Superficial Anatomy

The cerebrum is arbitrarily divided into five lobes: frontal, parietal, temporal, occipital, and the hidden insula. On the lateral surface, they are limited by the central sulcus, the posterior ramus of the sylvian fissure, the lateral parietotemporal line (from the impression of the parieto-occipital sulcus to the preoccipital notch), and the temporo-occipital line (from the posterior end of the posterior ramus of the sylvian fissure to the midpoint of the lateral parietotemporal line). The cerebrum has four main sulci that are 100% continuous—the sylvian fissure and the callosal, parieto-occipital, and collateral sulci—and two almost continuous (92%) sulci—the central and calcarine sulci. There are two 100% interrupted sulci: the precentral and inferior temporal sulci.¹ The central sulcus starts from the medial surface of the hemisphere above the cingulate sulcus and extends on the lateral surface of the hemisphere in a medial-to-lateral, superior-to-inferior, and posterior-to-anterior direction. It does not usually intercept the posterior ramus of the sylvian fissure and leaves a “bridge” connecting the precentral to the postcentral gyrus, known as pli de passage frontoparietal inferior, opercule rolandique, or the subcentral gyrus (Fig. 2-1A).

FIGURE 2-1 A, Lateral view of the left hemisphere. 1, “Omega” (motor hand area) and central sulcus; 2, superior frontal sulcus and gyrus; 3, precentral sulcus and gyrus; 4, postcentral sulcus and gyrus; 5, middle frontal gyrus; 6, inferior frontal sulcus; 7, supramarginal gyrus; 8, pli de passage; 9, anterior horizontal ramus; 10, pars triangularis; 11, ascending ramus and pars opercularis; 12, posterior ramus; 13, Heschl’s gyrus; 14, pars orbitalis; 15, superior temporal gyrus and sulcus; 16, middle temporal gyrus; 17, inferior temporal sulcus and gyrus. B, Upper left, “omega”sign. Lower left, surgical positioning. Right, the arrows indicate the “omega,” the black arrowheads indicate the superior frontal sulcus, and the blue arrowheads indicate the central sulcus. C, Posterolateral view of the left hemisphere. 1, Central sulcus; 2, postcentral gyrus and sulcus; 3, supramarginal gyrus; 4, intraparietal sulcus and superior parietal lobule; 5, superior temporal gyrus; 6, angular gyrus. D, Superolateral view of the left hemisphere. 1, Precentral gyrus; 2, foramen of Monro; 3, frontal horn; 4, head of the caudate nucleus and the superior limiting sulcus (insula); 5, pars orbitalis; 6, anterior limiting sulcus (insula); 7, inferior limiting sulcus (insula).

Frontal Lobe

The two main sulci are the superior and inferior frontal sulci, which are anteroposteriorly oriented and extend from the precentral sulcus to the frontal pole. At their posterior end, these two sulci are intercepted perpendicularly by the precentral sulcus, which has a direction very similar to that of the central sulcus. The precentral sulcus forms the anterior limit of the precentral gyrus. These two frontal sulci divide the lateral surface of the frontal lobe into three gyri: the superior, middle, and inferior frontal gyri (Fig. 2-1A). The anterior horizontal, the anterior ascending, and the posterior rami of the sylvian fissure divide the inferior frontal gyrus into three parts: the pars orbitalis, triangularis, and opercularis. The apex of the pars triangularis is usually retracted superiorly and leaves a space in the sylvian fissure that is generally the largest space in the superficial compartment of the sylvian fissure. The apex of the pars triangularis is directed inferiorly toward the junction of three rami of the sylvian fissure; this junctional point coincides with the anterior limiting sulcus of the insula in the depth of the sylvian fissure. It marks the anterior limit of the basal ganglia and the location of the anterior horn of the lateral ventricle. At the intercepting point between the superior frontal and precentral sulci, the precentral gyrus often has the morphology of the Greek letter “Ω” (omega), with its convexity pointing posteriorly. This is the most easily identifiable landmark of the motor strip and corresponds to the hand area (Fig. 2-1B).

