up to the beginning of the twentieth century, the nervous system was thought to control all communication within the body and the resulting integration of behavior. scientists had determined that nerves ran, essentially, on electrical impulses. these impulses were thought to be the engine for thought, emotion, movement, and internal processes such as digestion. however, experiments by william bayliss and ernest starling on the chemical secretin, which is produced in the small intestine when food enters the stomach, eventually challenged that view. from the small intestine, secretin travels through the bloodstream to the pancreas. there, it stimulates the release of digestive chemicals. in this fashion, the intestinal cells that produce secretin ultimately regulate the production of different chemicals in a different organ, the pancreas.

such a coordination of processes had been thought to require control by the nervous system; bayliss and starling showed that it could occur through chemicals alone. this discovery spurred starling to coin the term hormone to refer to secretin, taking it from the greek word hormon, meaning “to excite” or “to set in motion.” a hormone is a chemical produced by one tissue to make things happen elsewhere.

as more hormones were discovered, they were categorized, primarily according to the process by which they operated on the body. some glands (which make up the endocrine system) secrete hormones directly into the bloodstream. such glands include the thyroid and the pituitary. the exocrine system consists of organs and glands that produce substances that are used outside the bloodstream, primarily for digestion. the pancreas is one such organ, although it secretes some chemicals into the blood and thus is also part of the endocrine system.

much has been learned about hormones since their discovery. some play such key roles in regulating bodily processes or behavior that their absence would cause immediate death. the most abundant hormones have effects that are less obviously urgent but can be more far-reaching and difficult to track: they modify moods and affect human behavior, even some behavior we normally think of as voluntary. hormonal systems are very intricate

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