gray matter vs white matter

If you cut open a fresh brain, you would see two shades. The darker, pinkish-gray regions are gray matter, and the paler, almost ivory-white regions are white matter. They are not different organs but two ways the brain's tissue is organized. Gray matter is where the work of thinking gets done, and white matter is the wiring that carries the results from one place to another.

Gray matter is packed with the cell bodies of neurons (the brain's signaling cells) along with their bushy branches, where messages are received and decisions are made. White matter, by contrast, is full of axons: the long, cable-like tails that neurons use to send signals over distance. Each axon is wrapped in a fatty insulating sheath called myelin, and it is this pale fat that gives white matter its color and lets signals travel fast.

A useful picture is a city: gray matter is the buildings where people meet and get things done, while white matter is the highways and phone lines connecting one neighborhood to the next. In the wrinkled outer shell of the brain, gray matter sits on the surface and white matter lies beneath it, so the thinking layer is on the outside and its connecting cables run deep inside.