Parietal Lobe

The parietal lobe is limited anteriorly by the central sulcus, medially by the interhemispheric fissure, inferolaterally by the sylvian fissure and the temporo-occipital line, and posteriorly by the lateral parietotemporal line. Its two main sulci are the postcentral and intraparietal sulci. The postcentral sulcus is very similar to the central sulcus, except for its variable continuity. The postcentral sulcus is the posterior limit of the postcentral gyrus, and it can sometimes be double. The intraparietal sulcus starts at the postcentral sulcus and is directed posteriorly and inferiorly toward the occipital pole; its direction is often parallel and 2 to 3 cm lateral to the midline. The bottom of the intraparietal sulcus is related to both the roof of the atrium and the occipital horn. The intraparietal sulcus divides the lateral surface of the parietal lobe into two parts: the superior and inferior parietal lobules. The superior parietal lobule, which is the superomedial and smaller part, continues as the precuneus on the medial surface of the parietal lobe. The inferior parietal lobule is constituted by the supramarginal and angular gyri. The supramarginal gyrus, the posterior continuation of the superior temporal gyrus, turns around the posterior ascending ramus of the sylvian fissure. The angular gyrus is the posterior continuation of the middle temporal gyrus and turns superiorly and medially behind the posterior ramus of the sylvian fissure up to the intraparietal sulcus; it is sometimes limited between the two posterior terminations of the superior temporal sulcus, the angular and anterior occipital rami (Fig. 2-1C).

The postcentral and intraparietal sulci and the superior parietal lobule are a “mirror image” of the precentral and superior frontal sulci and the superior frontal gyrus, with the central sulcus being the “mirror.”

Temporal Lobe

The temporal lobe is limited superiorly by the posterior ramus of the sylvian fissure and posteriorly by the temporo-occipital and lateral parietotemporal lines. It has two main sulci, the superior and inferior temporal sulci, that divide the lateral surface of the temporal lobe into three gyri, the superior, middle, and inferior temporal gyri. The inferior temporal gyrus occupies the lateral and basal surfaces of the cerebrum. The superior and inferior temporal gyri converge anteriorly to form the temporal pole (Fig. 2-1A).

Occipital Lobe

The occipital lobe is located behind the lateral parietotemporal line and is composed of a number of irregular convolutions that are divided by a short horizontal sulcus, the lateral occipital sulcus, into the superior and inferior occipital gyri.

The “x-ray” vision concept can be demonstrated by the precentral gyrus, which begins on the medial surface of the cerebrum, above the level of the splenium of the corpus callosum, and passes above the body of the lateral ventricle, thalamus, posterior limb of the internal capsule, and posterior part of the lentiform nucleus to reach the sylvian fissure approximately midway between the anterior and posterior limits of the insula (Fig. 2-1D).

