The Discoverers of the Hippocampus If some authors could trace the discovery of the hippocampus back to early times – Galen according to Caspar Hofmann (1625), Hippocrates according to Isbrand van Diemerbroeck (1695) -, the apocryphal nature of Hippocratic and Galenic texts currently available for consultation does not let us ratify this assumption.

The term hippocampus, applied to the seahorse, was coined in the 1566 Lyons French translation of Pline's Historia naturalis by Antoine du Pinet, seigneur of Noroy. But surprisingly, both discoverers of the cerebral hippocampus are not remembered as forerunners in neuroanatomy : the first one, Aranzio, gave his name to the nodules of the aortic valve cusps and to the ligamentum venosum of the liver; the second one, Eustachi, to the flap-like valve of the inferior vena cava and to the pharyngotympanic tube.

The first accurate engraved plate depicting a cerebral hippocampus A rough sketch of the hippocampus is to be found in Constanzio Varoli's De nervis opticis (1573), but this author is much more remembered for another contribution to neurosciences : the image « bridge » he coined to describe the intermediate part of the brain stem : the pons, or anular protuberance. Thus, the first accurate plate depicting the hippocampus is probably to be found in Bartolomeo Eustachi's Tabulae anatomicae, edited by Giovani Maria Lancisi only in 1714, that is to say one hundred and forty years after Eustachi's death. The plates were drawn by the author himself and Pier Matteo Pini, and engraved in 1552 by Giulio de Musi.

The hippocampus becomes a ram, a phoenician god, and even a hippopotamus The morphology of the hippocampus led Jacques-Bénigne Winslow to compare it with a ram's horn (1732), and short after appeared the term Ammon's horn. Unlike what could still be read today (Standring, 2005), the term Ammon's horn is not related to the German nineteenth-century pathologist Friedrich August von Ammon, but refers to the ancient Phoenician god Amon. As far back as in 1742, the French surgeon René-Jacques Croissant de Garengeot already wrote : « (…) and that is why we call them Ammon's horns. » In 1779, Johann Christoph Andreas Mayer proposed to replace the term hippocampus by hippopotamus. Where could such an astonishing idea find its roots? Probably in Greek mythology that depicts Poseidon's chariot usually towed by seahorses, but also sometimes – as told us Pierre Belon in 1553 – by hippos.

Since that time, the metaphor coined by Aranzio proved to be very successful : the term hippocampus was extrapolated to at least fourteen cerebral formations : hippocampi accessorius, anterior, dorsalis, inversus, minor, major, nudus, pericallosalis, praecallosalis, praecommissuralis, proper, retrocommissuralis, supracallosalis and supracommissuralis (Olry and Martinoli, 1995).

Suggested readings Lewis F.T. (1922-1923) The significance of the term hippocampus. Journal of Comparative Neurology 35 : 213. Olry R. (1991) Métaphores zoologiques au sein des ventricules latéraux du cerveau, ou l'imagination au service de la linguistique. Histoire des Sciences Médicales 25 (3) : 221-224.

Bibliography Aranzio J.C. (1587) De humano foetu liber tertio editus, ac recognitus. Ejusdem Anatomicarum observationum liber : ac de tumoribus secundum locos affectos liber nunc primum editi. Venetiis : apud Jacobum Brechtanum. Belon P. (1553) De aquatilibus, libri duo cum eiconibus ad viuam ipsorum effigiem, quoad eius fieri potuit, expressis. Parisiis : apud Carolum Stephanum. Croissant de Garengeot R.J. (1742) Splanchnologie, ou l'anatomie des viscères, avec des figures originales tirées des cadavres, suivie d'une dissertation sur l'origine de la chirurgie. Paris : Ch. Osmont. Diemerbroeck I. van (1695) L'anatomie du corps humain. Lyon : Anisson & Posuel. Eustachi B. (1714) Tabulae anatomicae quas è tenebris tandem vindicatas. Romae : Ex Officina typographica Francisci Gonzagae. Hofmann C. (1625) Commentarii in Galeni De usu partium corporis humani lib. XVII. Francofurti ad Moenum : Typis Wechelianis, Apud Danielem & Davidem Aubrios, & Clementem Schleichium. Mayer J.C.A. (1779) Anatomisch-physiologische Abhandlungen vom Gehirn, Rückenmark und Ursprung der Nerven. Berlin Leipzig : G.J. Decker. Molinetti A. (1675) Dissertationes anatomico-pathologicae, quibus humani corporis partes accuratissime describuntur morbique singulas divexantes explicantur. Opus philosophis utile, medicis vero necessarium. Venetiis : Apud Paulum Balleonium. Olry R., Martinoli M.G. (1995) Hippocampal genealogy : analysis of a prolific semantic lineage. Society for Neuroscience Abstracts 21 (1) : 246. Standring S. (Ed) (2005) Gray's Anatomy. The Anatomical Basis of Clinical Practice. Edinburgh' : Elsevier Churchill Livingstone, 39th edition, p. 1551. Varoli C. (1573) De nervis opticis. Padua : P. & A. Meitti. Winslow J.B. (1732) Exposition anatomique de la structure du corps humain. Paris : G. Desprez.

Illustration A plate showing a cerebral hemisphere and the hippocampus. Taken from Johann Georg Duvernoy (1729) De sinibus cerebri, Commentarii Academiae scientiarum imperialis Petropolitanae 4 : 130-135.

Régis OLRY, MD Professeur Département de chimie-biologie Université du Québec à Trois-Rivières