Also called cornu ammonis, the Ammon's horn is one of the two interlocking gyri which make up the hippocampus of the brain; the other is the dentate gyrus. In anatomy "Ammon's horns" is abbreviated CA (for Cornu Ammonis). It is differentiated into four fields: CA1, CA2, CA3, CA4.
The Ammon's horn regions are also structured depthwise in clearly defined strata (or layers): 1) the alveus is the most superficial layer and contains the axons from pyramidal neurons, passing on toward the fimbria/fornix, one of the major outputs of the hippocampus; 2) stratum oriens (str. oriens) is the next layer below the alveus, where the cell bodies of inhibitory basket cells and horizontal trilaminar cells are located; 3) stratum pyramidale (str. pyr.) contains the cell bodies of the pyramidal neurons which are the principal excitatory neurons of the hippocampus; 4) stratum lucidum (str. luc.) is one of the thinnest strata in the hippocampus where mossy fibers from the dentate gyrus granule cells can be found; 5) stratum radiatum (str. rad.), like str. oriens, contains septal and commissural fibers as well as Schaffer collateral fibers; 6) stratum lacunosum (str. lac.) is a thin stratum that too contains Schaffer collateral fibers, but it also contains perforant path fibers from the superficial layers of entorhinal cortex; 7) stratum moleculare (str. mol.) is the deepest stratum in the hippocampus.
The term Ammon's horn is a metaphor which refers to the ram shaped horns' on the head representing the Egyptian God Amun, who protected the Pharoh Teharqa in the temple of Kawa. The Greek form of the name was Ammon.
Cross section slice showing Ammon's horn and dentate gyrus