Cerebral Anatomy at the Macroscopic Level

Although first described by the ancient Egyptians, the cerebral cortex was considered functionless until the beginning of the 19th century. For centuries, observers of the brain believed that this convoluted surface was unlikely to be the seat of any important function and localized mental activity within the ventricular structures, now known to contain only physiological fluid [1]. The gyri were compared to macaroni, or drawn like small intestines, without any precise pattern or organization. The only part of the cerebral hemisphere surface that was given a name was the Sylvian fissure.

In fact, most of the knowledge concerning the macroscopic architecture of the human brain came from the work of anatomists working at the end of the 19th century. At that time, these investigators had at their disposal only a few cadaver brains and no method of resectioning a brain in a different direction after an initial dissection. Therefore, they had to describe the coronal views using one hemisphere, the axial using another, and needed a second brain to describe the sagittal views. Additionally, removal of the brain from the skull resulted in deformations, and there was no formally standardized way to cut the brain into slices. Despite all these limitations, these anatomists did pioneering work and established the basis of modern brain anatomic labeling [2].

The beginning of modern cerebral anatomy can be set with the French neurological school and, in particular, with the brain descriptions given by Dejerine [2], who started a systematic anatomical nomenclature. Note, however, that heterogeneity of nomenclature is still present and obscures anatomical labeling. In particular, the same structure can be found labeled with different names. For example, the angular gyrus is also called "pli courbe" or inferior parietal gyrus and is often referred to as Brodmann's area 39, although this latter designation does not correspond to a macroscopic anatomical structure but rather to a cytoarchitectonic area that can only be defined on the basis of microscopic characteristics.

2.1 Brain Size and Gross Morphology

The human brain is a relatively small organ (around 1400 g) sitting within the skull and protected by membranes called the meninges, which include an external dense outer layer, called the dura mater, a thin inner layer, called the pia mater, and an intermediate layer, the arachnoid, constituted as a layer of fibers. The brain floats in a clear fluid, the cerebrospinal fluid (CSF), which has a protective role against trauma, as well as nourishing and draining functions.

The brain's basic subdivisions are the two cerebral hemispheres, the brainstem, and the cerebellum. The two cerebral hemispheres are separated from one another by a longitudinal fissure, also called the interhemispheric fissure, which contains the falx cerebri, a membranous septum that separates the hemispheres. The cerebral hemispheres are symmetrical to casual inspection, and their surfaces, called the cortex or gray matter, contain the cellular bodies of the neurons. The surfaces of the hemispheres are highly convoluted and can be described as a succession of crests, called the gyri, and fissures separating them, called the sulci. Underlying this gray mantle is the white matter, which is made of bundles of fibers emerging from the bodies of neurons. These fibers are the axons enveloped by their myelin. The two hemispheres are connected by a broad commissure of white-matter tracts: the corpus callosum. One should note that additional gray matter not belonging to the cortex is also found in the interior depths of the brain. This deep gray matter consists of large clusters of neurons, called the gray nuclei. These include the caudate nucleus and the lenticular nucleus, which can be subdivided into the putamen and pallidum, bilaterally within each hemisphere. Additional prominent deep gray matter nuclei include the thalamus and some smaller nuclei such as the subthalamic nucleus and the red nucleus.

The hemispheres are commonly subdivided into six lobes, four of which were named after the bones of the skull overlying them [3]. Here starts the kind of trouble one can meet with anatomical nomenclature: the limits between these lobes are partly arbitrary. We give here a description of the five lobes that constitute the human brain as seen on the external surface of one hemisphere (see Fig. 1a). The frontal lobe, located at the anterior tip of the brain, is clearly delimited both posteriorly by the Rolandic sulcus and inferiorly by the Sylvian fissure. In contrast, some boundaries of the parietal, temporal, and occipital lobes are not based on such clear anatomical landmarks. For example, as illustrated in Figure 1a, the limit between the occipital lobe and the parietal and temporal lobes is usually defined as a straight virtual line (yellow dashed line) that starts at the parieto-occipital sulcus (visible only on the internal surface of the brain) and runs downward to the incisure of Meynert [2]. Similarly, the boundary between the temporal lobe and the parietal lobe must be based on an arbitrary convention: Here we choose to draw a horizontal line starting from the point where the Sylvian fissure becomes vertical (blue dashed line) [4]. We must emphasize that these virtual limits do not have anatomical, histological, or functional support. The fifth lobe is the insula, a small triangular cortical area buried in the depth of the Sylvian fissure and therefore not visible in Fig. 1. The sixth lobe is the limbic lobe, which consists of large convolutions of the medial part of the hemisphere and includes the cingulate and subcallosal gyri as well as the hippocampal and parahippocampal gyri and the dentate gyrus, according to Broca [5] (Fig. 1b).

