These may be divided into three groups.
1. The superficial muscles arise from a common ori- gin on the lateral supracondylar line and the lat- eral epicondyle of the humerus [see Fig. 6.2]. They pass to either: (a) the bones of the forearm [Fig.
8.6]; (b) the bones of the hand [Fig. 8.22]; or (c) the bones of the fingers.
(i) Centrally placed in the superficial group are the muscles passing to the fingers—the ex- tensor digitorum and extensor digiti minimi.
(ii) On either side of the extensor digitorum and extensor digiti minimi are the three exten- sors of the carpals which are inserted into the Fig. 8.18 Superficial dissection of the back of the forearm.
Triceps Lateral intermuscular septum
Lateral epicondyle Olecranon Anconeus
Posterior surface of ulna Extensor carpi ulnaris
Extensor digiti minimi
Posterior interosseous A.
Extensor digitorum Ulna
Extensor pollicis longus Radial artery Extensor retinaculum Radius Extensor pollicis brevis Abductor pollicis longus
Extensor digitorum Extensor carpi radialis brevis Extensor carpi radialis longus Biceps Brachioradialis
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Extensor compartment of the forearm and hand
Fig. 8.19 Dissection of the lateral side of the left wrist and hand, showing the tendons in their synovial sheaths.
Adductor pollicis 1st dorsal interosseous Extensor digitorum Extensor pollicis longus Extensor carpi radialis brevis Radial A.
Capsule of wrist joint Extensor carpi radialis longus Extensor retinaculum
Extensor pollicis longus Abductor pollicis longus
Brachioradialis
Radial A.
Flexor pollicis longus Flexor carpi radialis Abductor pollicis longus Extensor pollicis brevis Abductor pollicis brevis
Extensor pollicis brevis
Fig. 8.20 Dissection of the back of the forearm, wrist, and hand, showing the tendons in their synovial sheaths.
Extensor indicis Extensor digitorum
1st dorsal interosseous Adductor pollicis
Extensor pollicis longus
Extensor pollicis brevis Abductor pollicis longus
Extensor carpi radialis longus Extensor carpi radialis brevis Radius
Abductor pollicis longus Abductor digiti minimi
Extensor digiti minimi
Extensor retinaculum Styloid process of ulna
Extensor digiti minimi Ulna Extensor carpi ulnaris Extensor digitorum
Radial A.
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The forearm and hand
side of the dorsal tubercle of the radius. More medially, the extensor indicis lies medial to the tendons of the extensor digitorum [Fig. 8.21].
3. A single deep muscle—the supinator—passes between the two forearm bones. It arises from the lateral surface of the fibrous capsule of the elbow joint and from the upper end of the ulna, distal to the radial notch. It curves round the posterior and lateral surfaces of the radius [Fig. 8.21] and is inserted on the radius between the attachments of the flexor digitorum superficialis [Fig. 8.4] and pro- nator teres. Figs. 8.23 and 8.24 show the relation- ship of muscles and tendons to the bones at the upper third of the forearm and wrist.
The nerve supply to all these muscles is from the radial nerve and its branches. The brachiora- dialis, the two extensor carpi radialis, and the su- pinator are supplied by branches which arise from the radial nerve anteriorly. The remaining muscles, except the anconeus, are supplied by the posterior interosseous nerve which is the continuation of the deep branch of the radial nerve. Thus, injury to the deep branch of the radial nerve in the supi- nator paralyses only these muscles. (The anconeus is supplied by a branch from the radial nerve to the medial head of the triceps.)
Actions of extensor muscles of the forearm
The actions of the extensor muscles of the fingers and thumb will be dealt with later when the muscles which act with them have been dissected. The ac- tion of the brachioradialis has been described earlier.
The extensor carpi radialis longus and bre- vis and the extensor carpi ulnaris act together to extend the wrist. They also stabilize the wrist in the extended position when the flexor muscles of the fingers act, so that these muscles can apply the full range of their contraction to the fingers.
An important point to remember is that, when the wrist is extended, the flexors of the wrist are relaxed.
When the wrist is flexed, the fist cannot be closed tightly, because the flexor muscles of the fingers cannot contract sufficiently and the extensor mus- cles of the fingers cannot stretch adequately to allow this movement. So unless the wrist is extended, a tight fist cannot be formed. Radial extensors of the wrist act with the radial flexor to produce abduc- tion of the wrist. This is an important movement, e.g. in raising a hammer prior to striking with it—
which requires the strength of two radial extensors.
The extensor and flexor carpi ulnaris act in a and the tendons of the lateral three muscles of the
superficial group—brachioradialis, and extensor carpi radialis longus and brevis—to the lateral as- pect of the base of the first metacarpal. Immediate- ly medial to these is the extensor pollicis lon- gus, the tendon of which curves round the medial Fig. 8.21 Deep dissection of the back of the forearm.
Triceps
Common extensor origin Annular ligament of radius Radial N., deep branch Anconeus
Posterior interosseous N. and A.
Posterior teres Ulna
Extensor carpi ulnaris
Extensor pollicis longus Extensor indicis
Dorsal carpal branch
Radial A.
Extensor retinaculum Extensor pollicis brevis
Abductor pollicis longus
Extensor carpi radialis brevis Extensor carpi radialis longus Brachioradialis
Supinator
2nd dorsal interosseous
1st dorsal interosseous Extensor carpi radialis longus
Extensor carpi radialis brevis
Extensor carpi ulnaris
4th dorsal interosseous 3rd dorsal interosseous
Fig. 8.22 Muscle attachments to the dorsal aspect of the right metacarpus.
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Extensor compartment of the forearm and hand
similar fashion to produce adduction of the wrist.
The flexors and extensors of the wrist act together to fix the wrist, so that fine movements of the fin- gers can take place on a stable hand.
The anconeus holds the ulna firmly against the humerus. It also contracts strongly during prona- tion, when the distal end of the ulna is displaced laterally by slight lateral rotation of the humerus.
This occurs in many movements, e.g. in using a screwdriver in the right hand to remove a screw.
The supinator supinates the forearm. It is a powerful muscle and acts with the biceps brachii.
It may be assisted by the abductor pollicis longus which arises, in part, from the ulna and runs paral- lel to the supinator.
The abductor pollicis longus is inserted into the anterolateral surface of the base of the first meta- carpal. It abducts the thumb at the carpometacar- pal joint and produces some extension. It also acts with the supinator, as described above.
Fig. 8.23 Section through the upper third of the left forearm. A = anterior; P = posterior; M = medial; L = lateral.
Pronator teres Brachioradialis
Radial N. (superficial branch) Extensor carpi radialis longus
Extensor carpi radialis brevis
Extensor digitorum
Posterior cutaneous N. of forearm
A
P M L Anconeus
Ulna
Flexor digitorum profundus
Flexor carpi ulnaris Flexor digitorum superficialis Palmaris longus Median N.
Flexor carpi radialis Radial A.
Ulnar N.
Radial N. (deep branch) Supinator
Fig. 8.24 Transverse section through the forearm above the flexor retinaculum, showing the relation of the synovial sheaths to the ten- dons. A = anterior; P = posterior; M = medial; L = lateral.
Flexor digitorum superficialis and profundus
Flexor carpi ulnaris
Ulnar N.
Median N.
Flexor pollicis longus Radial A.
Pronator quadratus
Radius
Extensor
pollicis longus Extensor digitorum
Extensor carpi ulnaris
Styloid process Ulna Ant. lig. of distal radio-ulnar joint
A
P M L
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The forearm and hand