Anatomically, the gray matter of the brain and spinal cord; gray substance composed mainly of nerve cell bodies and dendrites.
From Latin 'cinereus' (ashy, ash-colored), referring to the actual gray coloration of brain tissue. Used as scientific terminology in neuroanatomy since the 18th century.
Your brain's gray matter is literally ash-colored to the naked eye—ancient anatomists noticed this centuries before microscopes revealed it's packed with billions of neurons doing all your thinking, remembering, and feeling.
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