In scientific studies of the pineal gland, little progress was made until the second half of the nineteenth century. As late as 1828, Magendie could still advance the theory that Galen had dismissed and Qusta ibn Luca had embraced: he suggested that it is “a valve designed to open and close the cerebral aqueduct” (Magendie 1828). Towards the end of the nineteenth century, however, the situation started to change (Zrenner 1985). First, several scientists independently launched the hypothesis that the pineal gland is a phylogenic relic, a vestige of a dorsal third eye. A modified form of this theory is still accepted today. Second, scientists began to surmise that the pineal gland is an endocrine organ. This hypothesis was fully established in the twentieth century. The hormone secreted by the pineal gland, melatonin, was first isolated in 1958. Melatonin is secreted in a circadian rhythm, which is interesting in view of the hypothesis that the pineal gland is a vestigial third eye. Melatonin was hailed as a “wonder drug” in the 1990s and then became one of the best-sold health supplements. The history of pineal gland research in the twentieth century has received some attention from philosophers of science (Young 1973, McMullen 1979), but this was only a short-lived discussion.