cerebellum

The cerebellum is a fist-sized, densely wrinkled lump of brain tissue tucked underneath the back of your skull, behind and below the big folded brain you usually picture. Its name means "little brain" in Latin, and that is a fair description: it looks like a smaller, tightly pleated version of the main brain. Despite taking up only about a tenth of the brain's volume, it holds more than half of all the brain's nerve cells. Its main job is not to decide what you do, but to make sure that whatever you do comes out smooth, steady, and well-timed.

Think of the cerebellum as a backstage coach for movement. When you reach for a cup, walk a straight line, or speak a sentence, the cerebellum constantly compares what your body intended to do with what it is actually doing, and quietly nudges the muscles to fix the difference, moment by moment. It keeps your balance, coordinates the many muscles that must fire in the right order, and fine-tunes timing so motions are graceful instead of jerky. When it is damaged, people do not become paralyzed, but their movements turn clumsy and shaky: they stagger as if drunk, overshoot when they reach, and slur their words. This is why too much alcohol, which dulls the cerebellum, makes people wobble and lose coordination.