384
UNIT THREE
In some regions, the dura mater extends inward between
lobes of the brain and forms supportive and protective parti-
tions
(table 11.1)
. In other areas, the dura mater splits into
two layers, forming channels called
dural sinuses,
shown in
F gure 11.1
b
. Venous blood fl
ows through these channels as
it returns from the brain to vessels leading to the heart.
The dura mater continues into the vertebral canal as a
strong, tubular sheath that surrounds the spinal cord. It is
attached to the cord at regular intervals by a band of pia mater
(denticulate ligaments) that extends the length of the spinal
cord on either side. The dural sheath terminates as a blind sac
at the level of the second sacral vertebra, below the end of the
spinal cord. The sheath around the spinal cord is not attached
directly to the vertebrae but is separated by an
epidural space,
which lies between the dural sheath and the bony walls
(f g.
11.2)
. This space contains blood vessels, loose connective tis-
sue, and adipose tissue that pad the spinal cord.
A blow to the head may rupture some blood vessels associated with
the brain, and the escaping blood may collect beneath the dura
mater. This condition, called
subdural hematoma,
can increase pres-
sure between the rigid bones of the skull and the soft tissues of the
brain. Unless the accumulating blood is promptly evacuated, com-
pression of the brain may lead to functional losses or even death.
11.1
INTRODUCTION
The central nervous system (CNS) consists of the brain and
the spinal cord. The
brain
is the largest and most complex part
of the nervous system. It oversees many aspects of physiology,
such as sensation and perception, movement, and thinking.
The brain includes the two cerebral hemispheres, the dien-
cephalon, the brainstem (which attaches the brain to the spinal
cord), and the cerebellum, all described in detail in the sec-
tion 11.5 Brain. The brain includes about one hundred billion
(10
11
) multipolar neurons and countless branches of the axons
(nerve F bers) by which these neurons communicate with each
other and with neurons elsewhere in the nervous system.
The brainstem connects the brain and spinal cord and
allows two-way communication between them. The spinal
cord, in turn, provides two-way communication between the
CNS and the peripheral nervous system (PNS).
Bones, membranes, and fl
uid surround the organs of the
CNS. More speciF
cally, the brain lies in the cranial cavity
of the skull, whereas the spinal cord occupies the vertebral
canal in the vertebral column. Beneath these bony coverings,
membranes called
meninges,
located between the bone and
the soft tissues of the nervous system, protect the brain and
spinal cord
(f g. 11.1
a
)
.
11.2
MENINGES
The meninges (sing.,
meninx
) have three layers—dura mater,
arachnoid mater, and pia mater (F
g. 11.1
b
). The
dura mater
is the outermost layer. It is primarily composed of tough,
white, dense connective tissue and contains many blood ves-
sels and nerves. It attaches to the inside of the cranial cavity
and forms the internal periosteum of the surrounding skull
bones (see reference plate 13).
Scalp
Cranium
Cerebrum
Cerebellum
Vertebra
Spinal cord
Meninges
Meninges
Cerebrum
(b)
(a)
Tentorium
cerebelli
Gray matter
White matter
Subarachnoid space
Falx cerebri
Arachnoid
mater
Pia mater
Dura mater
Dural sinus (superior
sagittal sinus)
Arachnoid
granulation
Bone of skull
Subcutaneous tissue
Skin
FIGURE 11.1
±Meninges.±(
a
) Membranes called meninges enclose the brain and spinal cord. (
b
) The meninges include three layers: dura mater,
arachnoid mater, and pia mater.
TABLE
11.1
|
Partitions of the Dura Mater
Partition
Location
Falx cerebelli
Separates the right and left cerebellar hemispheres
Falx cerebri
Extends downward into the longitudinal ²
ssure, and
separates the right and left cerebral hemispheres (²
g. 11.1
b
)
Tentorium
cerebelli
Separates the occipital lobes of the cerebrum from the
cerebellum (²
g. 11.1
a
)
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