The appearance of the brain, a greyish-pink mass that is wrinkled like a walnut, does not do justice to its remarkable capacities. The brain accounts for around 2% of a person’s body weight and is made up of billions of nerve cells called neurones. It is one part of the encephalon (along with the diencephalon, brain stem and cerebellum) and is made up of two hemispheres of approximately equal sizes. The surface of these hemispheres is almost entirely covered with folds, known as gyri, separated by superficial ridges. The brain is mainly made of a whitish material (white matter), while its surface is covered with a thin layer of greyish material (cerebral cortex or grey matter). This difference in colour is due to the fact that the cortex mainly contains the neurones’ cellular bodies, whereas the white matter consists of a mass of axons that connect the various cortical areas together. At the centre of the brain, four ventricles (large C-shaped cavities) communicate with each other. The cerebrospinal fluid (or cephalorachidian fluid) inside these ventricles and around the brain, acts as a cushion and protects the brain from trauma.