The vertebral column is a flexible column, formed by a series of bones called vertebrae. It is part of the axial skeleton and consists of seven cervical, 12 thoracic, five lumbar, five sacral, and four coccygeal vertebrae. Its major function is to enclose and protect the spinal cord and provide structural support to the head and trunk.
The vertebral column—or spinal column—is composed of a series of 33 separate bones known as vertebrae. It is located in the trunk of the body and extends from the base of the skull to the pelvis. It belongs to the axial skeleton, meaning that portion of the skeleton associated with the central nervous system that also includes the bones of the cranium, ribs, and breastbone. The vertebral column consists of seven cervical—or neck—vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae, followed by the sacrum, composed of five fused vertebrae, and by four coccygeal vertebrae which are sometimes fused together and called the coccyx. The coccyx—or tailbone—is the last bone of the vertebral column.
Vertebrae are stacked on top of one another from the first cervical vertebra, called C1 or the atlas, to the sacrum. Only the first 24 vertebrae are considered movable. Both the superior and inferior surfaces of each vertebra are covered by a thin layer of cartilage joined to disk-shaped pads of fibrous cartilage, called intervertebral disks, that cushion the vertebrae and stabilize the vertebral column while allowing it to move. Each disk has a jelly-like core, the nucleus pulposus surrounded by a ring of tough fibrous tissue, the annulus fibrosus. The vertebrae are also bound together by two strong ligaments running the entire length of the vertebral column and by smaller ligaments between each pair of connecting vertebrae. Several groups of muscles are also attached to the vertebrae, providing additional support as well as movement control. The length of the vertebral column depends on the height of the vertebrae and the thickness of the intervertebral disks.
There are four normal curvatures in the vertebral column of the adult that align the head with a straight line through the pelvis. In the region of the chest and sacrum, they curve inwards and each is known as a kyphosis. In the lower back and neck regions, they curve outward and each is known as a lordosis.
All vertebrae have common features. A typical vertebra consists of two parts: an arch that encloses an opening called a vertebral foramen; and a body. Since the vertebrae are all stacked on top of one another, the foramina form the vertebral canal that houses the spinal cord from which the spinal nerves emerge. The body of a vertebra is a round, stocky part on the surface of which the inter-vertebral disk lies and it has two projections, called pedicles, that connect around the foramen to similar bony projections on the arch called facets. Besides enclosing the foramen with its facets, an arch also has three bony spikes, a spinous process located directly opposite the body and two transverse processes on each side of the foramen. These bony elements serve as important sites of attachment for deep back muscles. There are also differences between vertebrae, depending on their location in the column:
The cervical vertebrae. The seven cervical vertebrae are numbered C1 to C7. Together, they make up the bony axis of the neck. Typical cervical vertebrae have large vertebral foramina, and oval-shaped vertebral bodies. They are the smallest vertebrae of the column, but their bone density is higher than that of all the other vertebrae. The transverse processes of the cervical vertebrae are special because they also contain transverse foramina, which are passageways for arteries leading to the brain. The two first cervical vertebrae are special, because they provide a seat for the head. C1 directly supports and balances the skull. It has practically no body and looks like a ring with two transverse processes. On its upper surface, C1 also has two kidney-shaped facets that link it to the skull. The other special cervical vertebra is C2. It forms an axis which bears a tooth-like odontoid process on its body. This bony spike projects upward and lies in the ring of C1. As the head is turned from side to side, C1 thus pivots around the odontoid process of C2.
The thoracic vertebrae. The thoracic vertebrae are numbered T1 to T12 and are located in the chest area. They are larger than the cervical vertebrae. They have round foramina and long, pointed spinous processes that slope downward. Thoracic vertebrae have a unique feature, additional facets on the sides of their bodies that join them with the ribs. Starting with T3 and moving down, their bodies increase in size.
The lumbar vertebrae. The lumbar vertebrae are numbered L1 to L5. They feature large, massive bodies, triangular foramina, and robust spinous and transverse processes. Their facets are oriented so as to favor a wide range of bending flexibility. Lumbar vertebrae also contain small extra bony processes on their bodies that serve as sites of attachment for back muscles.
The sacrum. In the adult, the sacrum consists of five vertebrae that are fused together. It has a characteristically wide body curved upon itself and a triangular foramen. It is shorter and wider in the female than in the male. It links with L5 above and the coccyx below and it articulates on each side with the bones of the pelvis forming the sacroiliac joints with the iliac bones on either side. In addition to its characteristic shape, it contains two additional foramina through which spinal nerves pass.
The coccyx. The tailbone is a small triangular bone consisting of four fused rudimentary vertebrae. The number of coccygeal vertebrae may be five or three. They all lack pedicles and spinous processes, but a primitive body and transverse processes can be recognized in each of the first three vertebrae. The last vertebra is a mere small nodule of bone.
Monique Laberge Ph.D., The Gale Group Inc., Gale, Detroit,