The term uncus (unc) refers to a surface feature of the anterior parahippocampal gyrus (PHGa) where it "hooks around the hippocampal fissure to form a medially protruding convolution" in the human( Carpenter-1983 ) and the macaque ( Paxinos-2009a )). In the human, the topologically defined structures underlying the unc surface include the anterodorsal part of the entorhinal cortex where it covers the amygdala rostrally and the portion of the hippocampal complex (HPC) that bends back on itself caudally ( Mai-1997 ). In the macaque it is formed entirely by the hook portion of the HPC ( Paxinos-2009a ). Brain atlases of the rat and mouse show no equivalent feature. Many authors use the term 'uncus' and synonyms, such as 'uncinate gyrus', in reference to the combination of the topologically defined surface feature and the underlying histologically defined, volumetric components ( Anthoney-1994 ). In that sense, it is a hook-shaped substructure of the anterior-most part of the anterior parahippocampal gyrus ( Crosby-1962 ). NeuroNames treats the unc as a surface feature, because the boundaries of the volumetric 'uncinate gyrus' do not correspond to the boundaries of its internal, histologically defined structures, and because activation of the histologically defined structures correlates significantly better with functions of the individual structures ( Glasser-2016 ). It is useful as a gross anatomical landmark, but not as a volumetric structure for quantitative neuroanatomical analyses ( Martin-1996 ). . Updated 17 Jul 2026..