The cerebral hemispheres conceal most of the rest of an intact brain (Fig. 3-2A). Hemisection reveals many parts of the diencephalon, brainstem, and cerebellum, and additional features of the cerebral hemispheres as well (Fig. 3-2B). The cephalic flexure is visible at the junction between the brainstem and the diencephalon. The brainstem itself is subdivided into the midbrain, which is continuous with the diencephalon; the pons; and the medulla, which is continuous with the spinal cord. The two cerebral hemispheres are joined by a huge fiber bundle, the corpus callosum, which has an enlarged and rounded posterior splenium, a body, and an anterior, curved genu that tapers gently into a ventrally directed rostrum, which merges into the lamina terminalis (where development of the corpus callosum started).

Figure 3-2 Major regions of the adult brain in lateral (A) and medial (B) views. G, genu; R, rostrum; S, splenium (of the corpus callosum).

The nervous system develops embryologically from a neuroectodermal tube; the cavity of the tube persists in adults as a system of ventricles (see Fig. 5-1), part of which is apparent in the sagittal plane (Fig. 3-2B). Portions of the medial surfaces of the diencephalon form the walls of the narrow, slitlike third ventricle, which opens into the large lateral ventricle of each cerebral hemisphere through an interventricular foramen (foramen of Monro). Posteriorly, the third ventricle is continuous with a narrow channel through the midbrain, the cerebral aqueduct (aqueduct of Sylvius). The aqueduct in turn is continuous with the fourth ventricle of the pons and medulla, and the fourth ventricle is continuous with the microscopically tiny central canal of the caudal medulla and the spinal cord.

Humans, Relative to Other Animals, Have Large Brains

One impressive feature of the human brain is its size, and our distinctively human mental capacities are commonly attributed to this. The human brain weighs about 400 g at birth, and this weight triples during the first 3 years of life (resulting from the addition of myelin and the growth of neuronal processes rather than the addition of neurons). The rate of growth then slows, and the maximum brain weight of around 1400 g (Table 3-1) is reached at about age 11. This weight holds steady until about age 50, when a slow decline sets in (Fig. 3-3). The weight of 1400 g is only an average; brain weights for normal individuals range from 1100 g (or less) to around 1700 g. This large range is surprising, and its significance is not well understood; there is only a modest correlation between brain size and mental ability.

Table 3-1 Approximate Average Volumes of Intracranial Contents

	Volume (cm³)
Brain	1365
Gray matter	695
White matter	670
CSF	180
Cerebrum	1200
Basal ganglia^*	8
Cerebellum	135
Brainstem	30

Based on Anastasi et al (2006), Kruggel (2006), and Makris et al (2003).

* Caudate nucleus, putamen, and globus pallidus.

Figure 3-3 A, Average brain weights of human males and females at different ages. Notice how the brain grows rapidly after birth, doubling in the first year of life, before reaching its full size at about age 11 years. At all ages, male brains have a greater average weight than female brains. However, as indicated in B, adult female brains actually account for a greater percentage of body weight than do adult male brains. Brain growth is substantial in utero, and we are born with brains that are very large relative to body size. After the brain growth spurt of the first 1 to 3 years of life, body growth takes over, and the brain weight–body weight ratio declines progressively until about age 17.

(Plotted from data in Dekaban AS, Sadowsky D: Ann Neurol 4:345, 1978.)

Part of the reason for the large human brain is simply a reflection of body size: big animals tend to have big brains (Fig. 3-4). Elephants, for example, have 5000-g brains. Similarly, the size difference between the bodies of human males and females explains, at least in part, the fact that male brains are slightly larger than female brains (Fig. 3-3). However, this is not the whole story; many animals that are larger than humans nevertheless have smaller brains (Fig. 3-5). Overall, then, relative to body size, humans have larger brains than most other animals. It is tempting to attribute our mental abilities to our relatively large brains, but this is an oversimplification. Relative to body size, dolphins, some small primates and rodents, and even some fish have larger brains than we do. The key differences between human brains and those of other animals appear to be more complex neuronal interconnections and a selective increase in the size of certain areas of the cerebral cortex thought to be involved in higher functions (see Fig. 22-12).

Figure 3-4 Brains of a series of representative mammals, all reproduced at the same scale. Brain size is partly related to body size (e.g., cat versus lion, human versus elephant) and partly related to mental abilities (e.g., lion versus human). Not all parts of the brain change size in proportion to one another. For example, the olfactory bulbs of opossums and coyotes (blue arrows) are relatively large, those of monkeys and chimpanzees (green arrows) are proportionally much smaller, and those of humans are barely discernible at this magnification.

