Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord (together called the central nervous system, or CNS) and the routes of the nerves that connect to the rest of the body (known as the peripheral nervous system, or PNS). The delineation of distinct structures and regions of the nervous system has been critical in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions. For information about the composition of non-human animal nervous systems, see nervous system. For information about the typical structure of the Homo sapiens nervous system, see human brain or peripheral nervous system. This article discusses information pertinent to the study of neuroanatomy. History The first known written record of a study of the anatomy of the human brain is an ancient Egyptian document, the Edwin Smith Papyrus. In Ancient Greece, interest in the brain began with the work of Alcmaeon, who appeared to have dissected the eye and related the brain to vision. He also suggested that the brain, not the heart, was the organ that ruled the body (what Stoics would call the hegemonikon) and that the senses were dependent on the brain. The debate regarding the hegemonikon persisted among ancient Greek philosophers and physicians for a very long time. Those who argued for the brain often contributed to the understanding of neuroanatomy as well. Herophilus and Erasistratus of Alexandria were perhaps the most influential with their studies involving dissecting human brains, affirming the distinction between the cerebrum and the cerebellum, and identifying the ventricles and the dura mater. The Greek physician and philosopher Galen, likewise, argued strongly for the brain as the organ responsible for sensation and voluntary motion, as evidenced by his research on the neuroanatomy of oxen, Barbary apes, and other animals. The cultural taboo on human dissection continued for several hundred years afterward, which brought no major progress in the understanding of the anatomy of the brain or of the nervous system. However, Pope Sixtus IV effectively revitalized the