Pineal Gland - Anatomy & Physiology
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The pineal gland is an unpaired organ located immediately behind the thalamus. It is part of diencephalon and lies dorsally to it and is attched to it via the habenulae and a short peduncle. It's size varies between species and individuals. It is innervated by the postganglionic sympathetic fibres from the cranial cervical ganglion.
The pineal gland secretes melatonin in response to day length. The retina detects day length and sends an impulse via the hypothalamus, thoracic spinal cord and cranial cervical ganglia to the pineal gland. The gland effectively acts as the bodies biological clock and helps regulate the circadian rhythm, sleep and the seasonal and diurnal variations in reproductive behaviour.
In reptiles the pineal gland also contains photoreceptive cells and is sometimes called the third eye. In many eastern cultures and religions the third eye is of great significance.