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of the soul: by the soul, or for that matter by any other cause, it pushes the animal spirits which surround it to the pores of the brain.® spirit? in the brain's recesses and is not directly sensitive to light, at least in adults.° It is interesting to speculate that as the pineal assumes a more 60 +* THE BUILDING BLOCKS are expressions of, and proof for the existence of, our soul. Descartes believed that the pineal gland played an essential role in the expression Although the soul is joined with the entire body, there is one part of the body [the pineal] in which it exercises its function more than elsewhere. . . . [The pineal] is so suspended between the passages containing the animal spirits [guiding reason and carrying sensation and movement] that it can be moved by them . . . ; and it carries this motion on to the soul. . . . Then conversely, the bodily machine is so constituted that whenever the gland is moved in one way or another Descartes thus proposed that the pineal gland somehow was the "seat of the soul," the intermediary between the spiritual and physical. The body and the spirit met there, each affecting the other, and the repercus- sions extended in both directions. How close to the truth was Descartes? What do we know now about the biology of the pineal gland? Can we relate this biology to the nature of The pineal gland of evolutionarily older animals, such as lizards and am- phibians, is also called the "third" eye. Just like the two seeing eyes, the third eye possesses a lens, cornea, and retina. It is light-sensitive and helps regulate body temperature and skin coloration—two basic survival functions intimately related to environmental light. Melatonin, the pri- mary pineal hormone, is present in primitive pineal glands. As animals climbed the evolutionary ladder, the pineal moved in- ward, deeper into the brain, more hidden and removed from outside influences. While the bird pineal no longer sits on top of the skull, it remains sensitive to outside light because of the paper-thin surrounding bones. The mammalian, including human, pineal is buried even deeper