Introduction: Throughout history a number of speculations have been put forward concerning the functional role of the pineal gland, perhaps some of the most important being the neurophysiological proposals by Descartes.
Development: The psychophysiological role conferred on the pineal gland by Descartes in the 17th century, that is, as the seat of the human soul, had barely any scientific support at the time. Throughout the 18th century interest in the pineal gland gradually declined and it was considered to be just a vestigial evolutionary element until the second half of the 19th century, thanks to the boom of comparative anatomy, which confirmed the photoreceptive role of the parietal organ of the anurans, and of microscopic anatomy, which revealed the histological structure of the pineal gland of mammals.
Conclusions: In the early 20th century the first data were published in the scientific literature on the endocrine aspect of the pineal gland (on the relation between pineal tumours and precocious puberty), but its ultimate confirmation took place in 1958, with the isolation of melatonin by the team led by Aaron B. Lerner. Later, the term 'neuroendocrine transducer' was introduced to explain the principle of the pineal gland, that is to say, the transformation of information about light from the retina into an endocrine response consisting in the synthesis and release of the hormone melatonin. In turn, this hormone acts as a powerful neurotransmitter in the central nervous system, which makes the pineal gland a kind of 'biological clock'.