Lumbar Vertebrae

The spine is made up of three groups of bones called "vertebrae." There are five "lumbar vertebrae" in the small of the back (loins). Since the lumbars must support more weight than the vertebrae above them, they have developed larger and stronger bodies. The transverse processes of these vertebrae project backward at sharp angles, while their short, thick spinous processes are directed nearly horizontally.

Thoracic Vertebrae

The spine is made up of three groups of bones called "vertebrae." The twelve in the chest area are the "thoracic" vertebrae, which are larger than those in the cervical region. They have long, pointed spinous processes that slope downward, and have facets on the sides of their bodies that join with ribs. Beginning with the third thoracic vertebra and moving down, the bodies of these bones increase in size. This reflects the stress placed on them by the increasing amounts of body weight they bear.

A Vertebra

A typical vertebra has a drum-shaped "body" (centrum) that forms a thick, anterior portion of the bone. A longitudinal row of the bodies supports the weight of the head and trunk. The intervertebral disks, which separate joining vertebrae, are fastened to the roughened upper and lower surfaces of the bodies. These disks cushion and soften the forces created by walking and jumping, which might otherwise fracture the vertebrae or jar the brain. Each intervertebral disk is composed of a band of fibrous fibrocartilage (anulus fibrosus) that surrounds a gelatinous core, called the "nucleus pulposus." The bodies of adjacent vertebrae are joined on the front surfaces by "anterior ligaments" and on the back by "posterior ligaments." Projecting from the back of each body are two short stalks called "pedicles." They form the sides of the "vertebral foramen." Two plates (laminae) arise from the pedicles and fuse in the back to become "spinous process." The pedicles, laminae, and spinous process together complete a bony vertebral arch around the vertebral opening, through which the spinal cord passes. Between the pedicles and laminae of a typical vertebra is a "transverse process" that projects laterally and toward the back. Various ligaments and muscles are attached to the spinal process and the transverse process. Projecting upward and downward from each vertebral arch are "superior" and "inferior articulating processes." These processes bear cartilage-covered facets by which each vertebra is joined to the one above and the one below it. On the surfaces of the vertebral pedicles are notches that align to create openings, called "intervertebral foramina." These openings provide passageways for spinal nerves that proceed between joining vertebrae and connect to the spinal cord.

Cervical Vertebrae

The spine is made up of three groups of bones called "vertebrae." The seven "cervical vertebrae" comprise the bony axis of the neck. Although these are the smallest of the vertebrae, their bone tissues are denser than those in any other region of the column. The transverse processes of the cervical vertebrae are distinctive because they have "transverse foramina", which serve as passageways for arteries leading to the brain. Also, the "spinous processes" of the second through the fifth cervical vertebrae are uniquely forked. These processes provide attachments for various muscles. Two of the cervical vertebrae are of special interest. The first vertebra ("atlas") supports and balances the head. It has practically no body or spine and appears as a bony ring with two transverse processes. On its upper surface, the atlas has two kidney-shaped facets that unite with the occipital condyles of the skull. The second vertebra is the "axis," which bears a tooth-like "odontoid process" on its body. This process projects upward and lies in the ring of the atlas. As the head is turned from side to side, the atlas pivots around the odontoid process.

The Coccyx

The coccyx (or tail) is the lowest part of the vertebral column and is attached by ligaments to the margins of the sacral hiatus. When a person is sitting, pressure is exerted on the coccyx, and it moves forward, acting sort of like a shock absorber. Sitting down with too great a force may cause the coccyx to be fractured or dislocated.

Sacrum

The sacrum is a large triangular bone at the base of the lower spine. Its broad upper part joins the lowest lumbar vertebrae and its narrow lower part joins the coccyx or "tail bone". The sides are connected to the iliums (the largest bones forming the pelvis). The sacrum is a strong bone and rarely fractures. The five vertebrae that make up the sacrum are separated in early life, but gradually become fused together between the eighteenth and thirtieth years. The spinous processes of these fused bones are represented by a ridge of tubercles. The sacrum is wedged between the coxal bones of the pelvis and is united to them by fibrocartilage at the sacroiliac joints. The weight of the body is transmitted to the legs through the pelvic girdle at these joints.