The forebrain consists of two large, coiled and grooved cerebral hemispheres. The two cerebral hemispheres are collectively called the cerebrum. It is the largest part of the brain (80% of brain weight). The cerebrum has two layers. The outer one is called the cerebral cortex (3 cm thick) and the inner one is called the cerebral medulla. The cerebral cortex is composed of gray matter and the cerebral medulla is composed of white matter. The cerebral hemispheres are connected by a bundle of nerves called the corpus callosum. Its surface is folded to create high and low areas. The higher area is called the gyrus and the lower area is called the sulcus.
A few folds deepen to form 3 fissures (central, parieto-occipital, lateral). Each cerebral hemisphere is divided into 5 lobes (frontal lobe, parietal lobe, occipital lobe, temporal lobe and limbic lobe).