Accessory extensor carpi radialis muscle and interconnecting muscular bundle.

An accessory muscle and a muscular bundle were found and prepared in both forearms of a 55-year-old male cadaver. On the left side, the accessory muscle-originated from the medial aspect of the extensor carpi radialis brevis (ECRB) muscle, coursed downwards, crossed posterior to the tendon of ECRB, passed through the second chamber of the extensor retinaculum and inserted into the base of second metacarpal bone. Additionally, a muscular bundle was observed between the extensor carpi radialis longus (ECRL) and ECRB muscles. On the right side, both ECRL and ECRB had bifid tendons. The long and thin additional tendon of ECRL coursed downwards and joined the accessory tendon of the ECRB before entering the second chamber of the extensor retinaculum and the common tendon inserted into the base of the second metacarpal bone.