Overview
It has two heads, humeral and ulnar, that join in a common tendon. It forms the caudomedial border of the forearm, superficial to the digital flexors, and ends on the accessory carpal bone.
Origin
- Humeral head (caput humerale): medial epicondyle of the humerus.
- Ulnar head (caput ulnare): medial surface of the olecranon.
Insertion
Accessory carpal bone (os carpi accessorium).
Action
Flexes the carpus.
Innervation
Ulnar nerve (n. ulnaris).
Clinical relevance
Together with the ulnaris lateralis (m. ulnaris lateralis, formerly the extensor carpi ulnaris), which also ends on the accessory carpal bone, it pulls on that bone. In accessory carpal bone fractures this pull distracts the fragments, so they usually heal by fibrous rather than bony union.