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A core part of the brain made up of axons.
There’s a good chance you’ve overheard someone, at least once in your life, referring to a person’s intellect or lack thereof by using the term “gray matter.” Well, you’ll feel smarter to know that there’s also a thing called white matter. It’s the paler tissue of the brain and spinal cord made up mostly of impulse-conducting axons. These axons are covered in a fatty, protective substance called myelin, which gives the tissue its white/pale color.