The Body: A Guide for Occupants

by Bill Bryson

The most important component of the limbic system is a little powerhouse called the hypothalamus, which isn’t really a structure at all but just a bundle of neural cells. The name describes not what it does but where it is: under the thalamus. (The thalamus, meaning “inner chamber,” is a kind of relay station for sensory information and is an important part of the brain—there isn’t any part of the brain that isn’t important, obviously—but is not a component of the limbic system.) The hypothalamus is curiously unimposing. Though only about the size of a peanut and weighing barely a tenth of an ounce, it controls much of the most important chemistry of the body. It regulates sexual function, controls hunger and thirst, monitors blood sugar and salts, decides when you need to sleep. It may even play a part in how slowly or rapidly we age. A large measure of your success or failure as a human being is dependent on this tiny thing in the middle of your head. The