pineal gland This is a small structure, about the size of a pea, situated approximately in the centre of the head. Because it is one of the few obviously unpaired structures in the brain, the seventeenth-century French philosopher René Descartes (1596–1650) suggested that it was the seat of the soul, mediating subjective experience and intervening in the machinery of the brain in situations of free will and moral choice. In reality, the pineal gland is essentially part of the visual system. In mammals it responds indirectly to light because it receives messages along fibres from nerve cells of the suprachiasmatic nucleus of the hypothalamus, which themselves receive signals from the eye via fibres of the optic nerve. The suprachiasmatic nucleus is the body's major rhythm–generating centre — the heart of the body clock. In lower vertebrates the pineal gland is itself a ‘clock’ and its cells respond directly to light. The main output of the pineal gland is the ‘darkness’ hormone melatonin, which is normally made at night. Melatonin, by the duration of its secretion, serves to indicate to the body both darkness and the length of the night. This signal is used to regulate the timing of biological rhythms.

Josephine Arendt

Bibliography

Arendt, J. (1995) Melatonin and the mammalian pineal gland. Chapman and Hall, London, New York.

See also biological rhythms; body clock; hypothalamus; mind–body problem.

pineal gland (pĬn´eəl), small organ (about the size of a pea) situated in the brain. Long considered vestigial in humans, the structure, which is also called the pineal body or the epiphysis, is present in most vertebrates. It is sensitive to different levels of light and is essential to the functioning of an animal's biological clock. In many animals, including humans, the pineal gland synthesizes a hormone called melatonin in periods of darkness. Melatonin synthesis is halted when light hits the retina of the eye, sending impulses to the gland via the optic nerve. Besides influencing daily, or circadian, rhythms such those of as sleep and temperature, the pineal gland and melatonin appear to direct annual rhythms and seasonal changes in animals. The pineal gland and melatonin are now being studied for their roles in sleep, reproduction, aging, and seasonal affective disorder. In humans the pineal gland begins to produce melatonin at age 3 months; production falls steadily from puberty on.

pineal gland (pineal body) (pin-i-ăl) n. a pea-sized mass of nerve tissue attached by a stalk to the posterior wall of the third ventricle of the brain. It functions as a gland, secreting the hormone melatonin. Anatomical name: epiphysis.

pineal gland An outgrowth of the forebrain. In humans its functions are obscure, but in other vertebrates it acts as an endocrine gland, secreting the hormone melatonin.