Volumetric comparison of hippocampal regions in 44 primate species.

Volumes of retrocommissural hippocampal regions (Subiculum, CA 1, CA 2, CA 3, hilus region, and fascia dentata) were measured using serial sections of 44 species of prosimian and simian brains, including man. The volumes were compared allometrically with those of the least encephalized eutherian mammals, the madagassian tenrecs (Insectivora, Tenrecinae; 4 species). The retrocommissural hippocampus is 2.9 times larger in prosimians, 2.4 times in non-human simians, and 4.1 times in man. The different hippocampal regions do not enlarge uniformly to the same degree as the total hippocampus. The regions can be grouped into three categories, according to the degrees to which they enlarge in primates: a) Structures with the greatest degree of enlargement are the subiculum and the field CA 1. b) An enlargement similar to that of the total retrocommissural hippocampus is seen for the hilus region. c) No or only a slight enlargement as compared to their sizes in tenrecs is observed for the hippocampal fields CA 2, CA 3, and the fascia dentata.