Important Facts
The complicated folding of the surface of the cerebral hemispheres substantially increases the surface area and therefore the volume of the cerebral cortex. The folds or convolutions are called gyri, and the intervening grooves are called sulci. About two thirds of the cortex forms the walls of the sulci and is therefore hidden from surface view. Although some gyri are constant features of the cerebral surface, others vary from one brain to another and even between the two hemispheres of the same brain. Subtler depressions in the cerebral cortex are grooves and notches unrelated to the pattern of gyri and sulci. They are made by extracerebral structures such as the bones of the skull and the venous sinuses of the dura mater.
Whereas a sulcus is a groove that indents the surface of a cerebral hemisphere, a fissure is a cleft that separates different components of the brain. Despite the different definitions of sulci and fissures, the two terms are frequently used interchangeably for the deepest sulci.
At an early stage in studying human neuroanatomy, students should be able to delineate the lobes of the cerebral hemispheres and to recognize the major sulci, fissures, and gyri that are commonly referred to as landmarks. Of the smaller sulci and gyri, some are of great functional importance, but others have no known significance.
Major Sulci and Fissures
The lateral and parieto-occipital sulci appear early in fetal development and are especially deep in the mature brain. These, together with the central and circular sulci, are the boundaries for division of the cerebral hemisphere into the frontal, parietal, insular, temporal, and occipital lobes (Figs. 13-1 and 13-2).
The lateral sulcus (fissure of Sylvius or sylvian fissure) begins as a deep furrow on the inferior
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surface of the hemisphere. This is the stem of the sulcus, which extends laterally between the frontal and temporal lobes and divides into three rami when reaching the lateral surface. Whereas the posterior ramus is the main part of the sulcus on the lateral surface of the hemisphere, the anterior and ascending rami project for only a short distance into the frontal lobe. An area of the cortex called the insular lobe or insula (island of Reil) lies at the bottom of the lateral sulcus and is hidden from surface view. This cortex appears to have been bound to the underlying corpus striatum during late embryonic and early fetal development; growth of the surrounding cortex would then produce the deep lateral sulcus.
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FIGURE 13-1 Lobes of the cerebral hemisphere (lateral surface). |
The central sulcus (sulcus of Rolando; rolandic sulcus) is an important landmark for the sensorimotor cortex because the first somatic sensory area is immediately behind the sulcus, and the primary motor area is immediately in front of it. The central sulcus indents the superior border of the hemisphere about 1 cm behind the midpoint between the frontal and occipital poles. The sulcus slopes downward and forward, stopping just short of the lateral sulcus, and there are usually two bends along its course. The central sulcus is about 2 cm deep; its walls therefore constitute much of the sensorimotor cortex.
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FIGURE 13-2 Lobes of the cerebral hemisphere (medial and inferior surfaces). |
The calcarine sulcus on the medial surface of the hemisphere begins under the posterior end of the corpus callosum and follows an arched course to the occipital pole. In some brains, the sulcus continues over the pole for
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a short distance on the lateral surface. The calcarine sulcus is an important landmark for the visual cortex, most of which lies in the walls of the sulcus.
The parieto-occipital sulcus extends from the calcarine sulcus to the superior border of the hemisphere, which it intersects about 4 cm from the occipital pole.
The longitudinal and transverse cerebral fissures are external to the hemispheres and are therefore in a different category from the foregoing surface markings. The longitudinal cerebral fissure separates the hemispheres. A dural partition called the falx cerebri extends into the fissure. The corpus callosum, which constitutes the main cerebral commissure, crosses from one hemisphere to the other at the bottom of the longitudinal fissure. The transverse cerebral fissure intervenes between the cerebral hemispheres above and the cerebellum, midbrain, and diencephalon below. The posterior part of this fissure is located between the cerebral hemispheres and the cerebellum; it contains a dural partition known as the tentorium cerebelli. The anterior part of the transverse fissure intervenes between the corpus callosum and the diencephalon. It is triangular in outline, tapering anteriorly, and contains the tela choroidea, which consists of vascular connective tissue derived from the pia mater that covers the brain. The tela choroidea is continuous with the connective tissue core of the choroid plexuses of the lateral ventricles and the third ventricle, and the plexuses are completed by choroid epithelium derived from the ependymal lining of the ventricles. Choroid plexuses secrete cerebrospinal fluid (see Chapter 26).
