Brain

The brain is an organ present in all but the simplest of creatures which exerts control over all other organs in the body. Arthur Weasley considered it reasonable not to trust anything if one could not see where it kept its brain, advice he passed on to his children and repeated to Ginevra Weasley after he learned the amount of trust she had devoted to Tom Riddle's diary.

In a certain room of the Department of Mysteries, there exists a large tank containing a number of brains or brain-like objects. These brains exude thoughts as thin tendrils along their "bodies" and appear to have a certain amount of sentience on their own. They are used by Unspeakables to study human thought.

During the Battle of the Department of Mysteries, Ronald Weasley, under the influence of a Death Eater's spell that made him disoriented, used a Summoning Charm on one of these brains. The brain immediately attacked when it touched Ron, wrapping its tentacle-like thoughts around his body and attempting to suffocate him. Though he was eventually able to get free nad was treated to by Poppy Pomfrey, he suffered a great deal of damage and had scars from the events that would presumably last the rest of his life.