This is by definition the triangular area between pronator teres, brachioradialis and a line joining the humeral epicondyles. It possesses a roof and floor, and certain structures pass through it. The roof is formed by the deep fascia of the forearm, reinforced on the medial side by the bicipital aponeurosis. On the bicipital aponeurosis lies the median basilic vein with the medial cutaneous nerve of the forearm on its medial side; the aponeurosis separates these structures from the underlying median nerve and brachial artery. Laterally on the roof lie the lateral cutaneous nerve of the forearm and the median cephalic vein. The floor is formed in the main by the brachialis muscle; below and laterally the supinator clasps the neck of the radius. In the fossa, from medial to lateral side, are the median nerve, brachial artery, and the tendon of biceps. Farther laterally are the radial nerve and its posterior interosseous branch, but these are only seen when brachioradialis is retracted laterally. The artery is palpated here medial to the tendon to define the position for placing the stethoscope when taking the blood pressure. The radial nerve, having given branches to the brachialis and brachioradialis muscles, lower down gives another branch to extensor carpi radialis longus. It then divides into its two terminal branches, the superficial (cutaneous) branch and the deep branch, commonly called the posterior interosseous nerve. The latter gives branches to extensor carpi radialis brevis and supinator and disappears from the fossa by passing between the two heads of origin of the supinator muscle. The superficial branch passes down the forearm under cover of the brachioradialis.