Leonardo da Vinci, the Ventricles of the Brain, and the Foramen of Monro

Leonardo da Vinci’s neuroanatomical studies include a cast of the ventricles of an ox with melted wax. By depicting the result of this ingenious approach, he provided the first graphic representation of the connection between the cerebral ventricles. We question the widespread assumption that Leonardo already showed the fully paired lateral ventricles and the interventricular foramina today named after Alexander Monro. Instead, we demonstrate by graphical means and magnetic resonance imaging that this is in fact a longstanding misinterpretation of an unusual section plane and its depiction used by Leonardo. Such a midline section fails to show two lateral ventricles and their interventricular foramina both in human and ruminant brains. Our results correct numerous inaccurate statements made in the literature since 1930. With regard to Leonardo’s rendering of the ventricular system, we discuss the realism of his anatomical drawings, the visual techniques he employed, and the legitimation of our approach.