Spinal cord lies loosely in the vertebral canal. It extends from foramen magnum where it is continuous with medulla oblongata, above and up to the lower border of first lumbar vertebra below.
Coverings
Spinal cord is covered by
sheaths called meninges, which are membranous in
nature. Meninges are dura
mater, pia
mater and arachnoid mater. These coverings continue as coverings of brain. Meninges are responsible for protection and nourishment of the nervous tissues.
Shape and Length
Spinal cord is
cylindrical in shape. Length of the spinal cord is
about 45 cm in males and about 43 cm in females.
Enlargements
Spinal cord has two spindle-shaped
swellings, namely cervical and lumbar enlargements. These two
portions of spinal cord innervate upper and lower extremities respectively.
Conus Medullaris and
Filum Terminale
Below the lumbar enlargement,
spinal cord rapidly narrows to a cone-shaped termination called conus medullaris.
A slender non-nervous filament called filum terminale extends from conus medullaris downward to the
fundus of the dural sac at the level of second sacral vertebra.
Segments
Spinal cord is made up of
31 segments. In fact, spinal cord is a continuous structure. Appearance of the segment is by nerves arising from spinal cord, which are called spinal nerve.
Spinal Nerves
Segments of spinal cord
correspond to 31 pairs of spinal nerves in a
symmetrical manne
Nerve Roots
Each spinal nerve is
formed by an anterior (ventral) root and a posterior
(dorsal) root. Both the rootsr either
side leave the spinal cord and pass through the corresponding intervertebral
foramina. The
first
cervical spinal nerves pass through a foramen between occipital bone and first
vertebra, which is
called atlas. Cervical and thoracic roots are shorter whereas, the lumbar and
sacral roots are longer. Long nerves descend in dural sac to reach their
respective intervertebral foramina. This bundle of descending roots
surrounding the filum terminale resembles the tail of horse. Hence, it is
called cauda equina.
Fissure and Sulci
On the anterior surface of
spinal cord, there is a deep furrow known as anterior
median fissure. Depth of this fissure is about 3 mm.
Lateral to the anterior median fissure on either
side, there is a slight depression called the anterolateral
sulcus. It denotes the exit of anterior nerve
root. On the posterior aspect, there is a depression called posterior
median sulcus. This sulcus is continuous with a thin glial partition called the posterior
median septum. It extends inside the spinal cord for about 5
mm and reaches the gray matter. On either side, lateral to posterior median
sulcus, there is posterior intermediate sulcus. It is continuous with posterior
intermediate septum, which extends for about 3 mm into the spinal
cord. Lateral to the posterior intermediate sulcus, is the posterolateral
sulcus. This denotes the entry of posterior nerve
root.
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