This image shows several of the muscles on the upper arm of a male human cadaver.  The pectoralis major is a large triangular muscle covering the upper part of the chest.  Its action is to adduct and flex the humerus. The deltoid abducts the arm and aids in flexion and extension.  The triceps brachii consists of three heads and is the largest muscle of the brachium.  This muscle is located posterior to the humerus, and thus functions to extend the forearm. The brachialis is located on the anterolateral surface of the humerus.  It originates on the humerus and inserts on the ulna.  The action is to flex the forearm.  The biceps brachii has two heads of origin in the human-  a long head (comparable to the muscle in the cat) and a short head.  This muscle produces flexion of the forearm and supination of our hand.