Definition: The complex, internally convoluted structure that forms the medial margin (“hem”) of the cortical mantle of the cerebral hemisphere, bordering the choroid fissure of the lateral ventricle, and composed of two gyri (Ammon horn and the dentate gyrus), together with their white matter, the alveus and fimbria hippocampi. In monkeys, apes, and humans, the hippocampus is confined to the temporal lobe by the massive development of the corpus callosum. Cytoarchitecturally a unique form of allocortex (archicortex), the hippocampus forms part of the limbic system (formerly rhinencephalon). Its major afferent connections are with the entorhinal area of the parahippocampal gyrus and with the transparent septum; by way of the fornix, it projects to the septum, anterior nucleus of the thalamus, and mammillary body.
[G. hippocampos, seahorse]