Digestive hormone that is produced by the S cells lining the duodenum and jejunum when protein of partially digested food enters the intestine from the stomach. Stimulates the pancreas to produce a fluid high in salts but low in enzymes. First substance ever demonstrated to be a hormone.

SECRETIN: "The function of Secretin was discovered in 1902 by British physiologists William Maddock Bayliss (1860-1924) and Ernest Starling (1866-1927)."