The 'worm' in our brain. An anatomical, historical, and philological study on the vermis cerebelli.

The cell doctrine-the theory of ventricular localization of the mental faculties-includes Galen's idea of a locking or valve mechanism between the middle and the rear ventricle. The anatomical substrate was the vermiform epiphysis, known today as the . This entity played a significant role in brain physiology even though its appearance, texture, and location changed over time. This article tells the story of the "worm's" transformation from Galen to Vesalius and beyond. Until the time of Albertus Magnus (c. 1200-1280 ce), the worm corresponded to the . From the beginning of the fourteenth century, under the influence of Mondino's , the worm referred to the choroid plexus in the anterior ventricles; its Galenic heritage was abandoned. Contemporaneous illustrations confirm this anterograde movement. The contributions of post-Galenic natural philosophers and pre-Vesalian anatomists to this development are discussed. Today, the worm can serve as an example for different viewpoints and often deadlocked doctrines (religious, philosophic, scientific). In tracing beliefs about the worm from the Greeks to the Arabs and back to the Latin West, this article follows the history of neuroanatomy in the Middle Ages and the Renaissance.