The largest and most developed part of the brain, the site of most conscious and intelligent activities. Its main components are 2 large cerebral hemispheres that grow out from the upper part of the brainstem. Their surface is made up of a series of folds called gyri, separated by fissures called sulci, with a deep longitudinal fissure separating the 2 hemispheres. The 4 main surface regions of each hemisphere – the frontal, parietal, temporal, and occipital lobes – are named after their overlying bones. Each hemisphere has a central cavity, called a ventricle, filled with cerebrospinal fluid. This is surrounded by an inner layer, consisting of clusters of nerve cells called the basal ganglia. A middle layer of “white matter” is composed mainly of nerve fibres, which carry information between specific areas of the cortex and between areas of the cortex, central brain, and the brainstem. A thick band of fibres called the corpus callosum carries nerve signals between the 2 hemispheres. The outer surface layer of each hemisphere is the cerebral cortex – the “grey matter’’, where much of the sensory information from organs such as the eyes and ears is processed. Specific sensory processing takes place in separate regions. For example, visual perception is located in a part of the occipital lobe called the visual cortex. The cortex also contains “motor’’ areas concerned with the initiation of signals for movement by the skeletal muscles. Linked to the sensory and motor areas of the cortex are association areas, which integrate information from various senses and also perform functions such as comprehension and recognition, memory storage and recall, thought and decision making. Some of these cortical functions are localized to one “dominant’’ hemisphere (the left in almost all right-handed and many left-handed people). Two clearly defined areas in the dominant hemisphere are Wernicke’s area, responsible for the comprehension of words, and Broca’s area, which is concerned with language expression.


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