(From www.brainmuseum.org, courtesy Dr. Wally Welker; supported by NSF grant 0131028.)

Figure 3-5 The relative sizes of the brain of a rhinoceros and the alleged brain of the author. Although the rhino’s body weight is about 30 times greater, its brain weight is likely to be only half as great.

(Rhino, courtesy Albrecht Dürer. Author, courtesy Mr. and Mrs. Nolte. Suggested by an illustration in Cobb S: Arch Neurol 12:555, 1965.)

Named Sulci and Gyri Cover the Cerebral Surface

A striking aspect of human cerebral hemispheres is the degree to which their surface is folded and convoluted. Each ridge is called a gyrus, and each groove between ridges is called a sulcus; particularly deep sulci are often called fissures. This folding into gyri and sulci is a mechanism for increasing the total cortical area; each of us has about 2.5 ft² of cortex, two thirds of which is hidden from view in the walls of sulci. The appearance of different gyri and sulci varies considerably from one brain to another (Fig. 3-6), to the extent that they may not even be continuous structures (e.g., a particular gyrus may be transected by one or more sulci). Major features, however, are reasonably constant.

Figure 3-6 Superior and left lateral views of the brains of eight different individuals, with the age and gender of each subject indicated. These images were reconstructed from magnetic resonance imaging scans and show the range of sizes and shapes of the normal brain. The left central sulcus (green lines) is in about the same place in each brain and has roughly the same configuration, but the details differ from one brain to another. Other features (e.g., folding pattern of the superior frontal gyrus, configuration of the superior temporal sulcus) vary more substantially.

(Method from Tosun D et al: NeuroImage 23:108, 2004. Images, courtesy Dr. Jerry Prince; magnetic resonance data from Baltimore Longitudinal Study of Aging; participants provided by the Intramural Research Program of the National Institute on Aging.)

In the discussion that follows, the principal surface features of the hemispheres are described, with some broad generalizations regarding the function of various cortical areas. These functional descriptions are highly oversimplified and are offered primarily for purposes of initial orientation. Cortical function is discussed in more detail in Chapter 22.

Each Cerebral Hemisphere Includes a Frontal, Parietal, Occipital, Temporal, and Limbic Lobe

Four prominent sulci—the central sulcus, the lateral sulcus, the parietooccipital sulcus, and the cingulate sulcus—together with the preoccipital notch and parts of a few other sulci are used to divide each cerebral hemisphere into five lobes (Fig. 3-7):

1. The frontal lobe extends from the anterior tip of the brain (the frontal pole) to the central sulcus (sulcus of Rolando). On the lateral surface of the hemisphere, the lateral sulcus (fissure of Sylvius) separates it from the temporal lobe. On the medial surface of the brain, it extends to the cingulate sulcus and posteriorly to an imaginary line from the top of the central sulcus to the cingulate sulcus. Inferiorly it continues as the orbital part of the frontal lobe (named for its location above the orbit).

2. The parietal lobe extends from the central sulcus to an imaginary line connecting the top of the parietooccipital sulcus and the preoccipital notch. Inferiorly it is bounded by the lateral sulcus and the imaginary continuation of this sulcus to the posterior boundary of the parietal lobe. On the medial surface of the brain, it is bounded inferiorly by the subparietal and calcarine sulci, anteriorly by the frontal lobe, and posteriorly by the parietooccipital sulcus.

3. The temporal lobe extends superiorly to the lateral sulcus and the line forming the inferior boundary of the parietal lobe; posteriorly it extends to the line connecting the top of the parietooccipital sulcus and the preoccipital notch. On the medial surface, its posterior boundary is an imaginary line extending from the preoccipital notch toward the splenium of the corpus callosum, and part of its superior boundary is the collateral sulcus.

4. The occipital lobe is bounded anteriorly by the parietal and temporal lobes on both the lateral and medial surfaces of the hemisphere.

5. The limbic lobe is a strip of cortex that appears to more or less encircle the telencephalondiencephalon junction. It is interposed between the corpus callosum and the frontal, parietal, and occipital lobes and curves around to occupy part of the medial surface of what would otherwise be called the temporal lobe.

Figure 3-7 Lobes and sulci of the cerebral hemisphere. A, Boundaries of the frontal, parietal, occipital, and temporal lobes on the lateral surface of the hemisphere. B, Boundaries of the frontal, parietal, occipital, temporal, and limbic lobes on the medial surface of the hemisphere. C, Major sulci on the lateral surface of the hemisphere. D, Major sulci on the medial and inferior surfaces of the hemisphere. The subparietal sulcus of this hemisphere looks like a continuation of the cingulate sulcus, but in two thirds of brains they are separate sulci.

