Anatomical studies on the extensor pollicis et indicis accessorius and extensor indicis radialis muscles were conducted on 952 upper limbs from 476 Japanese adult cadavers. Each anomalous muscle occurred between the extensor pollicis longus proprius and extensor indicis proprius muscles in the dorsum of the forearm and hand.
The extensor pollicis et indicis accessorius muscle was present in 13 of the limbs (1.4%). It was not coexistent with another anomalous muscle of the extensor digitorum profundus mass in 9 cases, and it was coexistent with that muscle in 4.
The extensor indicis radialis muscle was present in 34 limbs (3.6%). It was accompanied by another anomaly of the extensor digitorum profundus mass in 5 cases, and not accompanied by that muscle in 29.
The extensor pollicis et indicis accessorius and extensor indicis radialis muscles were innervated by the posterior interosseus nerve of the radial nerve. It seems that the former anomalous muscle supplies the extensor control of the thumb and index finger, and the latter one of the index finger only. Each anomalous muscle was considered to be differentiating on the radial side of the extensor digitorum profundus mass in humans.