From each of the four profundus tendons a lumbrical muscle passes along the radial side of the metacarpophalangeal joint. On the palmar surface of the deep transverse metacarpal ligament, each develops a tendon which runs in a fibrous canal to reach the extensor expansion on the dorsum of the first phalanx.
Nerve supply of lumbricals:
Characteristically, the two ulnar lumbricals are innervated by the ulnar nerve and the two radial lumbricals by the median nerve. The proportion of ulnar and median distribution to the lumbricals follows that of the parent bellies of the tendons in the forearm. Variations of 1:3 and 3 :1 are not uncommon (each occurs in 20% of cases). It is interesting to note, though impossible to explain, that lumbricals supplied by the ulnar nerve are bicipital, each arising by two heads from adjacent profundus tendons, while those supplied by the median nerve are unicipital and arise from one tendon only.