These separations correspond only approximately to functional subdivisions, but they do provide a meaningful basis for discussion and reference.

An additional area of cerebral cortex not usually included in any of the five lobes discussed above lies buried in the depths of the lateral sulcus, concealed from view by portions of the frontal, parietal, and temporal lobes. This cortex, called the insula, overlies the site where the telencephalon and diencephalon fuse during embryological development (see Chapter 2). It can be revealed by prying open the lateral sulcus or by removing the overlying portions of other lobes (Fig. 3-8). The portion of a given lobe overlying the insula is called an operculum (Latin for “lid”); there are frontal, parietal, and temporal opercula. The circular sulcus outlines the insula and marks its borders with the opercular areas of cortex.

Figure 3-8 Location of the insula, demonstrated by prying open the lateral sulcus (A) and then cutting away the frontal, parietal, and temporal opercula (B). The surface of the insula is convoluted, like other cortical areas, typically into about three short gyri and two long gyri.

The Frontal Lobe Contains Motor Areas

Four gyri make up the lateral surface of the frontal lobe (Fig. 3-9). The precentral gyrus is anterior to the central sulcus and parallel to it, extending to the precentral sulcus. The superior, middle, and inferior frontal gyri are oriented parallel to one another and roughly perpendicular to the precentral gyrus. The superior frontal gyrus continues onto the medial surface of the hemisphere as far as the cingulate sulcus. The inferior frontal gyrus is visibly divided into three parts: (1) the orbital part, which is most anterior and is continuous with the inferior (orbital) surface of the frontal lobe; (2) the opercular part, which is most posterior and forms a portion of the frontal operculum; and (3) the wedge-shaped triangular part, which lies between the other two. The inferior, or orbital, surface of the frontal lobe is mostly occupied by a group of gyri of somewhat variable appearance that are usually referred to collectively as orbital gyri or orbitofrontal cortex. The only consistently named gyrus on this surface is the gyrus rectus (Greek for “straight gyrus”), which is most medial and extends onto the medial surface of the hemisphere. Between the gyrus rectus and the orbital gyri is the olfactory sulcus, which contains the olfactory bulb and tract. The medial surface of the lobe contains extensions of the superior frontal gyrus, precentral gyrus, and gyrus rectus; certain small cortical areas near the rostrum of the corpus callosum are part of the limbic lobe.

Figure 3-9 Lateral, medial, and inferior surfaces of the frontal lobe, seen from above and in front (A) and from medially and below (B). (The medial extensions of the precentral and postcentral gyri surround the end of the central sulcus; for this reason, they are sometimes referred to as the paracentral lobule. Using this terminology, the medial extension of the postcentral gyrus is also called the anterior paracentral lobule.)

The frontal lobe contains four general functional areas:

1. Much of the precentral gyrus is the primary motor cortex, which contains many of the cells of origin of descending motor pathways and is involved in the initiation of voluntary movements.

2. The premotor and supplementary motor areas occupy the remainder of the precentral gyrus, together with adjacent portions of the superior and middle frontal gyri; they are also functionally related to the initiation of voluntary movements.

3. Broca’s area, the opercular and triangular parts of the inferior frontal gyrus of one hemisphere (usually the left), is important in the production of written and spoken language.

4. The prefrontal cortex, a very large and somewhat confusingly named area (because it sounds like it is in front of the frontal lobe), occupies the remainder of the frontal lobe. Prefrontal cortex is involved with what are often referred to as executive functions, which can generally be described as personality, insight, and foresight.

The Parietal Lobe Contains Somatosensory Areas

The lateral surface of the parietal lobe is divided into three areas: the postcentral gyrus and the superior and inferior parietal lobules (Fig. 3-10). The postcentral gyrus is posterior to the central sulcus and parallel to it, extending to the postcentral sulcus. The intraparietal sulcus runs posteriorly from the postcentral sulcus toward the occipital lobe, separating the superior and inferior parietal lobules. The inferior parietal lobule in turn is composed of the supramarginal gyrus, which caps the upturned end of the lateral sulcus, and the angular gyrus, which similarly caps the superior temporal sulcus. The angular gyrus is typically broken up by small sulci and may overlap the supramarginal gyrus. The medial surface of the parietal lobe contains the medial extension of the postcentral gyrus and is completed by an area called the precuneus, which is bounded by the subparietal and calcarine sulci, the parietooccipital sulcus, and the marginal branch of the cingulate sulcus. The extensions of the precentral and postcentral gyri onto the medial surface of the hemisphere are sometimes referred to together as the paracentral lobule, which is partly in the frontal lobe and partly in the parietal lobe.