The hippocampus is subdivided into four parts according to width, cell size, and cell density:
! Field CA1 (A1) contains small pyramidalcells.
! Field CA2 (A2) is characterized by a nar-row, dense band of large pyramidal cells.
! Field CA3 (A3) is characterized by a wideloose band of large pyramidal cells.
! Field CA4 (A4) forms the loosely struc-tured inner zone. Recently, it has been called into question whether a separate CA4 region can be delimited from the CA3 region.
The narrow band of densely packed granule cells of the dentate gyrus (fascia dentata) (A5) surrounds the ending band of pyra-midal cells. The dentate gyrus is fused with the surface of the curled-up Ammon’s horn and appears only partially at the surface of the brain. It is separated by the hippocampalsulcus (A6) from the parahippocampal gyrus (A7) and by the fimbriodentate sulcus (A8) from the fimbria of the hippocampus (A9). The inner layer bordering on the ventricle is the alveus of the hippocampus (A10), in which the efferent fibers collect before leav-ing the hippocampus via the fimbria. The transitional area between Ammon’s horn and the bordering entorhinal cortex (A11) is called the subiculum (A12).