Flexor Mass of the Forearm
The flexor mass could be considered as one muscle with four tendons that originates on the medial epicondyle. Divisions between the muscles are slight even in a muscular arm, but the individual tendons can be quite distinct even in a slender arm. They all flex the wrist; one flexes the fingers as well.
Counting from brachioradialis to the ulna, the first long flexor is flexor carpi radialis. Its tendon follows the radius and ducks under the ball of the thumb, where it inserts at the base of the second and third metacarpals.
The second long flexor is palmaris (pal-MAIR-riss,
referring to the palm). Its tendon passes under where the ball of the thumb meets the heel of the hand, where it spreads across the metacarpals like, aptly, a palm frond.
The third long flexor is flexor digitorum, whose tendons spread to each of the four fingers, although only one medial tendon can be seen on the surface of the wrist.
Flexor digitorum underlies flexor carpi radialis and palmaris. In the middle third of a very muscular arm, one may see a bulky mass that seems to protrude between their tendons, meeting up with them at a serrated edge. This mass is flexor digitorum. Its bulk also lies below the tendons at the wrist.
The fourth and last long flexor is flexor carpi ulnaris, whose tendon goes under the heel of the hand to the pisiform bone. Its belly lies next to the ulna and attaches to it, and therefore can be seen on the dorsal side of the forearm, even though it inserts on the palmar side.
Flexor carpi ulnaris thinly covers deeper flexors on the forearm, and there may be a plane break on the upper half of the muscle where they push up near the ulna.
Between brachioradialis and the upper part of flexor carpi radialis lies pronator teres (TEH-reez,
the rounded pronator). It inserts into to the lateral side of the radius about midway down, and is responsible mostly for flipping it over the ulna, turning the forearm into pronation.