Lobes of the Cerebral Hemispheres
Each cerebral hemisphere has lateral, medial, and inferior surfaces on which the extent of the lobes of the hemisphere are now defined (see Figs. 13-1 and 13-2).
The frontal lobe occupies the entire area in front of the central sulcus and above the lateral sulcus on the lateral surface. The medial surface of the frontal lobe envelops the anterior part of the corpus callosum and is bounded posteriorly by a line drawn between the central sulcus and the corpus callosum. The inferior surface of the frontal lobe rests on the orbital plate of the frontal bone.
The natural boundaries of the parietal lobe on the lateral surface are the central and lateral sulci. The other boundaries consist of two lines; the first of these is drawn between the parieto-occipital sulcus and the preoccipital notch, and the second line runs from the middle of the one just established to the lateral sulcus. (The preoccipital notch, indicated inFigs. 13-1 and 13-2, is an inconspicuous indentation of the brain formed by the petrous part of the temporal bone.) On the medial surface, the parietal lobe is bounded by the frontal lobe, corpus callosum, calcarine sulcus, and parieto-occipital sulcus.
The temporal lobe is outlined on the lateral surface by the lateral sulcus and the lines previously noted. The inferior surface of the temporal lobe extends to the temporal pole from a line drawn between the anterior end of the calcarine sulcus and the preoccipital notch. Most of the occipital lobe appears on the medial surface of the hemisphere, where it is separated from the temporal lobe, as already described, and from the parietal lobe by the parieto-occipital sulcus. On the lateral surface, the occipital lobe consists of the small area posterior to the line that joins the parieto-occipital sulcus and preoccipital notch.
The portion of the great cerebral commissure in and near the midline is known as the trunk of the corpus callosum, and the fibers of the commissure that spread out within the centers of the hemispheres constitute the radiations of the corpus callosum. Names are assigned to certain regions of the trunk of the commissure (see Fig. 13-2); these regions are used as reference points further on. The enlarged posterior portion of the trunk is called the splenium. The anterior portion, or genu, curves ventrally and thins out to form therostrum. This is continuous with the lamina terminalis, which limits the third ventricle anteriorly.
Gyri and Sulci
Some surface markings of the hemisphere are landmarks for important functional areas; the central sulcus for the sensorimotor cortex and the calcarine sulcus for the visual cortex are examples. For the most part, the sulci and gyri
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serve only as a rough frame of reference for cortical areas whose functions may or may not be known. The markings can be identified according to lobes for the lateral surface, but this is not practicable for the medial and inferior surfaces.
The text and illustrations that follow apply to sulci and gyri of varying functional significance. Students may need to refer to this material when studying the localization of functions in the cerebral cortex (see Chapter 15).
LATERAL SURFACE
Frontal Lobe
The precentral sulcus (often broken into two or more parts) runs parallel to the central sulcus; these sulci outline the precentral gyrus, which is a landmark for the primary motor area of the cerebral cortex (Fig. 13-3). The remainder of the lateral surface of the frontal lobe is divided into superior, middle, and inferior frontal gyri by the superior and inferior frontal sulci. The anterior and ascending rami of the lateral sulcus divide the inferior frontal gyrus into opercular, triangular, and orbital portions. In the left hemisphere, the opercular and triangular portions consist of cortex of Broca's expressive or motor speech area. In the frontal lobe, as in the other lobes of the hemisphere, secondary gyri and sulci contribute to the variable topography of different brains.
