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Medical Definition of Locus cinereus
1. A shallow depression, of a blue colour in the fresh brain, lying laterally in the most rostral portion of the rhomboidal fossa near the cerebral aqueduct; it lies near the lateral wall of the fourth ventricle and consists of about 20,000 melanin-pigmented neuronal cell bodies whose norepinephrine-containing axons have a remarkably wide distribution in the cerebellum as well as in the hypothalamus and cerebral cortex. Synonym: substantia ferruginea, locus cinereus, locus ferrugineus. (05 Mar 2000)