The Central Nervous System
of the cerebral hemisphere, the
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:
of gray matter, which looks gray
in fresh brain tissue
, islands of gray matter situated deep within the
We consider these regions next.
is the “executive suite” of the nervous sys-
tem, where our
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
Describe the general function of the basal nuclei (basal
form the superior part of the brain
. 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
“twisters”)—separated by shallow grooves called
“furrows”)—mark nearly the entire surface of
the cerebral hemispheres. Deeper grooves, called
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
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
, which lies in the
frontal plane, separates the
Bordering the central sulcus are the
ĭ-tal), located more posteriorly on
the medial surface of the hemisphere, separates the
from the parietal lobe.
outlines the ﬂaplike
and separates it from the parietal and frontal lobes. A ﬁ²h lobe
(b) Left lateral view
(a) Anterior view
Ventricles of the brain.
Different regions of the large lateral ventricles are
labeled anterior horn, posterior horn, and inferior horn.