SURFACES OF
CEREBRUM
The cerebrum is divided into right and left hemispheres by a longitudinal fissure. Each hemisphere has three surfaces-superolateral, medial, and inferior-all of which have irregular fissures, or sulci, demarcating convolutions, or gyri. Although there are variations in arrangement between the two hemispheres in the same brain and in those from different persons, a basic similarity in the pattern allows the parts of the brain to be mapped and named.
On the superolateral surface, two
sulci, the lateral and the central, can be easily identified. The lateral
(sylvian) sulcus has a short stem between the orbital surface of the
frontal lobe and the temporal pole; in life, the lesser wing of the sphenoid
bone projects into it. At its outer end, the stem divides into anterior,
ascending, and posterior branches. The anterior and ascending rami are each
about 2.5 cm long; the former runs horizontally into the inferior frontal
gyrus, and the latter, vertically. The posterior ramus is about 7.5 cm long and
inclines upward as it extends backward to end in the supramarginal gyrus, which
is part of the inferior parietal lobule. These rami separate triangular areas
of cortex called opercula, which cover a buried lobe of cortex, the insula.
The central (rolandic) sulcus proceeds
obliquely down-ward and forward from a point on the superior border almost
halfway between the frontal and occipital poles. It is sinuous and ends above
the middle of the posterior ramus of the lateral sulcus. Its upper end usually
runs onto the medial surface of the cerebrum and terminates in the paracentral
lobule.
The parietooccipital sulcus is
situated mainly on the medial surface of the cerebrum, but it cuts the superior
margin and appears for a short distance on the superolateral surface about 5 cm
in front of the occipital pole. At about the same distance from the occipital
pole on the inferior margin, there is a shallow indentation, the preoccipital
notch, produced by a small ridge on the upper surface of the tentorium
cerebelli.
The above features divide the
cerebrum into frontal, parietal, occipital, and temporal lobes. The frontal
lobe lies in front of the central sulcus and anterosuperior to the lateral
sulcus. The parietal lobe lies behind the central sulcus, above the
posterior ramus of the lateral sulcus and in front of an imaginary line drawn
between the parieto-occipital sulcus and the preoccipital notch.
The occipital lobe lies
behind this same imaginary line. The temporal lobe lies below the stem
and posterior ramus of the lateral sulcus, and is bounded behind by the lower
part of the aforementioned imaginary line.
Frontal Lobe. The superolateral surface of the frontal lobe
is traversed by three main sulci and thus divided into four gyri. The precentral
sulcus runs parallel to the central sulcus, separated from it by the precentral
gyrus, the great cortical somatomotor area. The superior
and inferior frontal sulci curve across the remaining part of the
surface, dividing it into superior, middle, and inferior frontal gyri.
Parietal Lobe. The parietal lobe has two main sulci, which
divide it into three gyri. The postcentral sulcus lies parallel to the
central sulcus, separated from it by the postcentral gyrus, the great
somatic sensory cortical area.
The remaining, larger part of the
superolateral parietal surface is subdivided into superior and inferior
parietal lobules (gyri) by the intraparietal sulcus, which runs backward
from near the midpoint of the postcentral sulcus and usually extends into the
occipital lobe, where it ends by joining the transverse occipital sulcus.
Occipital Lobe. The outer surface of the occipital lobe is
less extensive than that of the other lobes and has a short transverse
occipital sulcus and a lunate sulcus; the latter demarcates the
visuosensory and visuopsychic areas of the cortex. The calcarine sulcus notches
the occipital pole.
Temporal Lobe. The temporal lobe is divided by superior and
inferior temporal sulci into superior, middle, and inferior temporal
gyri. The sulci run backward and slightly upward, in the same general direction
as the posterior ramus of the lateral sulcus, which lies above them. The
superior sulcus ends in the lower part of the inferior parietal lobule, and the
superjacent cortex is called the angular gyrus. The superior temporal gyrus
contains the auditosensory and auditopsychic areas.
Insula. The insula is a sunken lobe of cortex, overlaid
by opercula and buried by the exuberant growth of adjoining cortical areas. It
is ovoid in shape and is surrounded by a groove, the circular sulcus of
the insula. The apex is inferior, near the anterior (rostral) perforated
substance, and is termed the limen of the insula. The insular surface is
divided into larger and smaller posterior parts by the central sulcus of
the insula, which is roughly parallel to the central sulcus of the cerebrum.