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FIGURE 13-3 Gyri and sulci on the lateral surface of the right cerebral hemisphere. (A), (B), and (C) indicate the opercular, triangular, and orbital parts of the inferior frontal gyrus, respectively. |
Parietal Lobe
The postcentral sulcus runs parallel to the central sulcus; these sulci bound the postcentral gyrus, which is the landmark for the first somatic sensory
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(somesthetic) area of the cortex. The intraparietal sulcus extends posteriorly from the postcentral sulcus and divides that part of the surface not occupied by the postcentral gyrus into superior and inferior parietal lobules. The portions of the inferior parietal lobule that surround the upturned ends of the lateral sulcus and superior temporal sulcus are called the supramarginal gyrus and the angular gyrus, respectively. In the left hemisphere, these gyri are included in the receptive language area, which is necessary for perception and interpretation of spoken and written language.
Insular Lobe (Insula)
The regions that conceal the insula are known as the frontal, parietal, and temporal opercula; they must be spread apart or cut away to expose the insula (Fig. 13-4). The insula is outlined by a circular sulcus and is divided into two regions by a central sulcus. Several short gyri lie in front of the central sulcus, and one or two long gyri lie behind it. The inferior part of the insula in the region of the stem of the lateral sulcus is known as the limen insulae. The cortex of the insula is involved in involuntary activities such as the control of viscera by the autonomic nervous system. Cortical areas for the special visceral sensations of taste and smell also extend onto the insula.
The insula is an important landmark for certain structures inside the cerebral hemisphere. The lentiform nucleus, a component of the corpus striatum, is separated from the insula by two layers of white matter (the extreme and external capsules) and an intervening layer of gray matter (the claustrum).
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FIGURE 13-4 The insula of the left cerebral hemisphere, exposed by cutting away the frontal, parietal, and temporal opercula. |
Temporal Lobe
Superior and inferior temporal sulci divide the lateral surface of the temporal lobe into superior, middle, and inferior temporal gyri. Among variations in the temporal lobe, the inferior temporal sulcus may be discontinuous, making it difficult to identify. The inferior temporal gyrus is called the lateral occipitotemporal gyrus when viewed from the inferior aspect of the temporal lobe. The superior temporal gyrus has a large surface that forms the floor of the lateral sulcus. On the anterior part of this surface, transverse temporal gyri(also known as Heschl's convolutions) extend to the bottom of the lateral sulcus and mark the location of the primary auditory area of the cortex. The posterior part of the superior temporal gyrus is the planum temporale, which is larger on the left side in males but not in females. The planum temporale includes part of the receptive language area, which extends onto the parietal lobe.
Occipital Lobe
In the brains of primates other than humans and in some human brains, the calcarine sulcus continues for a short distance over the occipital pole. There is then a curved lunate sulcus around the end of the calcarine sulcus. Except for this inconstant marking, the small area of the occipital lobe on the lateral surface has minor grooves and folds of no special significance.
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The primary visual cortex occupies and surrounds the calcarine sulcus. The remainder of the occipital lobe consists of association cortex for interpretation of visual stimuli. The visual association cortex extends onto the parietal and temporal lobes (see also Chapters 15 and 20).
Medial and Inferior Surfaces
The cingulate gyrus begins beneath the genu of the corpus callosum and continues above the corpus callosum as far back as the splenium (Fig. 13-5). The cingulate sulcus intervenes between the cingulate gyrus and the medial frontal gyrus, which is continuous with the superior frontal gyrus on the lateral surface of the hemisphere. The cingulate sulcus gives off a paracentral sulcus and then divides into marginal and subparietal sulci in the parietal lobe. The region bounded by the paracentral and marginal sulci, which surrounds the indentation made by the central sulcus on the superior border, is called the paracentral lobule. The anterior and posterior parts of the paracentral lobule are, respectively, extensions of the precentral and postcentral gyri of the lateral surface of the hemisphere. The area above the
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subparietal sulcus is called the precuneus and is continuous with the superior parietal lobule on the lateral surface. The parieto-occipital and calcarine sulci bound the cuneus of the occipital lobe.
