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S.F. Davis
Demi-facet for
head of rib
articular facet
Facet for
tubercle of rib
Spinous process
Fig. 2.34 A typical thoracic vertebra
The Sacrum and Coccyx
The sacrum is a large triangular shaped bone that
is comprised of five fused sacral vertebrae. The
sacrum fits like a wedge in between the two hipbones of the pelvis. Inferior to the sacrum is the
coccyx or tailbone, which is comprised of four
fused coccygeal vertebrae.
The Spinal Cord
Superior articular
bral canal. As you progress toward L5 the number of nerve roots in the canal diminishes as they
begin to exit the vertebral column (Fig. 2.36).
The lumbar region of the spine is highly mobile
and is responsible for bearing the most compressive load. For this reason, the lumbar region is
also the most vulnerable to injury. Of all of the
lumbar vertebrae, L5 is the most common site of
injury and disease.
Spinous process
Fig. 2.35 A typical lumbar vertebra
Lumbar Vertebrae
The five lumbar vertebrae are the largest in size
and are characterized by the absence of both
transverse foramina and costal facets (Fig. 2.35).
The spinal cord usually ends at vertebral level
L1. At levels caudal to L1, the lumbar and sacral
nerve roots occupy the vertebral canal as they
descend toward their respective vertebral levels
to exit. At the most rostral lumbar segments,
there are more nerve roots occupying the verte-
The spinal cord is part of the central nervous system extending from termination of the medulla
oblongata to approximately the L1 vertebral
level. The function of the spinal cord is to transmit sensory and motor information to and from
the brain and periphery. Like the vertebral column, the spinal cord is divided into cervical, thoracic, lumbar, sacral, and coccygeal levels. The
cervical cord is divided into eight segments even
though there are only seven cervical vertebrae.
There are 12 thoracic segments, 5 lumbar segments, 5 sacral segments, and 1 coccygeal
segment. Each spinal cord segment gives rise to a
pair of spinal nerves for a total of 31 pair of spinal nerves.
The spinal cord is larger in diameter at the cervical and lumbar levels due to the increased number of cell bodies and nerve fibers dedicated to
the innervation of the limbs. These areas are
known as the cervical and lumbar enlargements (Fig. 2.37).
The spinal cord is contained within the dural
sac, often called the thecal sac, and is bathed by
cerebral spinal fluid. The spinal cord is anchored
at its caudal end to the thecal sac by an extension
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