Pronator quadratus
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Pronator quadratus is a square shaped muscle on the distal forearm that acts to pronate (turn so the palm faces downwards) the hand.
Its fibres run perpendicular to the direction of the arm, running from the most distal quarter of the anterior ulna to the distal quarter of the anterior radius.
When pronator quadratus contracts, it pulls the lateral side of the radius towards the ulna, thus pronating the hand. It also helps to keep the two bones in the forearm bound together.
As it is on the anterior side of the arm, it is innervated by a branch of the median nerve, the interosseous nerve. Arterial blood comes via the interosseous artery.