2.2 Sulcal and Gyral Anatomy

In this subsection, we will first give a short description of the sulci necessary to obtain the brain's lobar parcellation, namely

FIGURE 1 (a) Lobar parcellation of the external surface of a brain hemisphere. Red: frontal lobe, blue: temporal lobe, green: parietal lobe, yellow: occipital lobe. (b) Lobar parcellation of the internal surface of the hemisphere. The cingulate gyrus, which spans the hemisphere's entire internal boundary and which is part of the limbic lobe, is drawn in pink. The corpus callosum is the crescent-shaped structure nested just beneath the cingulate gyrus. The lobe known as the insula is not visible in this figure. The cerebellum, abutting the inferior surface of the brain, is in gray. (c) External hemisphere major sulci (to, transverse occipital; tl, superior temporal; sy_hor, horizontal branch of the Sylvian fissure; sy_asc, vertical branch of the Sylvian fissure; f2, inferior frontal sulcus; fl, superior frontal sulcus; post, postcentral sulcus; rol, Rolando; prec, precentral sulcus; ips, interparietal sulcus). (d) Internal hemisphere major sulci (rol, Rolando; pos, parieto-occipital sulcus; cal, calcarine fissure; cm, callosomarginal sulcus, including the cingulate sulcus and its marginal ramus; sps, subparietal sulcus). See also Plate 58.

FIGURE 1 (a) Lobar parcellation of the external surface of a brain hemisphere. Red: frontal lobe, blue: temporal lobe, green: parietal lobe, yellow: occipital lobe. (b) Lobar parcellation of the internal surface of the hemisphere. The cingulate gyrus, which spans the hemisphere's entire internal boundary and which is part of the limbic lobe, is drawn in pink. The corpus callosum is the crescent-shaped structure nested just beneath the cingulate gyrus. The lobe known as the insula is not visible in this figure. The cerebellum, abutting the inferior surface of the brain, is in gray. (c) External hemisphere major sulci (to, transverse occipital; tl, superior temporal; sy_hor, horizontal branch of the Sylvian fissure; sy_asc, vertical branch of the Sylvian fissure; f2, inferior frontal sulcus; fl, superior frontal sulcus; post, postcentral sulcus; rol, Rolando; prec, precentral sulcus; ips, interparietal sulcus). (d) Internal hemisphere major sulci (rol, Rolando; pos, parieto-occipital sulcus; cal, calcarine fissure; cm, callosomarginal sulcus, including the cingulate sulcus and its marginal ramus; sps, subparietal sulcus). See also Plate 58.

the Sylvian fissure and the Rolandic sulcus. We will then enter into the parcellation of each lobe, describing the major constant sulci, so as to provide the reader with a working knowledge of anatomical landmarks.

Sylvius and Rolando Sulci

The Sylvian Fissure: The Sylvian fissure (Fig. la,c), the second sulcus to appear during ontogenesis, is a very deep and broad sulcus. It is easy to identify, moving across the brain from the bottom toward the top as following an antero-posterior course. Its start marks the limit between the temporal pole and the frontal lobe, and, after an uninterrupted course, it ends posteriorly with a bifurcation into two sulci. One of these is a very small one with a downward curvature. The other larger one, which is called the terminal ascending segment of the Sylvian fissure, makes almost a 90° angle as it ascends upward into the parietal lobe.

The Rolandic Sulcus: The Rolandic sulcus, also called Rolando or the central sulcus, is a very important sulcus because it delimits the boundary between motor and the sensory cortices, as well as the boundary between the frontal and parietal lobes. There are several ways to identify it, and Fig. 2 shows three recipes that can be used to identify the Rolandic sulcus along its course in individual anatomical magnetic resonance images. On upper axial slices, the Rolandic sulcus is characterized by a typical notch [6,7] and never connects with any of surrounding sulci that run in a different direction such as the superior frontal sulcus or the intraparietal sulcus [8]. It is always located between two parallel sulci, namely the precentral and the postcentral sulci. These three sulci make a very typical pattern at the level of the vertex (see Fig. 2a). On the paramedial sagittal slices, at the top of the hemisphere, the Rolandic sulcus forms a notch just in front of the end of the ascending part of the callosomarginal sulcus [911] (see later subsection and Fig. 2b). On lateral sagittal slices, the Rolandic sulcus is the third sulcus encountered when starting from the ascending branch of the Sylvian fissure (which is located in the frontal lobe near the most anterior part of the Sylvian fissure) and moving backward (see Fig. 2c, top). One should note that the lower portion of the Rolandic sulcus never actually intersects the Sylvian fissure, but instead terminates just short of it as shown in Fig. 1c [12].

