Discovery of animal hormones — Starling (1905)

Starling and Bayliss studied how the pancreas starts to secrete pancreatic juice:

1. The pancreas begins to secrete pancreatic juice soon after food reaches the duodenum, even after the nerves supplying the pancreas have been cut.
2. So he concluded that there must be a non-nervous stimulus.
3. He then discovered that the mucous membrane lining the duodenum secretes a chemical substance (secretin) that passes into the blood and stimulates the pancreas to secrete pancreatic juice.
4. He named these substances hormones ("activators").

Logic of the experiment: cut nerve → secretion still occurs → so another route (blood) must carry the message; extract from duodenal lining injected into blood → pancreas secretes → the chemical messenger exists.