Fig. 9-1. Horizontal section through the middle of the arm. In A, the nerves and vessels are identified. B, shows the flexor muscles (supplied by the musculocutaneous nerve) anteriorly and the extensors (supplied by the radial nerve) posteriorly. B.B., biceps brachii; Br., brachial is. The three heads of the triceps are indicated. Click for high resolution image.