Sylvian Fissure

The sylvian fissure is the space between the frontal, parietal, and temporal opercula and the insula and extends from the basal to the lateral surface of the brain. It is composed of a superficial and a deep part. The superficial part has a stem and three rami; the stem extends medially from the semilunar gyrus of the uncus to the lateral end of the sphenoid ridge, where the stem divides into the anterior horizontal, anterior ascending, and posterior rami (Fig. 2-1A). The deep part is divided into a “sphenoidal compartment” and an “operculoinsular compartment.” The sphenoidal compartment, which arises in the region of the limen insulae lateral to the anterior perforated substance (APS), is a narrow space posterior to the sphenoid ridge between the frontal and temporal lobes that communicates medially with the carotid cistern, also called sylvian vallecula (see Fig. 2-4D).² The operculoinsular compartment is formed by two narrow clefts, the opercular cleft between the opposing lips of the frontoparietal and temporal opercula and the insular cleft, which has a superior limb located between the insula and the frontoparietal opercula and an inferior limb between the insula and the temporal operculum (Fig. 2-2A).³ The gyri that constitute the frontal and parietal opercula of the sylvian fissure are, from posterior to anterior, the supramarginal, postcentral, and precentral gyri and the pars opercularis, triangularis, and orbitalis (see Fig. 2-1A); the gyri that constitute the temporal operculum of the sylvian fissure are, from posterior to anterior, the planum temporale, Heschl’s gyrus, and the planum polare (Fig. 2-2B, left). Each gyrus of the frontoparietal operculum is related to its counterpart on the temporal side; the supramarginal gyrus is in contact with the planum temporale, the postcentral gyrus is in contact with Heschl’s gyrus, and the precentral gyrus and pars opercularis, triangularis, and orbitalis are related to the planum polare. The site on the posterior ramus of the sylvian fissure where the postcentral gyrus meets Heschl’s gyrus is projected in the same coronal plane as the external acoustic meatus. The medial wall of the sylvian fissure is the insula or island of Reil, which can be seen only when the lips of the sylvian fissure are widely separated. The insula has the shape of a pyramid with its apex directed inferiorly and has an anterior and a lateral surface. The anterior surface is triangular in shape and is constituted by the transverse and accessory gyri and the insular pole. The medial portion of the insular pole is marked by an arched ridge of variable prominence, the limen insulae, which is composed of fibers of the uncinate fasciculus covered by a thin layer of gray matter that extends from the anterior end of the long gyrus, passes through the medial part of the insular pole, and ends at the middle of the posterior orbital gyrus. “Limen” means threshold, and the limen insulae is the threshold between the carotid cistern medially and the sylvian fissure laterally (Fig. 2-2A). The insula is encircled and separated from the opercula by a deep furrow called the circular or limiting sulcus of the insula, which has three parts, the superior, anterior, and inferior parts (see Fig. 2-1D). From the limen insulae, the sulci and gyri of the insula are directed superiorly in a radial manner. The deepest sulcus, the central sulcus of the insula, is a constant sulcus that extends upward and backward across the insula, in the general line of the central sulcus of the cerebrum. It divides the lateral surface of the insula into a large anterior zone that is divided by several shallow sulci into three to five short gyri and a posterior zone that is formed by the anterior and posterior long gyri (Fig. 2-2D). From microsurgical and radiologic viewpoints, the insula represents the external covering of the central core and is constituted by the extreme, external, and internal capsules, the claustrum, the basal ganglia, and the thalamus (Fig. 2-3A, left; also see Fig 2-2A). The anterior, inferior, and posterior limits of the insula on the lateral projection correspond to the anterior, inferior, and posterior limits of the central core. The upper limit of the central core (caudate nucleus) is higher than the upper limit of the insula (Fig. 2-2A).

FIGURE 2-2 A, Coronal view. 1, Body of the caudate nucleus; 2, superior limiting sulcus; 3, opercular compartment; 4, insular compartment; 5, globus pallidus; 6, floor of the third ventricle and anterior commissure; 7, amygdala; 8, head of the hippocampus. B, Left, Anterosuperior view of the left temporal lobe. 1, Posterior transverse temporal gyrus; 2, middle transverse temporal gyrus; 3, parahippocampal gyrus; 4, Heschl’s gyrus; 5, fornix and dentate gyrus; 6, planum polare; 7, rhinal incisura. Right, Basal view of the roof of the lateral ventricle. 1, Septum pellucidum. The veins of the roof of the lateral ventricle drain toward the midline. C, Fiber dissection of the left hemisphere. 1, Corona radiata; 2, inferior occipitofrontal fascicle; 3, putamen; 4, superior longitudinal fascicle; 5, uncinate fascicle. D, Lateral view of the left lateral ventricle. 1, Corpus callosum; 2, septum pellucidum; 3, anterior septal and superior choroidal veins; 4, bulb of the callosum and medial atrial vein; 5, thalamostriate vein and thalamus (anterior tubercle); 6, column of the fornix and foramen of Monro; 7, calcar avis; 8, central sulcus of the insula; 9, choroid plexus and atrium; 10, apex of the insula; 11, temporal horn.

