Usually the four lumbrical muscles arise from the tendons of flexor digitor
um profundus and insert into the extensor expansions on the radial side of
the corresponding fingers. This special case showed a very rare variation o
f 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 metacarp
ophalangeal 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 i
nto the flexor digitorum superficialis tendon, as well as their bony attach
ment into the proximal and even more into the middle phalanx.