What is the brain?

From a structural perspective, there are ten basic parts of the vertebrate CNS that are almost universally agreed upon. These parts have been grouped in at least five different ways corresponding to five different theories about its basic plan or architecture. Two classical models that remain popular today are derived from (1) comparative anatomy and the body's segmental organization, and (2) comparative embryology and the neural tube's transverse and longitudinal organization. A new approach is concerned with deciphering the genetic program that assembles the nervous system during embryogenesis; how it will correspond to the other models remains to be determined. The simplest current model to explain the organization of the mammalian nervous system involves a segmental trunk that mediates reflex sensory-motor functions, and suprasegmental cerebral hemispheres and cerebellum.