The pineal gland is a tiny, unpaired structure deep in the center of the brain, and its solitary shape, the only major brain structure with no mirror twin on the other side, has made it an object of speculation for as long as anatomists have known it existed. In the seventeenth century the philosopher Rene Descartes proposed it as the seat of the soul, the single point where an immaterial mind could meet a physical body, precisely because it was the one structure in the brain that was not duplicated left and right. Modern physiology gives it a humbler but no less specific job. It secretes melatonin in a rhythm set by light reaching the eyes and relayed through the brain's own internal clock, so melatonin rises in darkness and falls with daylight, governing the timing of sleep. Some reptiles and amphibians carry a related structure that functions as a genuine light sensing third eye beneath the skull, a role the human pineal has lost even as it kept responding to light indirectly.
Hatha yoga's subtle body places its own culminating point at the same location. In Swami Satyananda Saraswati's synthesis in Asana Pranayama Mudra Bandha, the pineal correspondence sits with Sahasrara, the Crown chakra at the top of the head, the center the tradition reads as higher consciousness and union with the divine. Descartes's seat of the soul and the tradition's crown of consciousness both single out the same unpaired gland for a kind of ultimate significance, from two different systems of thought, neither folded into the other here.