Gray matter region : A recognizable volume of gray matter (Meckel, 1817) in the nervous system (Monro, 1783) that is distinguished by a unique set of neuron types (Bota & Swanson, 2007) with a unique spatial distribution. The entire gray matter is regionalized and individual regions may contain white matter (Meckel, 1817), including axons-of-passage, which are axons (K├Âlliker, 1896) passing through without forming synapses (Foster & Sherrington, 1897). The traditional way to view regionalization is with a Nissl stain, whose interpretation is based on methods that identify neuron types (Bota & Swanson, 2007) and their spatial distribution; see Brodal (1981, p. 4), Swanson (2004, p. 8). Examples of gray matter regions include cerebral cortical areas, thalamic nuclei, and peripheral nervous system ganglia. As a complete set, gray matter regions can be arranged in various different ways, for example, topographic arrangement of gray matter regions and subsystems arrangement of gray matter regions. Older synonyms include gray mass (Meynert 1872, p. 651), gray matter mass (Meynert, 1872, p. 654).

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Cell group : Often used as a synonym for gray matter region; see Cajal (1909, p. 41), Clark (1938, p. 9), Berman (1968, p. xi), Nauta & Haymaker (1969, pp. 141-142), Swanson (2003, p. 64). Synonyms include cellular aggregate (see Berman, 1968, p. xi), cell collection (see Berman, 1968, p. xi).