The flexor digitorum profundus (deep flexor of fingers) is a longitudinal muscle located deep in the forearm, underneath the flexor digitorum superficialis muscle. Its upper end is broader and thicker than its lower end, arising from the anterior and medial surface of the ulna. Running down along this bone, the flexor digitorum profundus ends up in four long tendons that travel through the carpal tunnel into the hand. These tendons extend along the four medial fingers.
Action/Function
The flexor digitorum profundus closes the four fingers, flexing the proximal and distal interphalangeal joints. It also flexes the metacarpophalangeal joints. Although it is situated in the forearm, it is an extrinsic (external) muscle of the hand, because its contractions act on the hand, closing the fingers and flexing in unison with flexor digitorum superficialis and the flexor pollicis longus of the thumb, when we want to grip or grasp something.
Innervation
The lateral half of the flexor digitorum profundus is innervated by the anterior interosseous nerve, which in turn originates from the median nerve. The medial half of this muscle is supplied by the ulnar nerve.
Blood supply
This forearm muscle of the hand receives oxygen-rich blood from the side branches of the ulnar artery (common interosseous arteries).
Below, drawing/diagram of deep muscles of forearm, showing the flexor digitorum profundus
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