Chapter 12
The Central Nervous System
431
12
of the cerebral hemisphere, the
insula
(in
9
su-lah; “island”), is
buried deep within the lateral sulcus and forms part of its floor
(Figure 12.4d). Te insula is covered by portions of the tempo-
ral, parietal, and frontal lobes.
Te cerebral hemispheres fit snugly in the skull. Te frontal
lobes lie in the anterior cranial fossa, and the anterior parts of the
temporal lobes fill the middle cranial fossa (see Figure 7.2b, c,
p. 201). Te posterior cranial fossa, however, houses the brain
stem and cerebellum. Te occipital lobes are located well superior
to that cranial fossa.
Each cerebral hemisphere has three basic regions:
A superficial
cerebral cortex
of gray matter, which looks gray
in fresh brain tissue
Internal
white matter
Basal nuclei
, islands of gray matter situated deep within the
white matter
We consider these regions next.
Cerebral Cortex
Te
cerebral cortex
is the “executive suite” of the nervous sys-
tem, where our
conscious mind
is found. It enables us to be aware
of ourselves and our sensations, to communicate, remember,
understand, and initiate voluntary movements.
Te cerebral cortex is composed of gray matter: neuron cell
bodies, dendrites, associated glia and blood vessels, but no fiber
tracts. It contains billions of neurons arranged in six layers.
Although only 2–4 mm (about 1/8 inch) thick, it accounts for
roughly 40% of total brain mass. Its many convolutions effec-
tively triple its surface area.
Modern imaging techniques allow us to see the brain in
action—PE± scans show maximal metabolic activity in the brain,
Differentiate between commissures, association fibers, and
projection fibers.
Describe the general function of the basal nuclei (basal
ganglia).
Te
cerebral hemispheres
form the superior part of the brain
(Figure 12.4)
. Te most conspicuous parts of an intact brain, to-
gether they account for about 83% of total brain mass. Picture how
a mushroom cap covers the top of its stalk, and you have a good
idea of how the paired cerebral hemispheres cover and obscure
the diencephalon and the top of the brain stem (see Figure 12.2c).
Elevated ridges of tissue called
gyri
(ji
9
ri; singular:
gyrus;
“twisters”)—separated by shallow grooves called
sulci
(sul
9
ki;
singular:
sulcus;
“furrows”)—mark nearly the entire surface of
the cerebral hemispheres. Deeper grooves, called
fissures
, sepa-
rate large regions of the brain (Figure 12.4c).
Te more prominent gyri and sulci are important anatomical
landmarks that are similar in all humans. Te median
longitudinal
fissure
separates the cerebral hemispheres (Figure 12.4a). Another
large fissure, the
transverse cerebral fissure
, separates the cerebral
hemispheres from the cerebellum below (Figure 12.4b, c).
Several sulci divide each hemisphere into five lobes—frontal,
parietal, temporal, occipital, and insula (Figure 12.4c, d). All but
the last are named for the cranial bones that overlie them (see
Figure 7.5, pp. 204–205). Te
central sulcus
, which lies in the
frontal plane, separates the
frontal lobe
from the
parietal lobe
.
Bordering the central sulcus are the
precentral gyrus
anteriorly
and the
postcentral gyrus
posteriorly. Te
parieto-occipital
sulcus
(pah-ri
0
ě-to-ok-sip
9
ĭ-tal), located more posteriorly on
the medial surface of the hemisphere, separates the
occipital
lobe
from the parietal lobe.
Te deep
lateral sulcus
outlines the flaplike
temporal lobe
and separates it from the parietal and frontal lobes. A fi²h lobe
Anterior horn
Interventricular
foramen
Inferior
horn
Lateral
aperture
(b) Left lateral view
Lateral ventricle
Septum
pellucidum
Third ventricle
Cerebral aqueduct
(a) Anterior view
Fourth ventricle
Central canal
Inferior
horn
Posterior
horn
Median
aperture
Lateral
aperture
Figure 12.3
Ventricles of the brain.
Different regions of the large lateral ventricles are
labeled anterior horn, posterior horn, and inferior horn.
previous page 465 Human Anatomy and Physiology (9th ed ) 2012 read online next page 467 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off