Functionally negligible, this muscle is of morphological interest. It is absent in 13% of arms. It is phylogenetically degenerating and shows the characteristics of this, i.e. short belly and long tendon (like plantaris in the leg). Replacement of its distal tendon by ligament (palmar aponeurosis) is a further characteristic of degeneration (like coccygeus and the sacrospinous ligament). It is supposed that the muscle once existed as a flexor of the proximal phalanges, with its tendons lying in the palm superficial to those of flexor digitorum superficialis and splitting around them to be attached to the proximal phalanges. The muscle arises from the common origin and its tendon is inserted, adherent across the front of the flexor retinaculum into the palmar aponeurosis. The main importance of the muscle is that its tendon slightly overlaps the median nerve from the ulnar side, and it must not be mistaken for the nerve.
Nerve supply of plamaris longus:
By the median nerve (C7 and 8).
Action of Palmaris longus:
It is a weak flexor of the wrist, and by its attachment to the palmar aponeurosis it may produce minimal flexion at the metacarpophalangeal joints of the fingers.