This strongly unyielding ligament is phylogenetically the degenerated tendon of palmaris longus. It extends, in continuity with the tendon, from the distal border of the flexor retinaculum, whence it fans out in a thick sheet towards the bases of the fingers. It divides into four slips, one for each finger. Each slip gives off superficial fibers that insert into the skin in the region of the crease at the base of the finger, while the main part divides into two bands over the proximal end of the fibrous flexor sheath; they are inserted into the deep transverse ligament of the palm, and into the bases of the proximal phalanges and the fibrous flexor sheaths. Some strands from the aponeurosis pass up on each side of the finger. When the fingers are forcibly extended the soft tissues of the palm can be seen bulging in the three intervals between the four slips, just proximal to the interdigital webs.
Over the hypothenar muscles the deep fascia is much thinner than the palmar aponeurosis and it is thinnest of all over the thenar muscles. This is in keeping with the increased mobility of the metacarpal bone of the thumb (note that the plantar aponeurosis over the big toe muscles is thick; the first metatarsal has very little freedom of movement). The function of the palmar aponeurosis is purely mechanical. It gives firm attachment to the skin of the palm to improve the grip, and it protects the underlying tendons. Contraction of the aponeurosis and its digital slips in Dupuytren’s contracture results in a fixed flexion of the fingers concerned (usually the ring and little fingers).