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Identify the opponens pollicis muscle [probe; cut ends of the abductor pollicis brevis muscle retracted], which lies deep to the abductor pollicis brevis muscle. The opponens pollicis muscle arises from the trapezium and flexor retinaculum and inserts into the entire length of the lateral side of the first metacarpal bone.
Finally, identify the more medially located flexor pollicis brevis muscle [probe]. It arises from the trapezium and flexor retinaculum and inserts into the base of the proximal phalanx of the thumb, just medial to the insertion of the abductor pollicis brevis muscle.Links and References:
Grant's: 6.90-6.95
Netter (1ed.): 433, 434, and 438 (2ed.): 429, 430, and 434
Rohen/Yokochi: 366-370 and 399