Course
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 antebrachial 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.
The palmaris longus tendon is responsible for exposing the claws in other mammals.
Read more about this topic: Palmaris Longus Muscle