Humero-ulnar head of flexor digitorum superficialis

Flexor digitorum superficialis (flexor digitorum sublimis) or flexor digitorum communis sublimis is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. It is in the anterior compartment of the forearm. It is sometimes considered to be the deepest part of the superficial layer of this compartment, and sometimes considered to be a distinct, "intermediate layer" of this compartment. It is relatively common for the flexor digitorum superficialis to be missing from the little finger, bilaterally and unilaterally, which can cause problems when diagnosing a little finger injury. (Wikipedia)