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M. Extensor Digitorum Communis.—The extensor digitorum communis arises from the common tendon, from the lateral epicondyle of the humerus, from the fascia over it, and from intermuscular septa on either side. Extending along the dorsum of the forearm it ends, proximal to the wrist, in four tendons, of which the most lateral often has a separate fleshy belly. After passing under the dorsal carpal ligament, in a compartment along with the extensor indicis proprius, the tendons separate on the dorsum of the hand, where
Triceps brachii (insertion) the three most medial tendons are joined together by two obliquely placed bands. One passes distally and laterally, and connects together the third and second tendons; the other is a broader and
(insertion) shorter band, which passes also distally and laterally, and joins the fourth to the third tendon.
Biceps brachii (insertion) In some cases a third band is
Supinator muscle present which passes distally and
(insertion) medially from the first to the second tendon; and, frequently, the tendon for the little finger is joined to the tendon for the ring digitorum finger, and separates from it only profundus
(origin) a short distance above the distal
Abductor pollicis longus end of the metacarpal bone.
(origin) The tendons are inserted in the following manner :-On the
(insertion) finger each tendon spreads out so as to form a membranous expansion over the knuckle and on the
(origin) dorsum of the first phalanx. The border of the tendon is indefinite over the metacarpo - phalangeal
Extensor pollicis brevis articulation, of which it replaces
(origin) the dorsal ligament. On thedorsum of the first phalanx the tendon receives at its sides the insertions of the interosseous and lumbrical muscles. At the distal end of the
quadratus first phalanx it splits into ill-de
(origin) fined median and collateral slips,
Brachioradialis which pass over the dorsum of the
(insertion) first inter-phalangeal articulation,
Groove for tendons of where they replace the dorsal ligament. The median slip is Groove for
pollicis longus inserted into the dorsum of the carpi ulnaris base of the second phalanx, while
Groove for extensor digitorum com
inunis and extensor indicis proprius the two lateral pieces become
Fig. 355.--MUSCLE-ATTACHMENTS TO THE RIGHT RADIUS united to form a membranous
AND ULNA (Dorsal Aspect). tendon on the dorsum of the second phalanx, which, after passing over the second inter-phalangeal articulation, is inserted into the base of the terminal phalanx.
The muscle is placed superficially in the forearm, between the extensors of the carpus and the proper extensor of the little finger.
Nerve-Supply.—The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
Actions. The muscle extends the elbow, wrist, and fingers. On account of the attachment together of the tendons to the third, fourth, and fifth fingers by accessory bands in the dorsum of the hand, these three fingers can only be fully extended together, while extension of the first finger
adial extensors carpus -Groove for extensor
can take place separately. In extension of the inter-phalangeal joints, the muscle is aided by the interossei and lumbrical muscles.
M. Extensor Digiti Quinti Proprius.- The extensor digiti quinti proprius has an origin, similar to and closely connected with that of the preceding muscle, from the common tendon, the fascia over it, and from intermuscular septa.
It passes along the dorsum of the forearm, as a narrow fleshy slip, between the extensor digitorum communis and the extensor carpi ulnaris, and ends in a tendon, which occupies a groove between the radius and ulna in a special compartment of the dorsal carpal ligament. On the dorsum of the hand the tendon, usually split into two parts, lies on the medial side of the tendons of the extensor digitorum communis, and is finally inserted into the expansion of the extensor tendon on the dorsum of the first phalanx of the little finger.
Nerve-Supply.—The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
M. Extensor Carpi Ulnaris.—The extensor carpi ulnaris has a double origin : (1) from the common tendon from the lateral epicondyle of the humerus, from the fascia over it, and from the intermuscular septa ; and (2), through the medium of the deep fascia, from the dorsal margin of the ulna in its middle two-fourths.
Lying in the forearm upon the dorsal surface of the ulna, it ends in a tendon which occupies a groove on the dorsal surface of the ulna in a special compartment of the dorsal carpal ligament, and is inserted into the medial side of the base of the fifth metacarpal bone (Fig. 353, p. 394).
