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).

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).

Gray matter ( GM ; Meckel, 1817 ) : Since the 16th century the nervous system (Monro, 1783) has been divided more and more precisely into gray matter and white matter (Meckel, 1817) based on their appearance in freshly dissected material observed with the naked eye-topographic macroarchitecture of nervous system; since the 19th century this differentiation has been made at the histological (subsystems microarchitecture of nervous system) level. Gray matter is the nervous system compartment that consists of the cell bodies (Deiters, 1865) of neurons (Waldeyer, 1891); the cytoplasmic neuron extensions: axons (Kölliker, 1896), dendrites (His, 1889), and amacrine extensions; and synapses (Foster & Sherrington, 1897) between the neuron extensions-as well as glia (Virchow, 1846) and parts of the circulatory system: vascular cells. Neuropil (Waldeyer, 1891) refers to the gray matter compartment exclusive of cell bodies and vascular cells (blood vessels) and thus consists of the cytoplasmic extensions of neurons and glia, including synapses. There is often a fuzzy border of variable and difficult to measure width between white matter and gray matter. For early history see Clarke & O'Malley (1996, Ch. 10); for modern histological interpretation see Peters et al. (1991), Swanson (2003, pp. 60-66). The general term "nervous system gray matter" as defined here was probably first used by Meckel for macrodissected adult humans (1817, see English translation, 1832 vol. 2, pp. 152-154, 166-167); also see Herrick (1915, p. 108).

Peripheral nervous system ( PNS ; Meckel, 1817 ) : In bilateral animals, a topographic division with condensations of the nervous system (Monro, 1783) consisting of nerves (Herophilus, c335-280 BC) and ganglia (Galen, c173). When a PNS is present, its obligate companion topographic division is a central nervous system (Meckel, 1817). There is no known isolated part of the PNS that does not have a connection to the rest of the nervous system; see Bullock & Horridge (1965, pp. 9-14). While the nerves were discovered by Herophilus (335-280 BC; see Solmsen, 1961, p. 185; von Staden, 1989, pp. 250-252), the term peripheral nervous system as currently understood was first used by Meckel (1817; see English translation 1832, p. 153).

Tegmentum ( Meckel, 1817 ) : The tegmentum (Swanson, 2000) without the cerebral peduncle (Tarin, 1753) white matter tract; see English translation (1832, vol. 2, p. 467), "Haube" in the original German. The Latin "tegmentum" (see Burdach, 1822, p. 101, who used the same definition) means "cap" or "covering" in English, and Meckel described it as such for the cerebral peduncle (Tarin, 1753). Hence the synonym for cerebral peduncle (Tarin, 1753), basis pedunculi (Meckel 1817; see English translation, 1832, vol. 2, p. 467); also see Ranson (1920, p. 158). Unfortunately, basis pedunculi later acquired a second meaning, the cerebral peduncle (Tarin, 1753) together with the substantia nigra (Soemmerring, 1791); see Henle (1871, p. 244), Strong & Elwyn (1943, p. 228), Crosby et al. (1962, pp. 221, 260).

White matter ( WM ; Meckel, 1817 ) : Since the 16th century the nervous system (Monro, 1783) has been divided more and more precisely into gray matter (Meckel, 1817) and white matter based on their appearance in freshly dissected material observed with the naked eye (macroarchitecture); since the 19th century this differentiation has been made at the histological (microarchitecture) level. White matter is the nervous system compartment that consists primarily of axons (Kölliker, 1896). Its name is derived from its appearance in fresh and fixed macroscopically observed material, its whitish color due to the presence of myelinated axons. However, white matter is often a mixture of myelinated axons and unmyelinated axons, and entire aggregates of axons can be unmyelinated. Therefore, as defined here white matter is a generic term for a nervous system volume where axons are the predominant neural component, although of course glia (Virchow, 1846) and parts of the circulatory system, vascular cells, are also present. White matter can contain scattered neurons (Waldeyer, 1891) that may be assigned either to an adjacent gray matter region, or to a new gray matter region embedded (nested) within the white matter; assignment depends on differentiable neuron types (Bota & Swanson, 2007) involved. There is often a fuzzy border of variable width and difficult to measure between gray matter and white matter. This use of the term was probably introduced by Meckel; see English translation (1832, vol. 2, pp. 152-154), also see Herrick (1915, p. 108). For early history see Clarke & O'Malley (1996, Ch. 10); for modern histological interpretation see Peters et al. (1991), Swanson (2003, pp. 60-66).