Lobes

The surfaces of the two cerebral hemispheres are furrowed by sulci, the intervening ridges being called gyri. Most of the cerebral cortex is concealed from view in the walls of the sulci. Although the patterns of the various sulci vary from brain to brain, some are sufficiently constant to serve as descriptive landmarks. Deepest sulci are the lateral sulcus (Sylvian fissure) and the central sulcus (Rolandic fissure) (Figure 2.1A). These two serve to divide the hemisphere (side view) into four lobes, with the aid of two imaginary lines, one extending back from the lateral sulcus, the other reaching from the upper end of the parietooccipital sulcus (Figure 2.1B) to a blunt preoccipital notch at the lower border of the hemisphere (the sulcus and notch are labeled in Figure 2.3). The lobes are called frontal, parietal, occipital, and temporal.

Figure 2.1 The five lobes of the brain. (A) Lateral surface of right cerebral hemisphere. (B) Medial surface of right hemisphere.

Figure 2.3 (A) Lateral and (B) medial views of the right cerebral hemisphere, depicting the main gyri and sulci.

The blunt tips of the frontal, occipital, and temporal lobes are the respective poles of the hemispheres.

The opercula (lips) of the lateral sulcus can be pulled apart to expose the insula (Figure 2.2). The insula was mentioned in Chapter 1 as being relatively quiescent during prenatal expansion of the telencephalon.

Figure 2.2 Insula, seen on retraction of the opercula.

The medial surface of the hemisphere is exposed by cutting the corpus callosum, a massive band of white matter connecting matching areas of the cortex of the two hemispheres. The corpus callosum consists of a main part or trunk, a posterior end or splenium, an anterior end or genu (‘knee’), and a narrow rostrum reaching from the genu to the anterior commissure (Figure 2.3B). The frontal lobe lies anterior to a line drawn from the upper end of the central sulcus to the trunk of the corpus callosum (Figure 2.3B). The parietal lobe lies behind this line, and it is separated from the occipital lobe by the parietooccipital sulcus. The temporal lobe lies in front of a line drawn from the preoccipital notch to the splenium.

Figures 2.3–2.6 should be consulted along with the following description of surface features of the lobes of the brain.

Figure 2.4 ’Thick slice’ surface anatomy brain MRI scan from a healthy volunteer.

(From Katada, 1990.)

Figure 2.5 Cerebrum viewed from inferior aspect, depicting the main gyri and sulci.

Figure 2.6 The diencephalon and its boundaries. CC, corpus callosum.

Diencephalon

The largest components of the diencephalon are the thalamus and the hypothalamus (Figures 2.6 and 2.7). These nuclear groups form the side walls of the third ventricle. Between them is a shallow hypothalamic sulcus, which represents the rostral limit of the embryonic sulcus limitans.

Figure 2.7 Sagittal MRI ‘slice’ of the living brain.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California, USA.)

Midline sagittal view of the brain

Figure 2.8 is taken from a midline sagittal section of the head of a cadaver, displaying the brain in relation to its surroundings.

Figure 2.8 Sagittal section of fixed cadaver brain.

(From Liu et al. 2003, with permission of Shantung Press of Science and Technology.)

Thalamus, caudate and lentiform nuclei, internal capsule

The two thalami face one another across the slot-like third ventricle. More often than not, they kiss, creating an interthalamic adhesion (Figure 2.9). In Figure 2.10, the thalamus and related structures are assembled in a mediolateral sequence. In contact with the upper surface of the thalamus are the head and body of the caudate nucleus. The tail of the caudate nucleus passes forward below the thalamus but not in contact with it.

Figure 2.9 Thalamus and corpus striatum, seen on removal of the trunk of the corpus callosum and the trunk of the fornix.

Figure 2.10 Diagrammatic reconstruction of corpus striatum and related structures. The vertical lines on the left in A and B indicate the level of the coronal sections on the right. (A) Ventricular system. (B) Thalamus and caudate nucleus in place. (C) Addition of projections to and from cerebral cortex. (D) Lentiform nucleus in place.

The thalamus is separated from the lentiform nucleus by the internal capsule, which is the most common site for a stroke resulting from local arterial embolism (blockage) or hemorrhage. The internal capsule contains fibers running from thalamus to cortex and from cortex to thalamus, brainstem, and spinal cord. In the interval between cortex and internal capsule, these ascending and descending fibers form the corona radiata. Below the internal capsule, the crus of the midbrain receives descending fibers continuing down the brainstem.

The lens-shaped lentiform nucleus is composed of two parts: putamen and globus pallidus. The putamen and caudate nucleus are of similar structure, and their anterior ends are fused. Behind this, they are linked by strands of gray matter that traverse the internal capsule, hence the term corpus striatum (or, simply, striatum) used to include the putamen and caudate nucleus. The term pallidum refers to the globus pallidus.

The caudate and lentiform nuclei belong to the basal ganglia, a term originally applied to a half-dozen masses of gray matter located near the base of the hemisphere. In current usage, the term designates four nuclei known to be involved in motor control: the caudate and lentiform nuclei, the subthalamic nucleus in the diencephalon, and the substantia nigra in the midbrain (Figure 2.11).

Figure 2.11 Nomenclature of basal ganglia.

In horizontal section, the internal capsule has a dog-leg shape (see photograph of a fixed-brain section in Figure 2.12, and the living-brain magnetic resonance image [MRI] ‘slice’ in Figure 2.13). The internal capsule has four named parts in horizontal sections: