The cerebrum, constituting about 87.5% of the brain weight, spreads over the diencephalon. The cerebral cortex is the outer layer of the brain and is composed of gray matter made up of nerve cell bodies. It is about 0.08 in (2 mm) thick and its surface area is about 5 sq ft (1.5 sq m)—around half the size of an office desk. White matter, composed of nerve fibers covered with myelin sheaths, lies beneath the gray matter. With the rapid growth of the brain during embryonic development, the gray matter grows faster than the white matter and folds on itself. The folds are called convolutions or gyri, and the grooves between them are known as sulci. A deep longitudinal fissure separates the cerebrum into a left and right hemisphere. Each cerebral hemisphere is divided into frontal, temporal, parietal, and occipital lobes. The corpus callosum, a large bundle of fibers, connects the two cerebral hemispheres. The thalamus and subcortical nuclei, or basal ganglia, are areas of gray matter that exist below the white matter.
Sensory areas of the cerebrum interpret sensory impulses. Spoken and written language are transmitted to a part of the cerebrum called Wernicke's area where meaning is extracted, and sent to Broca's area, one of the motor areas of the cerebrum. Motor areas of the cerebrum control muscle movements. Within Broca's area, thoughts are translated into speech, and muscles are coordinated for speaking. Impulses from other motor areas direct our hand muscles when we write, and our eye muscles when we scan the page for information.
Association areas of the cerebrum are concerned with emotions and intellectual processes, by connecting sensory and motor functions. In our association areas, innumerable impulses are processed that result in memory, emotions, judgment, personality, and intelligence.
Certain structures in the cerebrum and diencephalon make up the limbic system. These regions function in memory and emotions, and are associated with pain and pleasure.
By studying patients whose corpus callosum were destroyed, scientists realized that differences existed between the left and right sides of the cerebral cortex. The left side of the brain functions mainly in speech, logic, writing, and arithmetic. The right side of the brain, on the other hand, is more concerned with imagination, art, symbols, and spatial relations.
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