The Mechanism of Pancreatic Secretion
Acid in the gut sent a chemical through the blood to the pancreas — the first hormone, and the birth of endocrinology.
The body, everyone assumed, was wired like a telephone network. Two scientists in London found it also runs a postal service — and posted the first letter.
The big idea
Bayliss and Starling discovered that organs talk to each other not only through nerves, but through chemicals released into the blood. When acid arrives in the gut, the gut lining drops a chemical into the bloodstream; the blood carries it to the pancreas, which then pours out the juice that neutralises the acid. They called the chemical secretin.
Three years later Starling gave a name to this whole new class of blood-borne messengers: hormones. Every hormone you've heard of — adrenaline, insulin, testosterone — is the same kind of signal. This was the first one ever caught in the act, and it began the science of endocrinology.
How it came about
In 1902, the famous Russian physiologist Ivan Pavlov had convinced the world that digestion was run by nerves — reflexes triggered by food and acid. Bayliss and Starling, working together at University College London (and married into the same family — Bayliss had married Starling's sister), decided to test whether the gut-to-pancreas signal really needed nerves.
They took a loop of a dog's intestine and cut every nerve to it, leaving only its blood supply. Then they dripped acid inside. The pancreas, far away, secreted anyway. The signal couldn't be travelling by nerve — it had to be in the blood. To prove it, they scraped the gut's lining, soaked it in acid, and injected the liquid into a vein: the pancreas gushed. So thrilled were they that they reported it to the Royal Society just a week later.
Why it mattered
It revealed a second control system for the whole body. Nerves are like private phone lines — fast and aimed at one target. Hormones are like a message poured into a river: they reach everyone, but only the organ tuned to that message acts on it. Understanding hormones is what later let doctors treat diabetes with insulin, an underactive thyroid with thyroxine, and much more. It is one of the foundations of modern medicine.
A way to picture it
Imagine the gut needs to tell the pancreas, "acid's coming — send help." A nerve would be a phone call: a wire from one to the other. But Bayliss and Starling showed the gut instead tosses a sealed note into the bloodstream, like a message in a bottle dropped in a river. The note floats past every organ, but only the pancreas can read it — and when it does, it acts. Cut the phone lines and the note still arrives, because it was never travelling by wire.
Where it sits
This was a turning point between two giants' worldviews. Pavlov — also in this Library, for his work on conditioned reflexes — had made nerves the master of digestion; Bayliss and Starling showed chemistry shared the throne. The hormone idea they launched runs straight on to Banting and Best's insulin two decades later, to the discovery of adrenaline and the sex hormones, and to today's gut-hormone drugs like the GLP-1 agonists used for diabetes and weight loss. Endocrinology begins here.