Hormones discovered

In January 1902 the physiologists William Bayliss and Ernest Starling, at University College London, asked how the pancreas knows to release digestive juice when acidic food leaves the stomach. The accepted answer, from Ivan Pavlov's school, was a nerve reflex. They cut every nerve to a loop of a dog's small intestine, yet acid in the loop still made the pancreas secrete. They then ground the lining of the intestine with acid, injected the extract into the blood, and the pancreas responded. A substance made in the gut and carried in the blood did the job. They called it secretin.¹²

Earlier work had pointed this way. In 1849 Arnold Berthold showed that a rooster's testes work through the blood, Claude Bernard wrote of internal secretions in the 1850s, and in 1894 and 1895 George Oliver and Edward Schafer found that extract of the adrenal gland raises blood pressure. Cell theory had pictured glands as groups of cells, and the circulation of the blood described by Harvey in 1628 offered the delivery route. What Bayliss and Starling added was the idea of a second communication system, working by chemicals alongside the nerves. In 1905 Starling proposed the word hormone, from a Greek word meaning to arouse, for such messengers.²

Endocrinology grew quickly. Thyroxine was isolated in 1914, insulin was found and used to treat diabetes in 1921 and 1922, and sex hormones and steroids followed. The contraceptive pill of 1960 and modern treatments for diabetes, thyroid disease and growth disorders all rest on the idea that small chemical signals regulate the body.²