The body of the sternum meets the manubrium at the sternal angle, felt as a ridge at the most forward part of the sternum. The small xiphoid process below the body is the attachment site for some abdominal muscles.

Each of the 12 pairs of ribs attaches at its back (posterior) point to the thoracic vertebrae of the spine. In the front of the thoracic cavity, thin costal cartilage connects the first seven ribs directly to the sternum. These are designated as true ribs. Ribs 8–12 are termed false ribs because they lack this direct attachment to the sternum. Ribs 8–10 are connected to rib 7's costal cartilage. Ribs 11–12 do not connect to the sternum at all and are thus called the "floating ribs." Ribs also vary somewhat in structure. All ribs have a head that meets and articulates with the vertebral column. Ribs 1 to 10 each have a proximal head and tubercle (a small rounded projection or protuberance, especially on a bone connected by a narrow neck; ribs 11 and 12 have a head only. The tubercles form another point of articulation with the vertebral column. The first rib is flat and horizontal. Extending from the tubercle, ribs 2–10 make a sharp bend appropriately called the angle and then flatten into blades called shafts. Costal grooves in rib shafts form pathways for nerves and blood vessels. Ribs 1–10 articulate both with and between thoracic vertebrae. Because ribs 11–12 lack tubercles, they articulate only with vertebrae.

Structures of the Thoracic Cavity

In the thoracic cavity, 12 pairs of ribs attached to the vertebral column encircle the center of the body, protecting the heart, lungs, and other organs. Structures of the thoracic cage give the region strength and flexibility.