FIGURE 2-3 A, Left, Superior view. 1, Genu of the corpus callosum; 2, anterior limb of the internal capsule; 3, frontal horn; 4, genu of the internal capsule and thalamostriate vein; 5, striothalamic sulcus, thalamus, and superior choroidal vein; 6, bodies of the fornix and hippocampal commissure; 7, crus of the fornix and tail of the hippocampus; 8, collateral trigone; 9, calcar avis. Right, Roof of the third ventricle through a transchoroidal approach. 1, Head of the caudate nucleus and anterior caudate vein; 2, rostrum of the corpus callosum; 3, column of the fornix; 4, anterior septal vein; 5, foramen of Monro; 6, body of the fornix; 7, thalamostriate vein; 8, inferior membrane of the tela choroidea and choroid plexus of the third ventricle (the superior membrane of the tela has been removed); 9, body of the caudate nucleus and thalamostriate vein; 10, dorsal surface of the thalamus; 11, internal cerebral vein and medial posterior choroidal artery; 12, splenium of the corpus callosum. B, Superolateral view of the right hemisphere. 1, Bulb of the callosum; 2, thalamus; 3, internal capsule (genu); 4, internal capsule (anterior limb); 5, caudate nucleus; 6, calcar avis, 7, internal capsule (retrolenticular portion); 8, lentiform nucleus; 9, internal capsule (sublenticular portion). C, Basal view of the optic radiation. 1, Meyer’s loop; 2, optic tract; 3, middle part; 4, lateral geniculate body; 5, posterior part. D, Left, Superior view of the floor of the right temporal horn. 1, Uncus (anterior segment); 2, uncus (apex); 3, uncus (posterior segment); 4, head of the hippocampus; 5, inferior choroidal point; 6, body of the hippocampus and fimbria; 7, collateral eminence; 8, parahippocampal gyrus; 9, tail of the hippocampus; *, uncal recess. Right, Basal view. 1, amygdala; 2, temporal horn; 3, hippocampus.

Association Fibers of the Cerebrum

The association fibers are tracts of myelinated fibers that connect cortical areas of different lobes in the same hemisphere; they may be divided into short and long association fibers. The short association fibers connect adjacent gyri, whereas the long association fibers (fasciculi) connect distant gyri and form distinct compact bundles. The main fasciculi are (1) the superior longitudinal fasciculus, which is the largest, arches around the insula, and connects parts of the frontal, parietal, and temporal lobes; (2) the uncinate fasciculus, which lies in the depth of the limen insulae, has a marked curvature, and connects the basal parts of the frontal lobe with the temporal lobe; (3) the inferior occipitofrontal fasciculus, which connects the frontal and occipital lobes, as well as the posterior part of the temporal and parietal lobes; these fibers converge from the frontal lobe as a single bundle that runs lateral to the lentiform nucleus, where they are closely associated with the uncinate fasciculus (Fig. 2-2C); and (4) the cingulum, the fibers of which lie within the cingulate gyrus from below the rostrum of the corpus callosum to the parahippocampal gyrus.

