PINEAL GLAND

also called Pineal Body, or Epiphysis Cerebri, an endocrine gland found in vertebrates that regulates the production of the hormone melatonin. Though it is not part of the brain, the pineal gland develops from the roof of the diencephalon, a section of the brain. In some lower vertebrates the gland has a well-developed eyelike structure; in others, though not organized as an eye, it functions as a light receptor. Studies that were carried out in the 1980s suggested that the pineal gland was the evolutionary forerunner of the modern eye. The pineal gland is located within the third cerebral ventricle along the midline of the brain. Its name is derived from its shape, which is like that of a pine cone (Latin: pinea). In the adult human, it is about 0.64 cm (0.25 inch) long and is pinkish gray or white in colour. It weighs little more than 0.1 gram. The gland is large in children and begins to shrink with the onset of puberty. In adults, small deposits of such minerals as calcium make the pineal body visible on X rays. The apparent endocrine function of the gland is to elaborate the hormone melatonin. Studies of the pineal body in a number of vertebrates have conclusively proved that the pineal plays a major role not only in sexual development but also in hibernation and metabolism and seasonal breeding. In humans, however, its actual function is not understood very well; though humans have not responded as predictably to experimentation as have the lower vertebrates, evidence has suggested that the pineal gland plays a significant role in sexual maturation (the abundant production of melatonin in young children is believed to inhibit sexual development), circadian rhythm and sleep induction (secretion of melatonin is elevated at night), and so-called seasonal affective disease and depression (decreased daylight makes for longer periods of melatonin production).