Muscles of the Forearm.—The chief groups of muscles on the forearm are the flexors and pronators on the anterior surface and the extensors and supinators on the posterior surface. In general the flexors and pronators take their origin from on or around the internal condyle, while the extensors and supinators arise on or around the external condyle. Where not otherwise stated it will be understood that such is their origin. In a general way they may by grouped as follows:
| Anterior Surface. | ||
| Flexors of wrist |  | flexor carpi radialis |
| flexor carpi ulnaris | ||
| palmaris longus | ||
| Flexors of fingers |  | flexor sublimis digitorum |
| flexor profundus digitorum | ||
| Flexor of thumb | flexor longus pollicis | |
| Pronators of hand | | pronator radii teres |
| pronator quadratus | ||
Posterior Surface. | ||
| Extensor of forearm | anconeus | |
| Extensors of wrist | | extensor carpi radialis longior |
| extensor carpi radialis brevior | ||
| extensor carpi ulnaris | ||
| Extensors of thumb | | extensor ossis metacarpi pollicis |
| extensor primi internodii pollicis | ||
| extensor secundi internodii pollicis | ||
| Extensor of fingers | extensor communis digitorum | |
| Extensor of index finger | extensor indicis | |
| Extensor of little finger | extensor minimi digiti | |
| Supinators of hand | | supinator longus |
| supinator brevis | ||
Of the flexors of the wrist the flexor carpi radialis is inserted into the base of the index and usually of the third metacarpal bone, the flexor carpi ulnaris into the fifth metacarpal, the pisiform and the unciform bones, while the palmaris longus goes to the anterior annular ligament of the wrist and the palmar fascia of the hand. The flexor sublimis digitorum is inserted by four tendons into the second phalanges of the fingers, while the flexor profundus digitorum arises from the upper part of the ulna and is inserted into the last phalanges of the fingers. The flexor of the thumb arises from the middle of the radius and is inserted into the last phalanx of the thumb. Which joint is flexed by a muscle depends upon the origin and insertion of the muscle, all those included between being affected. Thus, the flexor sublimis digitorum, which has its origin in part at least from the condyle and is inserted in the second phalanges of the fingers, flexes the forearm, wrist, and all the finger-joints but the last, while the flexor profundus digitorum, arising from the ulna, though it flexes the wrist and fingers, has no power of flexing the forearm.
The pronator radii teres, besides arising from the supra-condylar ridge, rises from the coronoid process of the ulna. It is inserted into the middle of the outer surface of the radius and serves to pronate the forearm. The other pronator, the pronator quadratus, is a small quadrilateral muscle extending transversely across the radius and ulna just above their carpal extremities. It rises from the anterior surface of the ulna and is inserted into the anterior external border of the radius.
On the back of the forearm the anconeus serves to extend the forearm only, being inserted into the upper part of the posterior surface of the ulna. The extensors of the wrist are inserted into the bases of the various metacarpal bones and have some power to extend the forearm as well as the wrist. The extensors of the thumb, as their names imply, go one to the metacarpal bone and one to each of the phalanges, the longest one extending the whole thumb, the others only a part. They rise from the ulna and radius, not the condyle. The extensor communis digitorum goes to all the phalanges of all the fingers, the extensor minimi digiti to those of the little finger only, and the extensor indicis to those of the index finger, the last two arising short of the condyle.
Of the supinators the longer one is inserted into the styloid process of the radius, while the shorter one, the supinator brevis, is inserted into the upper part of the same bone, both thus serving to turn the radius on the ulna.
Where the tendons of the various muscles pass over the wrist, both front and back, they are covered with a synovial sheath and are held down by a broad ligament, which some of them perforate, the annular ligament. The strong fibrous band of the anterior annular ligament arches over the carpal bones in front. Beneath it pass the median nerve and the tendons of the flexors of the fingers and thumb. The posterior annular ligament is of less importance.
The deep palmar fascia forms a sheath for the muscles of the hand. In carpenters there sometimes occurs Dupuytren’s contraction of the palmar fascia, which draws the fingers up. As operation is not always successful, it is quite a serious matter.
The muscles of the hand itself include various abductor, adductor, and short flexor muscles of the thumb and little finger. There also extend between the metacarpal bones the lumbricales, four small muscles that aid the deep flexor muscles; likewise seven interossei, of which four are dorsal and three palmar. The dorsal interossei arise by two heads from the adjacent sides of the metacarpal bones and are inserted into the bases of the first phalanges, thus abducting the fingers; while the palmar interossei, arising from the palmar surface of the second, fourth, and fifth metacarpals, are inserted into the three corresponding first phalanges and adduct the fingers toward an imaginary line drawn through the middle finger.
Joints of the Upper Extremity.—The joints of the upper extremity, with the exception of the wrist-joint, are the most freely movable of any in the body, probably because the hand has the finest work to do and a greater number of motions are required. Even the wrist has much greater freedom of motion than the corresponding joint in the lower extremity.