The term area 18 of Brodmann (guenon) refers to a subdivision of the cerebral cortex of the guenon defined on the basis of cytoarchitecture. It is topographically and cytoarchitecturally homologous to area 18 of Brodmann (human) ( Brodmann-1909 ). Distinctive features ( Brodmann-1905 ): a wide, dense internal granular layer (IV); a distinct sublayer 3b of closely packed large pyramidal cells positioned in the external pyramidal layer (III) directly above layer IV; an almost cell free, narrow internal pyramidal layer (V) with no larger ganglion cells; a likewise very narrow, dense multiform layer (VI) composed of small polymorphic cells that form a distinct boundary with the underlying subcortical white matter. Like area 17 of Brodmann-1905, area 18 is relatively thin; the three deep layers are thin relative to the three outer layers; distinct boundaries between layers; abundance of granule cells; narrow layer VI; and sharp boundary between cortex and subcortical white matter.