This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The muscles which flex and extend the fingers of course also move the hand as a whole, but in addition to these muscles there are five others, - two flexor muscles and three extensor muscles, - which are inserted into the bones of the metacarpus and not into the phalanges. When these muscles contract they tend to move the whole hand and not the fingers alone. They are the flexor carpi radialis, flexor carpi ulnaris, extensor carpi radialis longior, extensor carpi radialis brevior, and extensor carpi ulnaris. The palmaris longus has already been described as a flexor of the fingers.
The two flexors of the wrist, the flexor carpi radialis and the flexor carpi ulnaris, are both superficial muscles lying directly beneath the skin. The flexor carpi radialis arises from the medial (internal) condyle of the humerus and intermuscular septa and lies between the pronator radii teres externally and the palmaris longus internally. It runs obliquely across the forearm, striking the wrist at about the junction of the middle and outer thirds. It lies next to and to the outer side of the palmaris longus tendon and to the ulnar side of the radial artery and inserts into the front of the base of the second metacarpal bone (Fig. 324).
The flexor carpi ulnaris arises by two heads, one from the common tendon of the medial (internal) condyle and the other from the olecranon process and upper two-thirds of the ulna. The two heads are separated by the ulnar nerve, which passes down in the groove between the medial condyle and olecranon process. The muscle passes straight down the anterior and inner surface of the ulna to insert first into the pisiform bone and unciform process and then to continue over to the base of the fifth metacarpal bone. The pisiform bone is a sesamoid bone in the tendon of the flexor carpi ulnaris muscle.
Both the flexor carpi radialis and the flexor carpi ulnaris flex the hand at the wrist. When the ulnaris alone acts it tends to tilt the hand inward; when the radialis acts alone it tends to incline the hand outward. Being superficial, these muscles are both important landmarks and guides to the arteries.
Fig. 324. - The flexor muscles of the wrist.
Fig. 325. - The extensor muscles of the wrist.
The extensor carpi radialis longior arises from the lower third of the external supracondylar ridge and the lateral (external) condyle and inserts into the back of the base of the second metacarpal bone. When it contracts it tends to tilt the hand toward the radial side as well as to extend it, and, being attached to the humerus above the line of the elbow-joint, it also aids in flexing the elbow.
The extensor carpi radialis brevior arises from the common tendon of the lateral condyle and fascia, and, running down parallel to the longior muscle, inserts into the base of the third metacarpal bone. It is covered by the extensor carpi radialis longior muscle and lies on the supinator (brevis). It acts as a pure extensor of the wrist (Fig. 325).
The extensor carpi ulnaris arises by two heads, one from the lateral (external) condyle and the other from the posterior surface of the ulna through the fascia common to it, to the flexor carpi ulnaris, and to the flexor profundus digitorum. It inserts into the base of the fifth metacarpal bone. It extends the wrist and tilts the hand toward the ulnar side.