A very rare case of the accessory palmaris longus muscle and clinical significance.

The palmaris longus muscle is located in the forearm region. Its morphological variability was noted during standard anatomical dissection of the upper limb. The muscle was characterised by a normal course, i.e. originating from the medial humeral epicondyle and inserting into the palmaris aponeurosis, but a small additional tendon attached to the flexor retinaculum was observed in its distal part. An accessory palmaris longus muscle was also observed nearby. Interestingly, this accessory muscle was reversed, and the first part was not muscular, but tendinous, represented by 2 tendons originating from the common muscular mass attached to the medial epicondyle of the humerus; these later connected together, creating one muscle belly distally attached to the flexor retinaculum. This additional structure was innervated by a neural branch from the median nerve, and the ulnar artery was responsible for blood supply. The course of the median nerve is also clinically important because, before entering the carpal tunnel, it was located directly under the accessory palmaris longus muscle. In turn, the ulnar artery passed through a special hole created by the flexor digiti minimi brevis and flexor retinaculum, before passing under the palmaris brevis muscle.