Nervous system ( NS ; Monro, 1783 ) : One system in the set of body systems. According to Bullock & Horridge, "A nervous system may be defined as an organized constellation of cells (neurons) specialized for the repeated conduction of an excited state from receptor sites or from other neurons to effectors or to other neurons." (1965, p. 6; also see Brusca & Brusca, 1990, p. 81). The structural arrangement or organization of connections between these neurons (Waldeyer, 1891), or more generally nodes, forms a neural network called the wiring diagram of the nervous system (Monro, 1783). In many animals the nervous system also contains glia (Virchow, 1846), and is invaded by the circulatory system. It is the chief system that integrates adjustments and reactions of the organism to internal and environmental conditions; see Dorland's (2003). In all animals the nervous system probably differentiates from the embryonic ectodermal layer; see Brusca & Brusca (1990, p. 103). The nervous system as such was probably first recognized (in writings that survive) by Rufus of Ephesus (fl. c100; see Clarke & O'Malley, 1996, p. 13); Monro (1783, p. 1) introduced the term as now used.