430
UNIT 3
Regulation and Integration of the Body
12
of the cerebral hemispheres causes their surfaces to crease and
fold into
convolutions
(Figure 12.2c), which increases their sur-
face area and allows more neurons to occupy the limited space.
Regions and Organization
Some textbooks discuss brain anatomy in terms of the
embry-
onic scheme
(see Figure 12.1c), but in this text, we will consider
the brain in terms of the medical scheme and the four adult
brain regions shown in Figure 12.2c: (1) cerebral hemispheres,
(2) diencephalon, (3) brain stem (midbrain, pons, and medulla
oblongata), and (4) cerebellum.
Te basic pattern of the CNS is a central cavity surrounded
by gray matter (mostly neuron cell bodies), external to which
is white matter (myelinated fiber tracts). Te spinal cord ex-
hibits this basic pattern, but the brain has additional regions
of gray matter not present in the spinal cord. Both the cerebral
hemispheres and the cerebellum have an outer layer or “bark”
of gray matter called a cortex. Tis pattern changes with descent
through the brain stem—the cortex disappears, but scattered
gray matter nuclei are seen within the white matter. At the cau-
dal end of the brain stem, the basic pattern is evident. Knowl-
edge of the basic pattern of the CNS will help you explore the
brain, moving from the most rostral region (cerebrum) to the
most caudal (brain stem). But first, let’s explore the central hol-
low cavities that lie deep within the brain—the ventricles.
Ventricles
Te brain
ventricles
are continuous with one another and with
the central canal of the spinal cord
(Figure 12.3)
. Te hollow ven-
tricular chambers are filled with cerebrospinal fluid and lined by
ependymal cells
, a type of neuroglia (see Figure 11.3c on p. 389).
Te paired
lateral ventricles
, one deep within each cerebral
hemisphere, are large C-shaped chambers that reflect the pat-
tern of cerebral growth. Anteriorly, the lateral ventricles lie close
together, separated only by a thin median membrane called the
septum pellucidum
(pĕ-lu
9
sid-um; “transparent wall”). (See
Figure 12.10, p. 440.)
Each lateral ventricle communicates with the narrow
third
ventricle
in the diencephalon via a channel called an
interven-
tricular foramen
.
Te third ventricle is continuous with the
fourth ventricle
via the canal-like
cerebral aqueduct
that runs through the mid-
brain. Te fourth ventricle lies in the hindbrain dorsal to the
pons and superior medulla. It is continuous with the central ca-
nal of the spinal cord inferiorly. Tree openings mark the walls
of the fourth ventricle: the paired
lateral apertures
in its side
walls and the
median aperture
in its roof. Tese apertures con-
nect the ventricles to the
subarachnoid space
(sub
0
ah-rak
9
noid),
a fluid-filled space surrounding the brain.
Check Your Understanding
1.
Which ventricle is surrounded by the diencephalon?
2.
Which two areas of the adult brain have an outside layer
of gray matter in addition to central gray matter and
surrounding white matter?
3.
What is the function of convolutions of the brain?
For answers, see Appendix H.
Cerebral Hemispheres
List the major lobes, fissures, and functional areas of the
cerebral cortex.
Explain lateralization of hemisphere function.
Metencephalon
Anterior (rostral)
Posterior (caudal)
Mesencephalon
Diencephalon
Midbrain
Midbrain
Cerebellum
Cerebellum
Pons
Diencephalon
Medulla
oblongata
Spinal cord
Cerebral
hemisphere
Cerebral
hemisphere
Outline of
diencephalon
Cervical
Spinal cord
Flexures
Telencephalon
Myelencephalon
(a) Week 5:
Two major flexures form, causing the telencephalon
and diencephalon to angle toward the brain stem.
(b) Week 13:
Cerebral hemispheres develop and grow
posterolaterally to enclose the diencephalon and the rostral brain
stem.
(c) Birth:
Shows adult pattern of structures and convolutions.
Brain stem
Midbrain
Pons
Medulla
oblongata
Figure 12.2
Brain development.
Initially, the cerebral surface is
smooth. Folding begins in month 6, and convolutions become more
obvious as development continues. See-through view in (b) and (c).
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