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.

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