Once the Rolandic sulcus has been identified, it becomes easy to locate the precentral and postcentral sulci that both run parallel to Rolando, anteriorly and posteriorly, respectively (Figs. 2a and 2c, bottom). One should note that the precentral sulcus is frequently composed of two parts, with an inferior segment lying more anteriorly (Fig. 1c) and often intersecting the superior frontal sulcus. This arrangement is

FIGURE 2 Illustration of the characteristics of the Rolandic sulcus on different MRI sections. (a) Identification of Rolando on upper axial slices. Rolando is in red, the precentral sulcus in yellow, the postcentral sulcus in blue, the intraparietal sulcus in light blue, the superior frontal sulcus in green, and the callosomarginal sulcus in orange. (b) Identification of the Rolandic sulcus on a paramedial sagittal slice. (c) Identification of the inferior part of the Rolandic sulcus on external parasagittal slices. See also Plate 59.

FIGURE 2 Illustration of the characteristics of the Rolandic sulcus on different MRI sections. (a) Identification of Rolando on upper axial slices. Rolando is in red, the precentral sulcus in yellow, the postcentral sulcus in blue, the intraparietal sulcus in light blue, the superior frontal sulcus in green, and the callosomarginal sulcus in orange. (b) Identification of the Rolandic sulcus on a paramedial sagittal slice. (c) Identification of the inferior part of the Rolandic sulcus on external parasagittal slices. See also Plate 59.

often reflected posteriorly on the other side of the Rolandic sulcus, where the postcentral sulcus often intersects the intraparietal sulcus (Fig. 2a).

Sulci for Panellation of the Different Lobes Frontal Lobe: Considering its size, the frontal lobe has relatively few constant sulci and gyri (Fig. la). The most posterior gyrus is the precentral gyrus, which lies between the Rolandic and the precentral sulci and contains the motor cortex and part of the premotor cortex. In addition to the precentral sulcus, two other major constant sulci can be identified in the frontal lobe: the superior frontal sulcus (fl) and the inferior frontal sulcus (f2), allowing the delineation of three gyri: the superior, middle, and inferior frontal gyri. The superior frontal sulcus is very deep on axial slices and frequently intersects the precentral sulcus (Fig. 2a). It is very symmetrical and can be easily identified on an external hemispheric surface reconstruction (Fig. lc). The inferior frontal sulcus merges with the precentral gyrus posteriorly on parasagittal slices (Fig. 2c) and from this point follows a horizontal course before ending low in the inferior frontal pole. The inferior frontal gyrus, located below the inferior frontal sulcus, corresponds to Broca's area on the left [13]. The ascending and horizontal branches of the Sylvian fissure divide it into three parts (Fig. lc): the pars opercularis, located posterior to the ascending branch, the pars triangu laris, located between the two branches, and the pars orbitaris, located below the horizontal branch.

Parietal Lobe: The parietal lobe is the region that shows the greatest interhemispheric and interindividual sulcal variability. It contains two main gyri: the superior parietal gyrus and the inferior parietal gyrus. The superior parietal gyrus (also called PI; see Fig. 3B) is easy to identify since it is limited anteriorly by the postcentral sulcus, internally by the internal limit of the two hemispheres and inferiorly by the intraparietal sulcus. In contrast, the inferior parietal gyrus, also called P2, is very complex and highly variable. It contains the supramarginal gyrus, which is the circumvolution surrounding the ending of the Sylvian fissure, the angular gyrus, and sometimes some intervening cortex [14]. The angular gyrus, also called the "pli courbe" by French anatomists such as Déjerine [2] or called Brodmann's area 39 by scientists accustomed to cytoarchitec-tonic nomenclature, exemplifies this variability and provides a good example of the difficulties one may encounter when trying to define homologies between subjects and even between hemispheres.

The angular gyrus can be quite easily identified when the superior temporal sulcus has a single posterior termination. In this case, it simply surrounds this single ending of the superior temporal sulcus (t1) at the limit between temporal, parietal, and occipital cortices. However, in 50% of cases the superior temporal sulcus has a double parallel ending in the left hemisphere. In that configuration, the angular gyrus is,

FIGURE 3 An example of difficulties defining homologies between subjects or hemispheres: the angular gyrus. (A) The angular gyrus, also named the "pli courbe'' or Brodmann's area 39, lies around the ending of the superior temporal sulcus (light blue). When the superior temporal sulcus has a single ending it is easy to identify, but in 50% of cases in the left hemisphere, it shows a double parallel ending (solid green and yellow). A. Some authors consider the anterior parallel ending to be the angular sulcus (solid green) that centers the angular gyrus (dashed green). (B) Other authors center the angular gyrus around the posterior parallel ending, namely the anterior occipital sulcus (yellow). In one configuration the angular gyrus is within the parietal lobe; in the other it lies within the occipital lobe. See also Plate 60.

