The authors report on the simultaneous occurrence of short supernumerary muscles on the palmar and dorsal sides of an adult human hand. The supernumerary muscle on the palmar side of the hand took its origin from the tendon of the m. flexor digitorum superficialis in the distal third of the forearm, sharing its insertion with that of the m. lumbricalis primus, and taking its innervation from the n. medianus. With regard to its origin, the muscle may be a supernumerary belly of the m. flexor digitorum superficalis which failed to make its way to the forearm during development. With regard to its insertion, though, it can just as well represent a detached part of the m. lumbricalis primus whose tendon became attached to the m. flexor digitorum superficialis at the time of simultaneous development of those muscles of the hand. By their origin, insertion, innervation and position, the muscles on the dorsal side of the hand were identified, in part, as mm. interossei dorsales accessorii in the 2nd and 3rd intermetacarpal spaces which are constantly found in man as a developmental feature to become rudimentary in some cases later on, in part, as an anomalous m. extensor digitorum brevis of the 2nd and 3rd fingers.