deep head of flexor pollicis brevis
deep head of flex·or pol·li·cis bre·vis[TA]
the head of short flexor of the thumb that arises from the trapezoid and capitate bones and transverse carpal ligaments. It is innervated by the deep ulnar nerve, and considered by many to be the first palmar interosseous muscle.
Synonym(s): caput profundum musculi flexoris pollicis brevis [TA]