Central nervous system ( CNS ; Meckel, 1817 ) : In animals with bilateral symmetry, a topographic division that is an obvious condensation of the nervous system (Monro, 1783) in the longitudinal plane (Henle, 1855), lying on or near the median plane (Henle, 1855). For invertebrates this longitudinal division consists of one or more nerve cords, whereas for vertebrates it consists of a single, hollow, and dorsal (Barckay, 1803) cerebrospinal axis (Meckel, 1817). In adult Echinoderms, which are radially symmetrical, a presumptive CNS is formed by a circular cord with associated radial cords, but there is no dominant ganglion (Galen, c173) that could be considered an invertebrate brain; see Bullock & Horridge (1965, pp. 9-14), Heinzeller & Welsch (2001, p. 41). When a CNS is present, its obligate companion topographic division is a peripheral nervous system (Meckel, 1817). While a continuous brain (Edwin Smith Surgical Papyrus, c1700 BC) and spinal cord (Galen, c162-c166) were known to Hippocrates in On the Sacred Disease and Fleshes (see translations by Adams, 1972, p. 351; and Potter, 1995, p. 139, respectively), the term central nervous system as currently understood for vertebrates was first used by Meckel (1817; see English translation, 1832, vol. 1, p. 152).
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