FIGURE 3 An example of difficulties defining homologies between subjects or hemispheres: the angular gyrus. (A) The angular gyrus, also named the "pli courbe'' or Brodmann's area 39, lies around the ending of the superior temporal sulcus (light blue). When the superior temporal sulcus has a single ending it is easy to identify, but in 50% of cases in the left hemisphere, it shows a double parallel ending (solid green and yellow). A. Some authors consider the anterior parallel ending to be the angular sulcus (solid green) that centers the angular gyrus (dashed green). (B) Other authors center the angular gyrus around the posterior parallel ending, namely the anterior occipital sulcus (yellow). In one configuration the angular gyrus is within the parietal lobe; in the other it lies within the occipital lobe. See also Plate 60.

according to some authors [2], centered around the more posterior of the two endings, known as the anterior occipital sulcus (see Fig. 3B), with the more anterior ending of the superior temporal sulcus defining the anterior limit of the angular gyrus. Meanwhile, according to other authors, the more anterior parallel ending is considered to be the angular sulcus, around which the angular gyrus is centered [15] (Fig. 3A). These differences in anatomical definitions are important since in one configuration the angular gyrus is within the parietal lobe while in the other it lies within the occipital lobe. Even Brodmann himself encountered difficulties in giving a cytoarchitectonic definition of the angular gyrus, and stated, "Its boundaries with occipital and temporal regions are ill-defined" [16]. In the brain figure constructed by Brodmann that so popularized cytoarchitectony as a basis for functional anatomy, Brodmann area 39 is located in the parietal lobe, limited posteriorly by the anterior occipital sulcus.

Sulci for Pareellation of the Temporal Lobe

Two sulci, the superior and inferior temporal sulci, divide the temporal lobe into three gyri: the superior, middle, and inferior temporal gyri. The superior temporal sulcus, although often separated into two different segments, is always present and quite easily identified by its horizontal course, parallel to the Sylvian fissure (Fig. 1c). The inferior temporal sulcus is divided into numerous segments, three on average [15], and can have different posterior endings: In 30% of the cases its ending on the right corresponds to a pli de passage between the two gyri it separates; it can also end at the preoccipital incisure of Meynert (Fig. 1c); and in 30% of the cases it can contribute to the anterior occipital sulcus, or continue with the lateral occipital sulcus. This exemplifies a case where it can be very difficult to define a standard way to recognize a sulcus and thus to find homologies between subjects and between hemispheres. As a consequence, the limits, especially posteriorly, between the middle and the inferior temporal gyri and between these and the inferior occipital gyrus will not be clear-cut. This sulcus should consequently not be considered as a reliable landmark for alignment between brains.

Constant Sulci of the Internal Surface of the Brain

On the internal surface of the hemisphere, three major sulci can be reliably identified: the callosomarginal or cingulate sulcus, the parieto-occipital sulcus, and the calcarine sulcus. As discussed in more detail later, they are primary sulci and do not present major variability. They are deep, follow a very typical course in the hemispheres, and can be identified using an isolated sagittal section, making their identification quite easy.

Parieto-Occipital Sulcus: The parieto-occipital sulcus is a very deep sulcus that crosses the posterior part of the hemisphere and divides the internal occipital lobe from the parietal and internal temporal lobes (Fig. 1d). It forms a notch on the external surface of the brain that serves as a landmark to draw the line that arbitrarily limits the occipital and parietal lobes externally (Fig. 1b) and from there goes downward and anteriorly following a linear path. At its midpoint it merges with the terminus of the calcarine sulcus.

Calcarine Sulcus: The calcarine sulcus also follows a linear course, running from the tip of the occipital lobe to the midpoint of the parieto-occipital sulcus. It can end in the occipital pole with a T shape, as is illustrated in Fig. 1d.

Callosomarginal or Cingulate Sulcus: The callosomarginal or cingulate sulcus is parallel to the superior surface of the corpus callosum. It starts under the rostrum of the corpus callosum (its most anterior part, which looks like a beak) and takes a posterior course to the posterior part of the corpus callosum, where it angles orthogonally to join the upper edge of the internal surface of the hemisphere just behind the Rolandic sulcus. It is often duplicated in the left hemisphere, and this second sulcus is called the paracingulate sulcus [17]. The subparietal sulcus continues the original course of the cingulate sulcus posteriorly along the corpus callosum (Fig. 1d).

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