Cerebrospinal axis ( CSA ; Meckel, 1817 ) : In popular usage it is a topographic division that corresponds to the vertebrate central nervous system (Meckel, 1817), although in fact it applies more generally to the chordate central nervous system (Meckel, 1817), a hollow tubular division of the nervous system (Monro, 1783) that lies in the median plane (Henle, 1855), dorsal (Barclay, 1803) to a notochord and flanked by a bilateral (Aristotle) series of segmental muscles (Nieuwenhuys, 2002). Since Classical Antiquity at least six fundamentally different ways to divide the cerebrospinal axis have been used, although today there is rather broad consensus about a set of major topographic divisions of cerebrospinal axis. The set of divisions is arranged in a hierarchical parceling scheme that is based primarily on structural differentiation of the neural tube (Baer, 1837) and its end product is adult macrostructure or gross anatomy (Swanson, 2000). The term cerebrospinal axis was introduced by Meckel (1817; see English translation, 1832, vol. 2, p. 410) for macrodissected adult humans although the feature itself was known to Herophilus (335-280 BC; see von Staden, 1989, p. 159).