387
CHAPTER ELEVEN
Nervous System II
magnum (see reference plate 15). The cord tapers to a point
and terminates near the intervertebral disc that separates the
F
rst and second lumbar vertebrae
(f
g. 11.5
a
)
.
Structure of the Spinal Cord
The spinal cord consists of thirty-one segments, each of which
gives rise to a pair of
spinal nerves.
These nerves branch to
various body parts and connect them with the CNS.
In the neck region, a thickening in the spinal cord, called
the
cervical enlargement,
supplies nerves to the upper limbs.
A similar thickening in the lower back, the
lumbar enlarge-
ment,
gives off nerves to the lower limbs. Just inferior to the
Chapters 9 and 10 distinguished between the term
nerve fiber,
which is part of a nerve cell, and
muscle fiber,
which refers to the
entire muscle cell. "Nerve F
ber" in the subsequent text is synony-
mous with axon.
11.4
SPINAL CORD
The
spinal cord
is a slender column of nervous tissue that is
continuous with the brain and extends downward through
the vertebral canal. The spinal cord originates where nervous
tissue leaves the cranial cavity at the level of the foramen
Choroid plexuses
of third ventricle
Choroid plexus of
fourth ventricle
Third ventricle
Fourth ventricle
Cerebral aqueduct
Blood-filled
dural sinus
Subarachnoid space
Arachnoid mater
Dura mater
Pia mater
Pia mater
Central canal of spinal cord
Subarachnoid space
Filum terminale
Arachnoid mater
Dura mater
Arachnoid
granulations
FIGURE 11.4
Choroid plexuses in ventricle walls
secrete cerebrospinal ±
uid. The ±
uid circulates
through the ventricles and central canal, enters
the subarachnoid space, and is reabsorbed into
the blood of the dural sinuses through arachnoid
granulations. (Spinal nerves are not shown.)
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