Nerve-Supply.-The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
Actions. --The muscle is an extensor of the wrist, and at the same time, acting with the flexor carpi ulnaris it is a powerful adductor of the wrist. Its humeral attachment makes it also a subordinate extensor of the elbow-joint.
M. Anconæus. The anconæus is a small triangular muscle. It arises, by a separate tendon, from the distal part of the dorsal surface of the lateral epicondyle of the humerus (Fig. 341, p. 380), and from the dorsal part of the capsule of the elbow-joint.
It covers part of the dorsal surface of the elbow-joint and proximal part of the ulna, and is inserted, by fleshy fibres, into a triangular surface on the lateral aspect of the olecranon and dorsal surface of the ulna, as far distally as the oblique line (Fig. 355, p. 397). It is also inserted into the fascia which covers it.
The epitrochleoanconæus is an occasional small muscle which arises from the dorsal surface of the medial epicondyle of the humerus, and is inserted into the medial side of the olecranon. It covers the ulnar nerve in its passage to the forearm.
Nerve-Supply.—The muscle is supplied by the terminal branch of the nerve to the medial head of the triceps muscle from the radial (C. 7. 8.).
Actions. The anconæus is an extensor of the elbow.
Deep Muscles. M. Supinator.--The supinator muscle (O.T. supinator radii brevis) is the most proximal of the deeper muscles. It is almost wholly concealed by the superficial muscles, and has a complex origin, -(1) from the lateral epicondyle of the humerus ; (2) from the radial collateral, and annular ligaments of the elbow-joint; (3) from the triangular surface on the shaft of the ulna just distal to the radial notch; and (4) from the fascia over it.
From this origin the muscle spreads laterally and distally, enveloping the proximal part of the radius, and is inserted into the volar and lateral surfaces of the bone, as far forwards as the tubercle of the radius, as far proximally as the neck, and as far distally as the oblique line and the insertion of the pronator teres (Figs. 348, p. 389, and 355, p. 397).
The muscle is divisible into superficial and deep parts with humeral and ulnar origins, between which the deep branch of the radial nerve passes in its course to the dorsal part of the forearm.
Nerve-Supply.–The supinator is supplied by a branch from the deep branch of the radial nerve, which arises from the nerve before the main trunk enters the muscle (C. 5. 6.).
Action.—The muscle is an extensor of the elbow, and the main supinator of the forearm. In the latter action it is assisted by the biceps.
M. Abductor Pollicis Longus.—The abductor pollicis longus (O.T. extensor ossis metacarpi pollicis) arises by fleshy fibres, distal to the supinator muscle, from the most proximal of the narrow impressions on the lateral half of the dorsal surface of the ulna; from the middle third of the dorsal surface of
the radius; and from the intervening portion of the interosseous membrane (Fig. 355, p. 397).
Becoming superficial in the distal part of the forearm, along with the extensor pollicis brevis, between the extensors of the wrist and the common extensor of the fingers, its tendon passes, with the latter muscle, under cover of the dorsal carpal ligament, to be inserted into the lateral side of the base of the first metacarpal bone (Fig. 356, p. 399). From the tendon, close to its insertion, a tendinous slip passes to the abductor pollicis brevis and the fascia over the thenar eminence, and another is frequently attached to the greater multangular bone.
Nerve-Supply.--The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
Actions. The muscle abducts the metacarpal bone of the thumb, and assists in abduction and extension of the wrist
M. Extensor Pollicis Brevis.-The extensor pollicis brevis (O.T. extensor primi internodii pollicis), an essentially human muscle, is a specialised portion of the previous muscle. It arises from a rhomboid impression on the dorsal surface of the radius, and from the interosseous membrane, distal to the abductor pollicis longus (Fig. 355, p. 397). It is closely adherent to that muscle, and accompanies it deep to the dorsal carpal ligament and over the radial artery to the thumb.
Its tendon is then continued along the dorsal surface of the first metacarpal bone, to be inserted into the dorsal surface of the base of the first phalanx of the thumb. Before reaching its insertion the tendon helps to form the capsule of the metacarpo-phalangeal joint.
