Arising from the common origin the muscle receives a further contribution from a wide aponeurosis which arises from the medial border of the olecranon and the upper three-fourths of the subcutaneous border of the ulna. This aponeurosis lies superficial to the belly of flexor digitorum profundus — the latter gives the bulk to the medial side of the upper forearm. It lies edge to edge, at the subcutaneous border of the ulna, with the aponeurosis of extensor carpi ulnaris. The ulnar nerve enters the flexor compartment of the forearm by passing between the humeral and ulnar heads of this muscle. The tendon of insertion of flexor carpi ulnaris passes to the pisiform. The
pisiform is morphologically a sesamoid bone in the tendon; by way of the pisohamate and pisometacarpal ligaments the muscle acts on a wider insertion. Its symmetry with the insertion of extensor carpi ulnaris into the base of the fifth metacarpal is maintained by way of the pisometacarpal ligament.
Nerve supply of flexor carpi ulnaris:
By the ulnar nerve; but in 95% of cases the fibers have come from the lateral cord or direct from the middle trunk by a communication in the axilla. The segmental value of this communication is C6, 7, not the ordinary C8, T1 or the remainder of the ulnar nerve.
Action of flexor carpi ulnaris:
It is a flexor of the wrist, an ulnar adductor when acting with extensor carpi ulnaris, and a fixator of the pisiform during contraction of the hypothenar muscles.