The pineal gland, located in the central portion of the brain, is believed to secrete a substance called melatonin.

Melatonin contributes to the process of skin pigmentation in lower animals, such as frogs and fishes. In mammals, melatonin is believed to affect the brain and influence the rate of gonad (ovary and testis) maturation.

THYMUS GLAND

The thymus gland, located behind the sternum in the mediastinum, resembles a lymph gland in structure. It contains lymphatic tissue and antibody-producing lymphocytes.

The gland produces a hormone, thymosin, and is important in the development of immune responses in newborns (it is large in children but shrinks in adults). Its endocrine function is not well understood. Removal of the thymus gland is helpful in treating a muscular-neurological disorder called myasthenia gravis.