Palmaris longus muscle

The palmaris longus is seen as a small tendon between the flexor carpi radialis and the flexor carpi ulnaris, although it is not always present.

It is a slender, fusiform muscle, lying on the medial side of the flexor carpi radialis.

It arises from the medial epicondyle of the humerus by the common flexor tendon, from the intermuscular septa between it and the adjacent muscles, and from the antibrachial fascia.

It ends in a slender, flattened tendon, which passes over the upper part of the flexor retinaculum, and is inserted into the central part of the flexor retinaculum and lower part of the palmar aponeurosis, frequently sending a tendinous slip to the short muscles of the thumb.

It can be palpated by touching the pads of the fifth and first fingers and flexing the wrist. The tendon, if present, will be very visible.

Variation
The palmaris longus is a variable muscle, absent in about 16 percent of Caucasians, and less frequently absent in other populations. It may be tendinous above and muscular below; or it may be muscular in the center with a tendon above and below; or it may present two muscular bundles with a central tendon; or finally it may consist solely of a tendinous band.

The muscle may be double.

Slips of origin from the coronoid process or from the radius have been seen.

Partial or complete insertion into the fascia of the forearm, into the tendon of the Flexor carpi ulnaris and pisiform bone, into the scaphoid, and into the muscles of the little finger have been observed.

Additional images
Due to its apparent unimportance, it is often used as a replacement for other tendons should injury arise.