The human brain, photographed in lateral view with the hindbrain at left, showing the heavily folded exterior of the cerebrum, the cerebral cortex (the grey matter). Comprising some 40% of the weight of the brain, the cerebral cortex is involved in consciousness, memory, mental ability and intellect; it is also responsible for initiating voluntary activity. The centres for motor and sensory control of the body are present throughout the cortex. Below the rear occipital lobe of the cerebral cortex (bottom left) protrudes the cerebellum, which acts as a coordination centre for muscular activity and balance which is needed for the synchronization of movement.