The brain is an organ weighting about \(1.4\) kg, with a volume of \(1.3\) l, and a total surface area of about \(2{,}500\) cm \(^{2}\) (e.g., \(0.25\) m \(^{2}\) , which are equivalent to four A4 paper sheets). The brain is part of the central nervous system (CNS) together with the medulla oblongata, the cerebellum, and the spinal cord (note that in the brain, the gray matter is external and the white matter is internal, while in the spinal cord, the gray matter is internal and the white matter is external). The peripheral nervous system (PNS) consists mainly of the nerves that leave the spinal cord and go to the various organs and districts of the body. Functionally, there is also the autonomic nervous system,which we will not deal with, however.