Ammon's horn is an alternative name for the hippocampus, a brain region involved in learning and memory; although most neuroscientists now use the latter term, you'll still see the former cropping up every now and again. Unfortunately, different scientists used the term in different ways--some used it to refer to the entire hippocampus, while others excluded the dentate gyrus from the definition.

In case you're wondering, Ammon was the ancient horned Egyptian god also known as Amon-Ra; however, the brain structure wasn't named after the god directly but instead got its name through a more circuitous route. Most body parts got their name during Greek or Roman times; in those days, "Ammon's horn" referred to a piece of curved metal jewelry worn around and behind the ear. (You can see it on some old coins, particularly those that show Alexander the Great.) Although the jewelry was probably named after the god, the brain part was likely named after the jewelry, which does in fact bear a startling resemblance to the brain structure--much more so than the seahorse from which the term "hippocampus" is derived.

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