Cerebral Hemispheres
The cerebral hemispheres are literally the two halves of the brain. They are identified on the outside or external surface by the folded hills (gyri) and valleys (sulci). The deep longitudinal fissure runs down the sagital (middle) plane of the brain and separates the two halves of the cerebrum. It clearly shows the separation between the two hemispheres. The hemispheres make up about 83% of the volume of the human brain. They are connected to one another by a thick bundle of nerve fibers, the corpus callosum.
Different lobes have been identified in the hemispheres that are named for the cranial bones that overlie them. The neuroanatomy of the hemispheres is simple in that the majority of the tissues are myelinated white matter covered by a thin layer of gray matter, the neocortex. However, the function of the regions of the hemispheres is complicated. Regions of the hemispheres are responsible for almost all higher functions of the brain such as reasoning and emotions, as well as motor skills. Sensory information is both received (afferent) and sent out (efferent) to various regions of the body. Many of the cranial nerves leave the brain at the base or ventral side of the hemispheres.
One study conducted recently showed that there are differences between male and female hemispheres in the number of neuronal densities. They are higher in males than in females. However, the neurons in females tend to be larger than those in males. Whether or how this leads to any difference in brain functioning is not known. This difference, however, has been identified in the cortex and upper white matter. This may suggest that these neuronal patterns can account for the differences in female's right-handedness, language advantage, and bilateral (both sides) brain activation patterns. In other words, these results may explain why women tend to use both sides of the brain during certain tasks while males use a dominant side.
The anatomy of the cerebral hemispheres has been well mapped. Most of the sulci and gyri have been identified and named. Regions tested for visual, auditory (hearing), gustatory (taste), and other sensory functions have been fairly well located. However, research on the brain's hemispheres remains difficult. Researchers believe they have identified many regions responsible for certain functions, but there are still many questions. Because it is not possible to experiment on human brain tissue as it is with mice or other traditional laboratory animals, much of the information researchers have is gained from work done on other mammals.
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