internal secretion

Before the word hormone existed, scientists had to name a strange observation: certain glands seemed to give something useful to the body without any visible duct or outlet. They called it internal secretion — secreting inward, into the body itself, rather than outward to a surface. That phrase was the seed from which the whole science of endocrinology grew.

Internal secretion is the historic term for the release of a substance from a gland directly into the bloodstream, as opposed to external secretion through a duct. It was popularized in the mid-1800s by the French physiologist Claude Bernard, who noticed the liver releasing glucose into the blood, and it framed the idea that glands could regulate the body chemically from within.

The concept was put on firm experimental footing in 1902 when Bayliss and Starling discovered secretin, the first molecule shown to be released into the blood and to act at a distance; Starling soon coined the word hormone for such substances. A caveat: today internal secretion is mostly of historical interest, having been absorbed into the modern vocabulary of hormones and endocrine signaling.