ABSTRACT
Historically the pineal gland has been viewed as being “a third eye,” as “the seat of the soul,” and at one time as a “sphincter to control the flow of thought.” Today, however, it is viewed as being a neuroendocrine transducer organ in mammals. This classification did not come easily, as early workers wanted to define pineal endocrine action along the lines of other endocrine gland functions known at the time. All were thought to be entirely dependent upon substances in the bloodstream both for their own control and for their effects on the remainder of the body. Endocrine glands secreted hormones into blood, and were themselves regulated by other hormones, which were delivered to them by the circulation. Secretory activity of a gland was thought to be maintained within narrow limits by simple homeostatic mechanisms: as the level of a particular hormone in blood rose, the gland invariably responded by decreasing its secretion of that hormone; when the level of the hormone fell, the gland increased its secretion. Studies conducted within the past 20 years indicate that the pineal gland is an intricate and sensitive “biologic clock,” converting cyclic nervous activity generated by light in the environment into endocrine secretions.
