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INTRODUCTION

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The cerebral hemispheres make us human. They include the cerebral cortex (which consists of six lobes on each side: frontal, parietal, temporal, occipital, insular, and limbic), the underlying cerebral white matter, and a complex of deep gray matter masses, the basal ganglia. From a phylogenetic point of view, the cerebral hemispheres, particularly the cortex, are relatively new. Folding of the cortex, in gyri separated by sulci, permits a highly expanded cortical mantle to fit within the skull vault in higher mammals, including humans. The cortex is particularly well developed in humans. There are multiple maps (motor, somatosensory, visual) of the body and the external world within the cortex. The cortex is highly parcellated, with different parts of the cortex being responsible for a variety of higher brain functions, including manual dexterity (the "opposing thumb" and the ability, eg, to move the fingers individually so as to play the piano); conscious, discriminative aspects of sensation; and cognitive activity, including language, reasoning, planning, and many aspects of learning and memory.

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DEVELOPMENT

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The telencephalon (endbrain) gives rise to the left and right cerebral hemispheres (Fig 10–1). The hemispheres undergo a pattern of extensive differential growth; in the later stages, they resemble an arch over the lateral fissure (Fig 10–2).

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FIGURE 10–1

Cross sections showing early development from neural groove to cerebrum.

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FIGURE 10–2

Differential growth of the cerebral hemisphere and deeper telencephalic structures.

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The basal ganglia arise from the base of the primitive telencephalic vesicles (Fig 10–3). The growing hemispheres gradually cover most of the diencephalon and the upper part of the brain stem. Fiber connections (commissures) between the hemispheres are formed first at the rostral portions as the anterior commissure, later extending posteriorly as the corpus callosum (Fig 10–4).

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FIGURE 10–3

Coronal sections showing development of the basal ganglia in the floor of the lateral ventricle.

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FIGURE 10–4

Dorsal view of developing cerebrum showing formation of the corpus callosum, which covers the subarachnoid cistern and vessels over the diencephalon.

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ANATOMY OF THE CEREBRAL HEMISPHERES

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The cerebral hemispheres make up the largest portion of the human brain. They appear as highly convoluted masses of gray matter that are organized into two somewhat symmetrical (but not totally symmetrical) folded structures. The crests of the cortical folds (gyri) are separated by furrows (sulci) or deeper fissures. The folding of the cortex into gyri and sulci permits the cranial vault to contain a large area of ...

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