Lateral Ventricles

Wrapping around the central core of the hemisphere are the lateral ventricles (Fig. 2-2D). Each ventricle has five components: a frontal horn, body, atrium, and occipital and temporal horns.⁴ The frontal horn is located in front of the foramen of Monro and has a roof, floor, and anterior, lateral, medial, and posterior walls. The transition between the genu and the body of the corpus callosum forms the roof, the rostrum of the corpus callosum forms the narrow floor, the septum pellucidum forms the medial wall, and the thalamus forms the posterior wall. The head of the caudate nucleus forms the majority of the lateral wall, but the most anterior part is constituted by the most anterior portion of the anterior limb of the internal capsule, and it is in close relation to the anterior limiting sulcus of the insula. The body of the lateral ventricle is located behind the foramen of Monro and extends to the point where the septum pellucidum, corpus callosum, and fornix meet. It has a roof, floor, and lateral and medial walls. The body of the corpus callosum forms the roof, the septum pellucidum above and the body of the fornix below form the medial wall, the body of the caudate nucleus forms the lateral wall, and the thalamus forms the floor. The caudate nucleus and the thalamus are separated by the striothalamic sulcus, the groove in which the stria terminalis and the thalamostriate vein course. The atrium has a roof, floor, and anterior, medial, and lateral walls. The roof is formed by the body, splenium, and tapetum of the corpus callosum. The floor is formed by the collateral trigone, a triangular area that bulges upward over the posterior end of the collateral sulcus. The medial wall is formed by two roughly horizontal prominences: the upper prominence, or the bulb of the callosum, is formed by a large bundle of fibers called the forceps major that connects the two occipital lobes; the lower prominence, or the calcar avis, overlies the deepest part of the calcarine sulcus. The lateral wall has an anterior portion formed by the caudate nucleus as it wraps around the lateral margin of the pulvinar, as well as a posterior portion formed by the fibers of the tapetum as they sweep anteroinferiorly along the lateral margin of the ventricle and separate the ventricular cavity from the optic radiation. The anterior wall has a medial part composed of the crus of the fornix as it wraps around the posterior portion of the pulvinar and a lateral part formed by the pulvinar of the thalamus. The occipital horn extends posteriorly into the occipital lobe from the atrium. It varies in size from being absent to extending far posterior in the occipital lobe. The bulb of the callosum and the calcar avis form its medial wall, the tapetum forms the roof and the lateral wall, and the collateral trigone forms the floor (Fig. 2-3A and B).⁴ The temporal horn extends forward and inferiorly from the atrium into the medial part of the temporal lobe and has a roof, floor, and anterior, lateral, and medial walls. The tapetum, the tail of the caudate nucleus, part of the retrolentiform and sublentiform components of the internal capsule, and the amygdaloid nucleus form the roof. The retrolentiform component is the posterior thalamic radiation that includes the optic radiation (Fig. 2-3C); the sublentiform component is formed mainly by the acoustic radiation. The amygdaloid nucleus constitutes the most anterior portion of the roof of the temporal horn and is located above and in front of the head of the hippocampus (Fig. 2-3D, right), anterior to the inferior choroidal point, which is the most anterior site of attachment of the choroid plexus in the temporal horn.⁵ There is no clear separation between the roof of the temporal horn and the thalamus because all fibers of the optic radiation come from the lateral geniculate body. Therefore, it is reasonable to consider the roof of the temporal horn a lateral extension of the thalamus.⁵ The attachment site of the choroid plexus can be a surgical landmark to separate the thalamus from the roof of the temporal horn (see Fig. 2-6D, right). The tapetum and the optic radiation form the lateral wall, the amygdaloid body forms the anterior wall, the head of the hippocampus forms the anterior third of the medial wall, and the choroidal fissure forms the posterior two thirds of the medial wall.⁵ The floor is formed medially by the hippocampus and laterally by the collateral eminence (Fig. 2-3D, left). The temporal horn is projected onto the middle temporal gyrus on the lateral view.

The structures related to the lateral ventricle are the foramen of Monro, internal capsule, corpus callosum, fornix, thalamus, caudate nucleus, hippocampus, temporal amygdala, and choroidal fissure.

Fornix

The fornix is a C-shaped structure that wraps around the thalamus in the wall of the lateral ventricle. The initial portion of the fornix, the fimbria, arises from the alveus, which is the subcortical white matter of the hippocampal allocortex, and thickens along the medial edge of the hippocampus; it is separated from the dentate gyrus by the fimbriodentate sulcus. The fimbria then passes posteriorly to become the crus of the fornix, which is the subcortical radiation of the hippocampal allocortex. In the atrium the crus wraps around the posterior surface of the pulvinar of the thalamus and arches superomedially toward the lower surface of the splenium of the corpus callosum; at the junction between the atrium and body of the lateral ventricle, the paired crura meet to form the body of the fornix. At the anterior margin of the thalamus, the body of the fornix separates into two columns that arch along the superior and anterior margins of the foramen of Monro. The columns of the fornix then split, pass predominantly posterior to the anterior commissure, and are directed inferiorly and posteriorly through the lateral wall of the third ventricle to reach the mamillary bodies at the floor of the third ventricle. In the area below the splenium, the two crura of the fornix are united by the hippocampal commissure (Fig. 2-4A; also see Fig. 2-3A and D, left).

FIGURE 2-4 A, Intraoperative view of the right temporal horn. 1, Amygdala; 2, head of the hippocampus; 3, fimbria and taenia fimbriae; 4, choroid plexus; 5, body of the hippocampus. B, Medial view of the right thalamus. 1, Caudate nucleus; 2, thalamus (dorsal surface); 3, fornix (column); 4, taenia thalami; 5, foramen of Monro; 6, thalamus (pulvinar); 7, anterior commissure; 8, massa intermedia; 9, hypothalamic sulcus; 10, lamina terminalis; 11, pineal gland; 12, hypothalamus; 13, mamillary body; 14, midbrain; 15, quadrigeminal plate; 16, optic nerve and oculomotor nerve. C, Left, Posterosuperior view of the anterior wall and floor of the third ventricle. 1, Fornix (column); 2, fornix (column); 3, anterior commissure; 4, lamina terminalis; 5, optic recess; 6, infundibular recess; 7, tuber cinereum; 8, midbrain. Right, Anterosuperior view of the posterior wall of the third ventricle. 1, Fornix (reflected); 2, suprapineal recess and pineal gland; 3, internal cerebral vein and choroid plexus; 4, habenular commissure; 5, posterior commissure; 6, aqueduct and midbrain; 7, massa intermedia; 8, thalamostriate vein; 9, anterior septal vein. D, Frontal view. 1, Parieto-occipital artery; 2, calcarine artery; 3, vein of Galen and P3; 4, sylvian point and atrium; 5, parahippocampal and dentate gyri; 6, lateral posterior choroidal artery; 7, crus cerebri and basal vein; 8, anterior choroidal artery and inferior ventricular vein; 9, deep middle cerebral vein. Arrows indicate the limen insulae.

Third Ventricle

The third ventricle is a narrow, funnel-shaped, unilocular midline cavity. It communicates at its anterosuperior margin with each lateral ventricle through the foramen of Monro and posteriorly with the fourth ventricle through the aqueduct of Sylvius (Fig. 2-4B). It has a roof, a floor, and an anterior, posterior, and two lateral walls.⁹ The roof extends from the foramen of Monro anteriorly to the suprapineal recess posteriorly and is constituted superiorly to inferiorly by five layers (see Fig. 2-3A). The first layer is the fornix; the body of the fornix is the anterior portion of the roof of the third ventricle, and the crura and the hippocampal commissure are the roof of the posterior portion. The second layer is the superior membrane of the tela choroidea, which is the part of the tela choroidea that passes thorough the forniceal side of the choroidal fissure to cover the choroid plexus of the lateral ventricle. The third layer is a vascular layer located in a space between the superior and inferior membranes of the tela choroidea called the velum interpositum; it contains the internal cerebral veins and branches of the medial posterior choroidal arteries. The fourth layer, the inferior membrane of the tela choroidea, forms the floor of the velum interpositum. It is attached anterolaterally to the taenia thalami, a small ridge on the free edge of a fiber tract, the striae medullaris thalami, that extends along the superomedial border of the thalamus from the foramen of Monro to the habenular commissure (Fig. 2-4B). The posterior part of the inferior membrane of the tela choroidea is attached to the superior surface of the pineal body. The fifth layer is the choroidal plexus of the third ventricle and is usually represented by two parallel strands of choroid plexus projecting backward on each side of the midline. The floor extends from the optic chiasm, anteriorly, to the orifice of the aqueduct of Sylvius posteriorly, and it is constituted, from anterior to posterior, by the optic and infundibular recesses, the tuber cinereum, the mamillary bodies, the posterior perforated substance, the midbrain, and the aqueduct (Fig. 2-4B). The anterior wall is formed by the lamina terminalis and the posterior wall is represented, from inferior to superior, by the posterior commissure, pineal recess, habenular commissure, pineal gland, and suprapineal recess (Fig. 2-4C). At the inner angle formed by the roof and the anterior wall is the anterior commissure.¹⁰ Frequently, there is another commissure in the cavity of the third ventricle located posterior to the foramen of Monro called the massa intermedia, which connects both thalami. The lateral wall of the third ventricle is constituted by the thalamus above and by the hypothalamus below, both separated by the hypothalamic sulcus, a shallow groove extending from the foramen of Monro to the aqueduct. The hypothalamic sulcus is the rostral continuation of the sulcus limitans of the brainstem (Fig. 2-4B).