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FIGURE 13-5 Gyri and sulci on the medial and inferior surfaces of the right cerebral hemisphere. (A) Uncus. (B) Isthmus (retrosplenial cortex) connecting the cingulate and parahippocampal gyri. |
On the medial surface of the frontal lobe, underneath the rostrum of the corpus callosum, is the subcallosal gyrus, also known as the parolfactory area. This is part of the septal area, a component of the limbic system (see Chapter 18).
On the inferior surface of the hemisphere (see Figs. 13-5 and 13-6), a convolution extends from the occipital pole almost to the temporal pole. The posterior part of the convolution consists of the lingual gyrus. The anterior part forms the parahippocampal gyrus, which hooks sharply backward on its medial aspect as the uncus, a region where fibers of the olfactory tract end. The collateral sulcus defines the lateral margin of the lingual and parahippocampal gyri. The short rhinal sulcus, at the lateral edge of the parahippocampal gyrus anteriorly, delimits the entorhinal area, which belongs to the olfactory and limbic systems. The medial occipitotemporal gyrus, also commonly called the fusiform gyrus, lies along the lateral side of the collateral sulcus. It is broken up by several small, variable sulci. The occipitotemporal sulcus intervenes between the medial occipitotemporal gyrus and the lateral occipitotemporal gyrus. The latter is continuous with the inferior temporal gyrus of the lateral surface of the hemisphere.
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FIGURE 13-6 Gyri and sulci on the inferior surface of the right cerebral hemisphere. (A) Uncus. |
The inferior surface of the frontal lobe is commonly known as the orbitofrontal cortex. The olfactory bulb and olfactory tract (see Fig. 13-6) conceal most of the olfactory sulcus. The gyrus rectus is located medially to the olfactory sulcus. The large area lateral to the olfactory sulcus typically consists of four irregular orbital gyri (medial, anterior, posterior, and lateral) separated by an H-shaped arrangement of sulci.
Limbic Lobe
The cingulate and parahippocampal gyri are connected by a narrow isthmus (more often called retrosplenial cortex) behind and beneath the splenium of the corpus callosum. The connected
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gyri form the limbic lobe. This is part of the limbic system of the brain, which incorporates several additional structures, most prominently the hippocampus, the dentate gyrus and the amygdaloid body (in the temporal lobe), the hypothalamus, the septal area, and some nuclei of the thalamus (see Chapter 18). The term limbic system is still in use but may soon become obsolete. It embraces neural circuitry involved in such diverse functions as learning, remembering, defensive and reproductive behavior, and responses to subjective feelings.
Suggested Reading
Chiavaras MM, Petrides M. Orbitofrontal sulci of the human and macaque monkey brain. J Comp Neurol 2000;422:35-54.
Haines DE. Neuroanatomy. An Atlas of Structures, Sections and Systems, 7th ed. Baltimore: Williams & Wilkins, 2007.
Hanke J. Sulcal pattern of the anterior parahippocampal gyrus in the human adult. Ann Anat 1997;179:335-339.
Kulynych JJ, Vladar K, Jones DW, et al. Gender differences in the normal lateralization of the supratemporal cortex: MRI surface-rendering morphometry of Heschl's gyrus and the planum temporale. Cereb Cortex 1994;4: 107-118.
Montemurro DG, Bruni JE. The Human Brain in Dissection, 2nd ed. New York: Oxford University Press, 1988.
Naidich TP, Valavanis AG, Kubik S. Anatomic relationships along the low-middle convexity, 1: normal specimens and magnetic resonance imaging. Neurosurgery 1995;36: 517-532.
Nieuwenhuys R, Voogd J, van Huijzen C. The Human Central Nervous System. A Synopsis and Atlas, 3rd ed. Berlin: Springer-Verlag, 1988.
Nolte J, Angevine JB. The Human Brain in Photographs and Diagrams, 3rd ed. St. Louis: Mosby, 2007.