Sometimes referred to as the ‘third eye’, it haslong been associated with the claim made by René Descartes (1596-1650) that the gland was the‘seat of the soul’. It is a smallendocrine gland located in the epithalamus whose primary function is to producemelatonin and serotonin (a precursor of melatonin). The pineal gland is richly innervated byadrenergic neurons that play an active role in this function. Because it is the‘manufacturing center’ for melatonin, the gland plays a important role in theregulation of the circadian rhythms and thus the patterning of sleep via thesuprachiasmatic nucleus. In humans, thegland gradually increases in size from prenatal to postnatal life until about 2 years ofage (adult: 7x8x6 mm), with an increase in weight during and after puberty (adult: 172 mg). Melatonin is noticeably reduced duringpuberty, which tends to coincide with the onset of sexual activity. The figure below indicates the location of the pineal gland relative to other sub-cortical structures.
