The first part of the abductor pollicis muscle is a deep part which originates from the ulna, interosseous membrane, and radius and is covered by Extensor Digitorum Longus. It has numerous subdivisions of the belly (all arranged in a parallel fashion) which terminates in a central tendon. The fibers of the first part are short, obliquely attached to a tendon in a pennate fashion. After its passage through the first compartment of the extensor retinaculum, the deep tendon separates into several branches and is ultimately inserted into the trapezium, abductor pollicis brevis, opponens pollicis, capsule, and anterior oblique ligament.