The cerebral hemisphere is one half of the brain, either the right or the left. In humans, the two hemispheres are symmetrical, but not perfectly so. Making up the external part of the brain, and surrounded by a grey matter called the cortex, these hemispheres are lined with grooves that outline the different brain areas, or lobes. Each hemisphere is the site of functions controlling perception and motor function of the opposite side of the body
. This is why every person is said to have a dominant hemisphere.