"Lack of hippocampal volume change in long-term heavy cannabis users."
http://marijuana.researchtoday.net/archive/2/4/358.htm

It forms a part of the limbic system and plays a part in memory and navigation.
In Alzheimer's disease, the hippocampus becomes one of the first regions of the brain to suffer attack; memory problems and disorientation appear amongst the first symptoms.
the hippocampus is among the phylogenetically (the evolutionary development of an organ) oldest parts of the brain.
http://encyclopedia.tfd.com/hippocampus

Images of Hippocampus
http://brainmaps.org/index.php?i=Hippocampus