n. a hormone produced by the upper small intestine in response to the arrival of hydrochloric acid from the stomach. Secretin stimulates secretion of pancreatic juice by the pancreas and of bile by the liver. Its function was discovered in 1902 by British physiologists William Maddock Bayliss (1860–1924) and Ernest Starling (1866–1927), and it was the first substance demonstrated to be a hormone.