Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Serotonin is a hormone and neurotransmitter which is found in many tissues of humans and animals. Moreover, it is also present in fungi and plants. The chemical which is produced by the human body was discovered in the 1930s by Dr. Vittorio Erspamer and the discovery of the Italian pharmacologist and chemist has contributed to the understanding of mental illnesses as well as their causes and treatment. Learn more about serotonin and its functions in this book.