The human spine is divided into three sections, based on the anatomy and function of spinal vertebrae. The upper seven vertebrae are known as the cervical spine and make up the neck region, which supports the base of the skull. The middle portion, or thoracic spine, comprises the twelve rib-forming vertebrae, which house many of the important internal organs of the chest. The lowest part of the spine consists of the five lumbar vertebrae. Each of the 24 vertebrae are labelled based on the section of the back in which they are located, and their position within that section. The seven cervical vertebrae are thus named C1-C7, with C1 being uppermost, the twelve thoracic vertebrae are T1-T12, and the five lumbar vertebrae take the labels L1-L5.
Spinal vertebrae become progressively larger and heavier from top to bottom as a consequence of how much weight they have to bear. The lumbar vertebrae are therefore the largest and strongest bones in the spinal skeleton. The intervertebral discs (rings of tissue that act as shock absorbers for the spine as well as allowing movement) are also correspondingly large.
Directly beneath the lumbar spine, five vertebrae fuse to form the sacral bone, or sacrum. These are labelled in similar convention to the remainder of the spine as S1-S5. The sacrum supports the much larger pelvic bone, and the hip joints are located on either side of the sacral vertebrae. Because of its position, it is the link between the upper torso and lower limbs of the body.
The sacrum forms a wedge shape that decreases in width from top to bottom and, just as there are gender-based differences in pelvic skeletal anatomy, so the female sacrum is shorter and wider than is found in male subjects. The front of the sacrum is marked by five transverse (horizontal) lines, which delineate the boundaries between the five vertebrae. To the rear is a ridge of bone called the median sacral crest, which runs vertically down the center of the back of the sacrum. This is the result of the fusion of the transverse processes of the sacral vertebrae. Fusion of the sacral bones typically starts to occur at some point in the late teens and is usually completed by age 30.
The lowest end of the spine is completed by a structure known as the coccyx, or tailbone. This is a small bone shaped like an upside-down triangle, which is formed by the fusion of the four coccygeal vertebrae (Co1-Co4) during the third decade of life. In females the coccyx points to the rear, in males to the front.
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