The fourth lumbrical muscle in the hand: a variation of insertion on the fifth finger.

Usually the four lumbrical muscles arise from the tendons of flexor digitorum profundus and insert into the extensor expansions on the radial side of the corresponding fingers. This special case showed a very rare variation of a unipennate fourth lumbrical muscle of the right hand; the muscle fibre bundles originated on the radial side of the flexor digitorum profundus and coursed horizontal on its radial side, deep to the palmar aponeurosis and in front of the deep transverse metacarpal ligament over the fifth metacarpophalangeal joint. At the level of this joint, its tendon divided into one radial and one ulnar slips. Both heads surrounded the tendons of the flexor digitorum superficialis and profundus muscles, and found their insertion into the flexor digitorum superficialis tendon, as well as their bony attachment into the proximal and even more into the middle phalanx.