Through painstaking study, scientists have established how and where various functions are localized in the brain. The largest portion of the human brain is the cerebrum (SER-ee-bruhm). The cerebrum is easily identified by its highly folded outer layer. It is composed of two cerebral hemispheres, as shown in Figure 49-6. The two cerebral hemispheres are connected by the corpus callosum (KAWR-puhs kuh-LOH-suhm). The corpus callosum is a band of axons that lies deep in the central groove that separates the right hemisphere from the left hemisphere. Other grooves separate each hemisphere into four lobes: the frontal, parietal (puh-RIE-uh-tuhl), temporal, and occipital (ahk-SIP-i-tuhl) lobes.
The folded outer layer of the cerebrum is the cerebral cortex, which contains 10 to 20 percent of the brain's total number of neurons. The many folds allow the surface area of the cortex to fit within the skull. As shown in Figure 49-6, different parts of the cerebral cortex control information and sensations from the body and motor responses. For example, the area of the cortex that interprets touch information lies in the parietal lobe.
Some functions are not symmetrically localized in the cerebrum. Brain centers involved in speech and language reside primarily in the left hemisphere. Brain centers involved in processing spatial information and certain kinds of reasoning are located primarily in the right hemisphere. These functions are located variably in left-handed people.
Below the folded surface of the cerebral cortex lies the white matter, which is composed of myelinated axons. These axons link specific regions of the cortex with each other and with other neural centers. Because of the crossover of these axons as they enter the brain from the body, many impulses originating in the right half of the body are processed in the left half of the brain, and vice versa.
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