Origin and insertion?
The muscle originates from several bony structures:
- Medial epicondyle;
- Coronoid process of the ulna (processus coronoideus);
- Proximal part of the radius (radius).
Proceeding toward the hand, the muscle belly transitions into tendons that insert onto the middle phalanges of digits II–V.
⚙️ Structural feature: at the level of the proximal phalanges, the tendons split into two slips, forming bifurcated tendons. The tendon of the flexor digitorum profundus (m. flexor digitorum profundus) passes through the resulting cleft. This anatomical formation is called the tendinous chiasm (chiasma tendineum).
Topography on the forearm
Flexor digitorum superficialis forms the second layer of muscles in the medial compartment of the anterior forearm.
- Anteriorly (superficially): the muscle is covered by the first layer, which includes, from medial to lateral: flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres. In the middle and lower thirds of the forearm, its belly may be visible in the intervals between the muscles and tendons of the first layer.
- Posteriorly (deeply): the deep surface of the muscle is adjacent to the deep lamina of the forearm fascia. This lamina separates the first and second layers from the third, deep layer.
- Vessels and nerves: the ulnar artery (a. ulnaris) passes between the flexor digitorum superficialis and flexor digitorum profundus. In the middle and lower thirds of the forearm, it runs in the sulcus between the flexor carpi ulnaris and flexor digitorum superficialis. The median nerve (n. medianus) is located centrally.
Topography in the hand and digits
In the digits, the tendon of the flexor digitorum superficialis inserts onto the middle phalanx (phalanx media). The tendon of the flexor digitorum profundus extends to the distal phalanx (phalanx distalis).
The fibrous digital sheath of the flexor tendon (vagina fibrosa tendinis flexoris) consists of annular (pars anularis) and cruciate (pars cruciformis) parts. The synovial sheath of the flexor tendon (vagina synovialis tendinis flexoris) is also described.
Function, blood supply, and innervation
- Function: the muscle flexes the middle phalanges of digits II–V and assists in wrist flexion.
- Innervation: supplied by the median nerve (n. medianus). Segments: C7–Th1.
- Blood supply: provided by the radial (a. radialis) and ulnar (a. ulnaris) arteries.
Class and exam questions
Which phalanges do the tendons of the flexor digitorum superficialis attach to?
They insert onto the middle phalanges of digits II–V as bifurcated tendons.
Which nerve innervates this muscle?
The muscle is innervated by the median nerve (n. medianus), segments C7–Th1.
What is the tendinous chiasm (chiasma tendineum)?
This is the site at the level of the proximal phalanges where the tendon of the superficial flexor splits into two slips to allow the tendon of the deep flexor to pass through.
In which muscle layer of the forearm is the superficial flexor located?
It forms the second muscle layer of the anterior forearm compartment, lying deep to the superficial muscles of the first layer.
What else to study on this topic
- Flexor digitorum profundus muscle
- Median nerve (n. medianus)
- Ulnar artery (a. ulnaris)
- Common flexor sheath
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