The pineal body (corpus pineale), or the pineal gland, is a small, pinecone-shaped endocrine gland located at the posterior portion of the third ventricle of the brain. The pineal gland, which was known to physicians in Antiquity, has been a mystifying subject to scientists. René Descartes, who, while maintaining that the soul and the body, including the brain, are two essentially different substances, identified the pineal body as the point of contact which enables the immaterial soul to affect the material body. At one time, the pineal gland was thought to be a vestigial third eye, due to the fact that in some lower animals, such as fishes and amphibians, the pineal gland is comprised of cells that are similar to the specialized, neuroepithelial cells of the retina that respond to light. Fossilized skulls of ostraderms, a type of jawless fish, reveal holes for three...