Each part is further subdivided by minor sulci into short and long insular
gyri. The claustrum and lentiform nucleus lie deep to the insula.
MEDIAL SURFACE OF CEREBRAL HEMISPHERES
The medial surfaces of the cerebral
hemispheres are flat, and, although separated for most of their extent by the
longitudinal fissure and falx cerebri, they are connected in parts by the
cerebral commissures and by the structures bounding the third ventricle.
Corpus Callosum. The corpus callosum is the largest of the
cerebral commissures, and forms most of the roof of the lateral ventricle. In a
median sagittal section, it appears as a flattened bridge of white fibers, and
its central part, or trunk, is convex upward. The anterior end is
recurved to form the genu, which tapers rapidly into the rostrum.
The expanded posterior end, or splenium, overlies the midbrain and adjacent part of the
cerebellum. The corpus callosum is about 10 cm long and 2.5 cm wide between the
points where it sinks into the opposing hemispheres in the depths of the corpus
callosal sulcus. Its fibers diverge to all parts of the cerebral cortex.
Fornix. Below the splenium and trunk of the corpus
callosum are the symmetric arching bundles (crura of the fornix) that meet to form the body of
the fornix and separate again to become the columns of the fornix,
curving downward to the mammillary bodies. The body of the fornix lies in the
roof of the third ventricle, and the tela choroidea is subjacent; the lateral
fringed margins of this double fold of pia mater are the choroid plexuses of
the central parts of the lateral ventricles, while an extension from the
underside of the fold in the midline forms the choroid
plexus of the third ventricle.
Cingulate Sulcus. The cingulate sulcus is easily identified on the
medial surface, lying parallel to the corpus callosum. It begins below the genu
of the corpus callosum and ends above the posterior part of the trunk by
turning upward to cut the superior margin of the hemisphere. Opposite the
middle of the trunk is another vertical branch sulcus, and the area of cortex
between these ascending sulci is the paracentral lobule, which contains parts
of the motor and sensory cortical areas. The cingulate sulcus separates the medial
frontal and cingulate gyri, and below the genu and rostrum of the
corpus callosum are small parolfactory sulci separating the subcallosal
(parolfactory) areas and paraterminal gyrus. Posterior Medial
Surface. The posterior part of the medial surface has two deep sulci.
The upper parietooccipital sulcus inclines backward and upward to cut
the superior border. The lower calcarine sulcus extends forward from the
occipital pole to end beneath the splenium of the corpus callosum, and the
isthmus of cortex between them connects the cingulate and parahippocampal
gyri. The wedge-shaped region between the parietooccipital and calcarine sulci
is the cuneus, while the area between the parietooccipital sulcus and
the paracentral lobule is the precuneus. The main visuo- sensory area is
located in the walls of the calcarine sulcus and
in the adjacent cortex.
INFERIOR SURFACE OF CEREBRAL HEMISPHERE
The inferior surface is divided by
the stem of the lateral sulcus into smaller, orbital and larger, tentorial
surfaces.
The orbital surface rests on the roofs of the orbit and nose and is
marked by an H-shaped orbital sulcus, as well as by a straight groove on
the medial side, the olfactory sulcus, which lodges the olfactory bulb
and tract. The orbital sulcus demarcates the orbital gyri; the small
con- volution medial to the olfactory sulcus is the straight gyrus.
The tentorial surface lies partly on the floor of the middle cranial
fossa and partly on the tentorium cerebelli. It has two anteroposterior
grooves, the collateral and occipitotemporal sulci. Both run
almost directly forward from the occipital pole to the temporal pole; like
other sulci, they may be subdivided, and the anterior
end of the collateral sulcus is called the rhinal sulcus. The parahippocampal
and lingual gyri lie medial to the collateral sulcus. The dentate
gyrus, a narrow fringe of cortex with transverse markings, occupies the
groove between the parahippocampal gyrus and the fimbria of the hippocampus.
The anterior end of the parahippocampal gyrus becomes recurved to form the uncus, which is partly occupied by the cortical
olfactory area. The medial occipitotemporal gyrus is fusiform in shape,
and lies between the collateral and occipitotemporal sulci. The lateral
occipitotemporal gyrus lies lateral to the occipitotemporal sulcus and is
continuous with the inferior temporal around the inferior margin of the hemisphere.
