First (Superficial) Layer
The most superficial layer is represented by muscles that share a common point of origin — the medial epicondyle of the humerus.
- Pronator teres (m. pronator teres). Has two heads: humeral (from the medial epicondyle) and ulnar (from the coronoid process). It inserts into the lateral surface at the middle of the radius. Its function is pronation and flexion of the forearm. Notably, this muscle forms the medial boundary of the cubital fossa, passing between the brachioradialis and the flexor carpi radialis. It is innervated by the median nerve (C6–C7) and supplied by the brachial and radial arteries.
- Flexor carpi radialis (m. flexor carpi radialis). Originates from the same common site and inserts onto the base of the 2nd metacarpal bone (sometimes sending a slip to the 3rd). It performs wrist flexion and radial deviation (abduction). It is innervated by the n. medianus (C6–C8) and supplied by the radial artery.
- Palmaris longus (m. palmaris longus). Starts from the medial epicondyle and blends into the palmar aponeurosis (aponeurosis palmaris). It tensions this aponeurosis and assists in wrist flexion. Innervation: n. medianus (C8–Th1). Blood supply: radial and ulnar arteries.
Second (Intermediate) Layer
This layer contains a single, highly important muscle.
Flexor digitorum superficialis (m. flexor digitorum superficialis). It has an extensive origin: the medial epicondyle, the coronoid process of the ulna, and the proximal part of the radius. Its tendons travel to digits II–V and insert into their middle phalanges.
⚙️ Clinically relevant detail: at the level of the proximal phalanges, each tendon splits (forming two slips) to allow the tendon of the deep flexor to pass through it. This site is called the chiasma tendineum (chiasma tendineum). The muscle flexes the middle phalanges of these digits and participates in wrist flexion. It is controlled by the median nerve (C7–Th1) and receives blood from the ulnar and radial arteries.
Deep Layers
Deep to the superficial flexor lie the muscles of the third and fourth layers.
The third layer includes:
- Flexor digitorum profundus (m. flexor digitorum profundus). Originates from the anterior and medial surfaces of the ulna and the interosseous membrane. It inserts into the distal phalanges of digits II–V. It flexes the distal phalanges and the entire wrist. This is the only muscle in the compartment with dual innervation: the medial part is innervated by the ulnar nerve (n. ulnaris), while the lateral part is innervated by the median nerve (C8–Th1). It is supplied by the ulnar and anterior interosseous arteries.
- Flexor pollicis longus (m. flexor pollicis longus). Starts from the anterior surface of the radius and the interosseous membrane. It inserts into the base of the distal phalanx of the thumb. Accordingly, it flexes the thumb and the wrist. Nerve: n. medianus (C7–C8). Vessels: anterior interosseous, ulnar, and radial arteries.
The fourth (deepest) layer consists of a single short muscle:
- Pronator quadratus (m. pronator quadratus). Originates on the anterior surface of the distal third of the ulna and passes horizontally to the anterior surface of the distal third of the radius. Its function is forearm pronation and stabilization of the distal radioulnar joint. Innervation: n. medianus (C7–Th1). Blood supply: anterior interosseous artery.