Flexion of the fingers is produced by two long muscles, flexor digitorum profundus, and flexor digitorum superficialis.
Here’s the deep finger flexor, flexor digitorum profundus. It arises from the anterior and medial surface of the ulna, and from the interosseous membrane. Here are its four tendons, entering the carpal tunnel. We’ll follow them in a minute. This adjoining muscle we’ll see later on. It’s flexor pollicis longus, the long thumb flexor.
Now let’s add the superficial finger flexor, flexor digitorum superficialis to the picture. Here it is. It lies right on top of the profundus. It has two heads of origin, a radial head and a humero-ulnar head. The humero-ulnar head arises, as part of the common flexor tendon, from the medial epicondyle of the humerus, and also from the adjoining ulna. Its radial head arises from this long oblique line on the radius. Between the two heads there’s a gap, which the median nerve and the ulnar artery both pass through.
The four separate tendons of flexor digitorum superficialis are bundled together as they enter the carpal tunnel. Before we follow the superficialis and profundus tendons into the hand, we’ll bring the forearm to the upright position.
As the flexor tendons pass through the carpal tunnel, they’re all enfolded within this common synovial sheath which extends into the palm of the hand. Just as the flexor tendons emerge from the carpal tunnel, the four profundus tendons give rise to these four intrinsic muscles, the lumbricals. We’ll be looking at these later. For now we’ll remove them to simplify the picture.
Just before reaching the MP joint, the superficialis and profundus tendons of each finger enter the flexor tendon sheath together. To follow them we’ll remove the sheath. Over the proximal phalanx, the superficialis tendon splits into two halves, which pass around the profundus tendon. We’ll remove the profundus tendon for a moment. The two halves of the superficialis tendon re-unite, and as they do so they insert here, on the middle phalanx.
The profundus tendon (here it is back in place) emerges between the two halves of superficialis, and continues distally to insert here, on the base of the distal phalanx.
The action of flexor digitorum superficialis is to flex the proximal IP joint, and the MP joint. The action of flexor digitorum profundus is to flex both the IP joints, and the MP joint.