681 Int. J. Morphol., 28(3):681-684, 2010. Superficial Portion of Abductor Pollicis Brevis Muscle, Morphological Study and Literature Review. Case Report Fascículo Superficial del Músculo Abductor Corto del Pulgar, Estudio Morfológico y Revisión de la Literatura. Reporte de Caso * Guillermo Méndez R.; ** Daniela Zavando; * Mario Cantín & * Iván Suazo Galdames MÉNDEZ, R. G.; ZAVANDO, D.; CANTÍN, M. & SUAZO, G. I. Superficial portion of abductor pollicis brevis muscle, morphological study and literature review. Case report. Int. J. Morphol., 28(3):681-684, 2010. SUMMARY: Abductor pollicis brevis muscle (APB) belongs to the foreground of the subfascial muscle thenar region, which is of great importance in the movement of the thumb on its two-joint arrangement. In this article, we report the presence of a superficial portion of the APB muscle and its relationship and discuss the available literature and the clinical implications of the presence of this variation. KEY WORDS: Abductor pollicis brevis muscle; Thenar eminence; Thumb; Movement of the thumb. INTRODUCTION The differentiation from prehominids (Australophitecus afarensis) to hominids (Homo habilis) is due to the use of tools, which was only possible through the development of the brain and the opposition of the thumb (precision grip), being considered as evolutionary characteristics. Because of the significance of this fact, the structure and function of the brain and thumb are still currently widely investigated (Susman, 1994; Miralles & Miralles, 2005). In relation to the human thumb, when we look at its constitution, strength, and fine motor control of their movements, we found six different muscles that are interrelated, prepared prior to the metacarpophalangeal joint: flexor pollicis longus (FPL), 1st interossei palmar (of Henle) (1st IP), deep fascicle of flexor pollicis brevis (FPBdeep), superficial fascicle of flexor pollicis brevis (FPBsup), adductor pollicis (ADP), and abductor pollicis brevis (APB) muscles. In these muscles, the APB and ADP muscles plus an extrinsic muscle of the hand, the abductor pollicis longus (APL), are responsible for the claim and thumb abduction (Llusá et al., 2004; Miralles & Miralles). In particular, we distinguish the APB muscle, which also acts in the abduction and medial rotation of the proximal phalanx during early stages of opposition (Moore & Dalley, 2006) and the extension of the latter on the distal phalanx through its expansion into the APL tendon (Napier, 1952; Kapandji, 2001; Llusá et al.). These actions are explained from the relationship and trajectory of the APB muscle on carpometacarpal and metacarpophalangeal joints of the thumb, which are traversed by that muscle. The classic anatomy texts indicated as proximal attachment or origin of the APB muscle tubercle of the scaphoid bone, the flexor retinaculum, and an expansion of the APL tendon (Latarjet & Ruiz-Liard, 1999; Feneis & Dauber, 2000; Llusá et al.; Rouvière & Delmas, 2005), description to which some authors add the Trapezium tubercle (Drake et al., 2005, Moore & Dalley). The distal attachment of the APB muscle is the lateral tubercle of the base of the proximal phalanx and an expansion to the tendon of the extensor pollicis brevis muscle (EPB) (Latarjet & Ruiz- Liard; Llusá et al.; Drake et al.) to the extensor pollicis longus muscle (EPL) (Rouvière & Delmas, 2005), as well as in the sesamoid bone of the thumb (Llusá et al.; Feneis & Dauber, 2000). * Universidad de Talca, Chile. ** Universidad Autónoma de Chile, Sede Talca, Chile.