398
UNIT THREE
The region the midbrain produces continues to be called the
midbrain
in the adult structure, and the hindbrain gives rise
to the
cerebellum, pons,
and
medulla oblongata
(f g. 11.15
and
table 11.4)
. Together, the midbrain, pons, and medulla
oblongata comprise the
brainstem
(bra
¯n
stem), which
attaches the brain to the spinal cord.
On a cellular level, the brain develops as specif c neu-
rons attract others by secreting growth hormones. In the
embryo and Fetus, the brain overgrows, and then apoptosis
(programmed cell death) destroys excess cells.
Structure of the Cerebrum
The
cerebrum
(ser
e
¯-brum), which develops From the ante-
rior portion oF the Forebrain, is the largest part oF the mature
brain. It consists oF two large masses, or
cerebral hemi-
spheres
(ser
e
˘-bral hem
i-sFe
¯rz), which are essentially mirror
images oF each other (
f
g. 11.16
and reFerence plate 9). A
deep bridge oF nerve f bers called the
corpus callosum
con-
nects the cerebral hemispheres. A layer oF dura mater called
the
falx cerebri
separates them (see f
g. 11.1
b
).
Many ridges or convolutions, called
gyri
(ji
ri) (sing.,
gyrus
), separated by grooves, mark the cerebrum’s surFace.
Generally, a shallow to somewhat deep groove is called a
sulcus
(sul
kus; pl.
sulci,
sul
si)
,
and a very deep groove is
called a
F
ssure.
The pattern oF these elevations and depres-
sions is complex, and it is distinct in all normal brains. ±or
example, a
longitudinal F
ssure
separates the right and leFt
cerebral hemispheres; a
transverse F
ssure
separates the cere-
brum From the cerebellum; and sulci divide each hemisphere
into lobes (see f
gs. 11.15 and 11.16).
A fetus or newborn with
anencephaly
has a face and lower brain
structures but lacks most higher brain structures. A newborn with this
anomaly survives only a day or two. Sometimes the parents donate
the organs.
Anencephaly is a type of neural tube defect (NTD). It occurs at
about the twenty-eighth day of prenatal development, when a sheet
of tissue that normally folds to form the neural tube, which develops
into the CNS, remains open at the top. In
spina bif
da,
an opening is
farther down the neural tube, causing a lesion in the spine. Paralysis
may occur from that point downward. Sometimes surgery can par-
tially correct spina biF
da. Taking folic acid supplements just before
and during pregnancy can lower the risk of a neural tube defect.
In a disorder called
lissencephaly
(“smooth brain”), a newborn has
a smooth cerebral cortex, completely lacking convolutions. Absence
of a protein early in prenatal development prevents certain neurons
from migrating in the brain, which blocks formation of convolutions.
The child is profoundly mentally retarded, with frequent seizures and
other neurological problems.
The lobes oF the cerebral hemispheres (f
g. 11.16) are
named aFter the skull bones that they underlie. The lobes
include the Following:
Brain Development
The basic structure oF the brain refl
ects the way it Forms dur-
ing early (embryonic) development. It begins as the neural
tube that gives rise to the CNS. The portion that becomes the
brain has three major cavities, or vesicles, at one end—the
forebrain
(prosencephalon),
midbrain
(mesencephalon), and
hindbrain
(rhombencephalon)
(f g. 11.14)
. Later, the Fore-
brain divides into anterior and posterior portions (telenceph-
alon and diencephalon, respectively), and the hindbrain
partially divides into two parts (metencephalon and myel-
encephalon). The resulting f
ve cavities persist in the mature
brain as the fl uid-f lled
ventricles
and the tubes that connect
them. Cells oF the tissue surrounding the spaces diFFerentiate
into the structural and Functional regions oF the brain.
The wall oF the anterior portion oF the Forebrain gives
rise to the
cerebrum
and
basal nuclei,
whereas the posterior
portion Forms a section oF the brain called the
diencephalon.
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
Neural tube
Telencephalon
Diencephalon
Mesencephalon
Myelencephalon
Metencephalon
Neural tube
Cerebral
hemispheres
Diencephalon
Midbrain
Medulla
oblongata
Pons and Cerebellum
Spinal cord
(a)
(b)
(c)
FIGURE 11.14
Brain development. (
a
) The brain develops from
a tubular structure with three cavities. (
b
) The cavities persist as the
ventricles and their interconnections. (
c
) The wall of the tube gives rise
to various regions of the brain, brainstem, and spinal cord.
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