385
CHAPTER ELEVEN
Nervous System II
11.3
VENTRICLES AND
CEREBROSPINAL FLUID
Interconnected cavities called
ventricles
(ven
trı˘-klz) lie in
the cerebral hemispheres and brainstem (
f g. 11.3
and refer-
ence plates 13 and 14). These spaces are continuous with the
central canal of the spinal cord and are F
lled with CS±.
The largest ventricles are the two
lateral ventricles.
The
F rst ventricle is in the left cerebral hemisphere and the sec-
ond ventricle is in the right cerebral hemisphere. They extend
anteriorly and posteriorly into the cerebral hemispheres.
A narrow space that constitutes the
third ventricle
is in the
midline of the brain beneath the corpus callosum, which is a
bridge of axons that links the two cerebral hemispheres. This
ventricle communicates with the lateral ventricles through
openings (
interventricular foramina
) in its anterior end.
The
fourth ventricle
is in the brainstem, just anterior to
the cerebellum. A narrow canal, the
cerebral aqueduct
(aque-
duct of Sylvius), connects it to the third ventricle and passes
lengthwise through the brainstem. This ventricle is continuous
with the central canal of the spinal cord and has openings in
its roof that lead into the subarachnoid space of the meninges.
Tiny, reddish caulifl owerlike masses of specialized cap-
illaries from the pia mater, called
choroid plexuses
(ko
roid
plek
sus-ez), secrete CS±. These structures project into the
cavities of the ventricles
(f g. 11.4)
. A single layer of special-
ized ependymal cells (see chapter 10, p. 361) joined closely by
tight junctions covers the choroid plexuses. In much the same
way that astrocytes provide a barrier between the blood and
The
arachnoid mater
is a thin, weblike membrane that
lacks blood vessels and is located between the dura and pia
maters. It spreads over the brain and spinal cord but gener-
ally does not dip into the grooves and depressions on their
surfaces. Many thin strands extend from its undersurface
and are attached to the pia mater. Between the arachnoid
and pia maters is a
subarachnoid space,
which contains the
clear, watery
cerebrospinal fl
uid
(ser
e
˘-bro-spi
nal fl oo
id),
or
CSF.
The
pia mater
is thin and contains many nerves, as well
as blood vessels that nourish the underlying cells of the brain
and spinal cord. The pia mater is attached to the surfaces
of these organs and follows their irregular contours, passing
over the high areas and dipping into the depressions.
Meningitis
is an inflammation of the meninges. Bacteria or viruses
that infect the cerebrospinal F
uid are typical causes of this condition.
Meningitis may a±
ect the dura mater, but it is more commonly limited
to the arachnoid and pia maters. Meningitis most often a±
ects infants
and children and is serious. Complications include loss of vision, loss
of hearing, paralysis, and mental retardation. It may be fatal.
PRACTICE
1
Describe the meninges.
2
Name the layers of the meninges.
3
Explain the location of cerebrospinal F
uid.
FIGURE 11.2
Meninges of the spinal cord. (
a
) The dura mater ensheaths the spinal cord. (
b
) Tissues forming a protective pad around the cord ²
ll
the epidural space between the dural sheath and the bone of the vertebra.
Spinal cord
Spinal cord
Pia mater
Arachnoid mater
Dura mater
Ventral root
Dorsal root
Dorsal root
Spinal nerve
Spinal
nerve
Dorsal root
ganglion
Dorsal root
ganglion
Subarachnoid
space
Thoracic
vertebra
Body of
vertebra
Epidural
space
Epidural space
(a)
(b)
Dorsal branch
(dorsal ramus)
Ventral branch
(ventral ramus)
Ventral root
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