Chapter 12
The Central Nervous System
459
12
In several places, the meningeal dura mater extends inward
to form flat partitions that subdivide the cranial cavity. Tese
dural septa
, which limit excessive movement of the brain within
the cranium, include the following (Figure 12.23a):
Falx cerebri
(falks ser
9
ĕ-bri). A large sickle-shaped (
falx
5
sickle) fold that dips into the longitudinal fissure between the
cerebral hemispheres. Anteriorly, it attaches to the crista galli
of the ethmoid bone.
Falx cerebelli
(ser
0
ĕ-bel
9
i). Continuing inferiorly from the
posterior falx cerebri, this small midline partition runs along
the vermis of the cerebellum.
Tentorium cerebelli
(ten-to
9
re-um; “tent”). Resembling a
tent over the cerebellum, this nearly horizontal dural fold ex-
tends into the transverse fissure between the cerebral hemi-
spheres (which it helps to support) and the cerebellum.
Arachnoid Mater
Te middle meninx, the
arachnoid
mater
(ah-rak
9
noid), forms
a loose brain covering, never dipping into the sulci at the cerebral
surface. It is separated from the dura mater by a narrow serous
cavity, the
subdural space
, which contains a film of fluid. Beneath
the arachnoid membrane is the wide
subarachnoid space
. Spider-
web-like extensions span this space and secure the arachnoid mater
to the underlying pia mater (
arachnida
means “spider”). Te sub-
arachnoid space is filled with cerebrospinal fluid and also contains
the largest blood vessels serving the brain. Because the arachnoid
mater is fine and elastic, these blood vessels are poorly protected.
Knoblike projections of the arachnoid mater called
arach-
noid villi
(vil
9
i) protrude superiorly through the dura mater
and into the superior sagittal sinus (see Figure 12.22). Tese villi
absorb cerebrospinal fluid into the venous blood of the sinus.
(cerebrospinal fluid). Furthermore, the blood brain barrier pro-
tects the brain from harmful substances in the blood. We de-
scribed the cranium, the brain’s bony encasement, in Chapter 7.
Here we will consider the other protective elements.
Meninges
Te
meninges
(mĕ-nin
9
jēz;
mening
5
membrane) are three
connective tissue membranes that lie just external to the CNS
organs. Te meninges:
Cover and protect the CNS
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid
Form partitions in the skull
From external to internal, the meninges (singular:
meninx
) are
the dura mater, arachnoid mater, and pia mater
(Figure 12.22)
.
Dura Mater
Te leathery
dura mater
(du
9
rah ma
9
ter), meaning “tough
mother,” is the strongest meninx. Where it surrounds the brain,
it is a two-layered sheet of fibrous connective tissue. Te more
superficial
periosteal layer
attaches to the inner surface of the
skull (the periosteum). (Tere is no dural periosteal layer sur-
rounding the spinal cord.) Te deeper
meningeal layer
forms
the true external covering of the brain and continues caudally in
the vertebral canal as the spinal dura mater. Te brain’s two du-
ral layers are fused together except in certain areas, where they
separate to enclose
dural venous sinuses
that collect venous
blood from the brain and direct it into the internal jugular veins
of the neck
(Figure 12.23)
.
Superior sagittal sinus
Skin of scalp
Periosteum
Falx cerebri
(in longitudinal
fissure only)
Blood vessel
Arachnoid villus
Pia mater
Arachnoid mater
Dura mater
• Meningeal la
yer
Periosteal layer
Subdural space
Subarachnoid space
Bone of skull
Figure 12.22
Meninges: dura mater, arachnoid mater, and pia mater.
The meningeal
dura forms the falx cerebri fold. A dural sinus, the superior sagittal sinus, is enclosed by the
dural membranes superiorly. Arachnoid villi return cerebrospinal fluid to the dural sinus. (Frontal
section.)
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