From Concealed Neuroanatomy in Michelangelo’s Separation of Light From Darkness in the Sistine Chapel in the May, 2010, issue of Neurosurgery:
In the winter of 1511, Michelangelo entered the final stages of the Sistine Chapel project and painted 4 frescoes along the longitudinal apex of the vault, which completed a series of 9 central panels depicting scenes from the Book of Genesis. It is reported that Michelangelo concealed an image of the brain in the first of these last 4 panels, namely, the Creation of Adam. Here we present evidence that he concealed another neuronanatomic structure in the final panel of this series, the Separation of Light From Darkness, specifically a ventral view of the brainstem.
Could be pareidolia, but given Michelangelo’s breakthroughs in anatomical rendering and that God is depicted here with a rather non-standard bearded neck, who knows?