Neuroanatomy is the study of the structure and organizationof the nervous system. In contrast to animals with radial symmetry, whosenervous system consists of a distributed network of cells, animals withbilateral symmetry have segregated, defined nervous systems. Their neuroanatomyis therefore better understood. In vertebrates, the nervous system issegregated into the internal structure of the brain and spinal cord (togethercalled the central nervous system, or CNS) and the routes of the nerves that connectto the rest of the body (known as the peripheral nervous system, or PNS). Thedelineation of distinct structures and regions of the nervous system has beencritical in investigating how it works. For example, much of whatneuroscientists have learned comes from observing how damage or"lesions" to specific brain areas affects behavior or other neuralfunctions.