The corpus callosum is the band, known as a commissure, of white matter that provides a connection between the two hemispheres of the cerebrum. It consists of about 200 million axons. The upper surface of the corpus callosum has a relatively thin layer of gray matter. The underside forms the roof of the lateral and third ventricles.
Humans and, to a much lesser extent, great apes, have evolved an asymmetry of the brain whereby the two hemispheres are specialized in different functions and this, perhaps, is partly responsible for our great intellectual abilities. The corpus callosum is the main channel through which nerve transmissions between the two pass. Although it is unclear exactly what the roles of these transmissions are, it is clear that motor, sensory, visual and other functions are involved.
The other main connection between the two hemispheres of the brain is called the anterior commissure.