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Spine Overview
The atlas is the first of the seven cervical vertebrae, and is called such because it bears the direct weight of the skull, just as the mythical Greek hero Atlas bore the world on his shoulders. The atlas vertebra meets with the occipital condyles which flank the foramen magnum in the basilar part of the occipital bone of the skull. This junction forms the atlanto-occipital joint, and is responsible for the primary articulation between the spine and the skull. It is the only vertebra in the spine which has no vertebral body. The atlas vertebra, in turn rests upon the axis vertebra, which is the second of the cervical vertebra in the spine, with the articulation between these two vertebra occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding structure on the axis called a dens.
The axis is the second of the seven cervical vertebrae, and is called such because it allows axial (rotational) movement of the skull. The axis lies directly beneath the atlas vertebra, their junction occurring at lateral articular surfaces and an unique juncture between a concave facet (on the atlas) and an upward-protruding dens (on the axis). This articulation is regulated by the alar ligament, which attaches to both atlas and axis.
The cervical vertebrae are the first (upper) seven in the vertebral column. The first cervical vertebra is the atlas, so called because it directly bears the weight of the skull. The second cervical vertebra is called the axis, because it admits the rotation of the skull by allowing the atlas to pivot upon it. The other five cervical vertebrae have no names, but are called by their number (i.e., third cervical vertebra). Each of the cervical vertebra features a body (anterior, or frontal, portion) and an arch (posterior, or rear, portion). The body of each vertebra in the column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord. Every cervical vertebra has a foramen (opening) in each of its transverse processes (lateral protrusions). The arch of the vertebra features a small knob or prominence, called an anterior tubercle. The anterior tubercles on the sixth cervical vertebra are particularly large and are known as the carotid tubercles.
The coccyx (or "tailbone") is composed of from three to five rudimentary vertebrae. Often, the first of these coccygeal vertebrae is separate, while the remainder are fused together. The articulation between the coccygeal vertebrae and the sacrum allow some flexibility in the coccyx, which is particularly beneficial in taking the stresses of sitting and falling. The coccyx is extremely susceptible to shock fracture, as might be induced from a fall. Furthermore, since a number of nerve pathways pass near this area, damage to the coccyx threatens damage to the nerves of the lower body. The juncture of the first coccygeal vertebra with the sacrum occurs at the lower facet of the sacrum.
Cartilage disks are located between the vertebrae, and serve to cushion the spinal column from shock. Each disk features an inner, pulpy center, called the nucleus pulposus, and a fibrous outer ring, called the annulus fibrosus, which is visible in a lateral view of the spine. These intervertebral disks are easily torn or dislocated when the vertebra column is subjected to inordinate stresses, such as those encountered in lifting a heavy load improperly, or twisting the back sharply, as occurs in many sporting injuries. Such a "slipped" disk is only one of many causes of back pain. Others include arthritis, spinal meningitis, and inflammation of a tendon or muscle. Such back problems can cause extreme pain, which may be increased by changes in the weather or poor diet, making lifting, walking, and sitting an excruciating ordeal. Since so many muscles place stress on the spine when they operate, simple activities such as going to the bathroom, coughing, laughing, and even breathing may be intolerable with such a condition. More recently, however, medical advances have enabled a number of these ailments to be alleviated and chiropracty, arthroscopic surgery, and other forms of physical therapy make much suffering unnecessary.
The lumbar vertebrae are the five vertebrae which are below the thoracic vertebrae and above the fused vertebrae of the sacrum. The lumbar vertebrae feature no facets on the body or transverse processes (as the thoracic vertebrae have) and the bodies of the lumbar vertebrae are much larger than those of the cervical or thoracic vertebrae. The vertebral foramen is usually triangular, while the spinous process points backward and is rectangular or hatchet-shaped. The transverse processes of the lumbar vertebrae (which also represent their rib elements) lack the foramina which characterize the cervical vertebrae. The large body of each lumbar vertebra bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.
The sacrum is the portion of the vertebral column between the lumbar vertebrae and the structures of the coccyx. It is composed of five vertebrae which are fused together to form a single bone structure. The sacrum features a median crest (running down the back, or posterior of the sacrum) which is made of the fused spinous processes of its component vertebrae. Beneath this crest is the sacral canal, a tunnel which runs lengthwise from the top of the sacrum to a hiatus (opening) near the bottom. Four pairs of holes (sacral foramina) pierce the sacrum, flanking the medial (center) line, where the intermediate sacral crests are formed by the fused articular processes of the component vertebrae. To the outside of the intermediate sacral crests are the lateral crests, formed by the fused transverse processes of the component vertebrae. In the sacrum, therefore, unlike the upper vertebrae in the spine, the intertransverse ligaments have been replaced by fusion of these processes together. The crests are not represented on the front (pelvic) surface of the sacrum, though the sacral foramina are evident.
The thoracic vertebrae are the middle twelve in the vertebral column. Most of the thoracic vertebrae feature costal (relating to the ribs) facets on the body and transverse processes of the vertebra, no foramina in the transverse processes (as the cervical vertebra have), a spinous process which points back and down, and a round vertebral foramen. The body of each thoracic vertebra in the spinal column bears the weight of the vertebrae above it (and the skull), while the arch serves to create a canal-like area along the spine to house and protect the spinal cord.
Most vertebrae exhibit pronounced lateral protrusions (or processes), one on each side of the vertebra. These transverse processes serve as the attachment sites for ligaments (intertransverse ligaments) and muscles, which control the bending and twisting of the vertebral column. The base of each transverse process in most vertebrae is just off of the main body of the vertebra, located instead at the pedicle. The pedicle is part of the ring-like structure of a vertebra, which also includes the body and lamina of a vertebra, forming the vertebral foramen which protects the spinal cord.
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