DAVID KRONEMYER: The Codex Gigas is a medieval manuscript containing a version of the Bible and various other then-current tracts. It is most notable for its large depiction of the devil:
The primary motor cortex is a region of the brain that is controls muscles and executes movements when given a brain command to do so. Different parts of it correspond to different functional areas of activity. It typically is depicted as a “homunculus” or “little man.” It is not to scale in the sense that some areas of the body (such as the hands) require more refined muscle control than others. They are assigned a correspondingly greater area of the primary motor cortex. There is of course no actual homunculus inside of the brain that is directing motor activity. Rather it is a representation of how the various parts of the primary motor cortex are assigned to separate muscle groups. Here is a picture of how this looks:
It occurred to me how much the depiction of the devil in the codex resembled that of the homunculus in modern neuroanatomy. Perhaps our medieval counterparts had inadvertently stumbled onto something. Consider for example the maxim that “idle hands are the devil’s playground.” Given the close resemblance in the depiction of the hands perhaps this aphorism has some neurophysiological basis.