Nerve-Supply.-- The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
Actions. The muscle extends the wrist and thumb (or the metacarpo-phalangeal joint), and assists in abduction of the wrist and thumb.
M. Extensor Pollicis Longus.— The extensor pollicis longus (O.T. extensor secundi internodii pollicis) arises from the lateral part of the dorsal surface of the ulna, in its middle third, and from the interosseous membrane, distal to the abductor pollicis longus (Fig. 355, p. 397). Its tendon grooves the dorsal surface of the radius, and occupies a special compartment under cover of the dorsal carpal ligament.
Extending obliquely across the dorsal surface of the hand, the tendon crosses the radial artery, helps to form the capsule of the first metacarpo-phalangeal articulation, and is inserted into the dorsal surface of the base of the second phalanx of the thumb.
At the wrist the tendons of the muscles of the thumb, the abductor pollicis longus and extensor pollicis brevis laterally, and the extensor pollicis longus medially, bound a hollow (the “anatomical snuff-box") best seen in extension and abduction of the thumb, which corresponds to the position of the radial artery as it winds round the wrist to reach the palm of the hand.
Nerve-Supply. The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
M. Extensor Indicis Proprius.—The extensor indicis proprius (O.T. extensor indicis) arises, distal to the extensor pollicis longus, from the most distal impression on the dorsal surface of the ulna, extending distally from the middle of the body to within two inches of its distal end, and sometimes also from the interosseous membrane (Fig. 355, p. 397). Its tendon passes through a compartment of the dorsal carpal ligament along with the tendons of the extensor digitorum communis.
On the dorsum of the hand the tendon lies on the medial side of the tendon of the common extensor destined for the forefinger, and is inserted into the forefinger, joining the membranous expansion of the tendon of the extensor digitorum communis on the dorsum of the first phalanx.
Nerve-Supply.-The dorsal interosseous nerve (C. (5.) 6. 7. 8.).
Actions of the Muscles of the Forearm and Hand. These muscles are concemed in the movements of the elbow, wrist, and digits. In the majority of cases the muscles act upon more than one joint.
1. Action on the Elbow-Joint.-It has been shown already that flexion and extension of the elbow are assisted by certain of these muscles. The flexor muscles are the pronator teres, and the flexor muscles of the wrist and fingers. In the position of pronation, the movement of flexion is aided by the brachioradialis and extensor muscles of the wrist and fingers. The extensors are the supinator muscle and anconæus, and the extensor muscles of the wrist and fingers.
2. Pronation and supination of the hand are performed by special muscles, aided by muscles which act also upon other joints. The brachioradiális assists in flexion and pronation on the one hand, and in extension and supination on the other hand. In the supine position it assists pronation, and in the prone position it assists supination, in each case bringing the hand into the position intermediate between pronation and supination.
3. Action on the Wrist-Joint.—The movements at the wrist-joint are flexion and extension, abduction and adduction. Flexion and adduction are much more extensive movements than extension and abduction, on account of the form of the wrist-joint. The following muscles produce these movements :
4. Movements of the Fingers.—Two separate series of movements occur in relation to the articulations of the fingers : flexion and extension (at the metacarpo-phalangeal and interphalangeal joints), and abduction and adduction (only at the metacarpo-phalangeal joints). The movements and the muscles concerned are given in the following tables :
Flexor digitorum sublimis
Extensor digitorum communis
Extensor indicis proprius
phalangeal articulations) Lumbricales ] (acting on the inter-pha-
Interossei Slangeal articulations)
Flexion is more powerful and complete than extension of the fingers. The flexor digitorum profundus alone acts on the terminal phalanges; the flexor sublimis and flexor profundus together tiex the proximal inter-phalangeal joint; and flexion of the metacarpo-phalangeal articulation is effected by these muscles, assisted by the interossei, lumbricales, and flexor digiti quinti brevis. Extension of the phalanges is effected by the united action of the extensors of the digits, the interossei and lumbricales; extension of the fingers at the metacarpo-phalangeal joints is produced rolely by the long extensor muscles. Separate extension of the index finger only is possible; the three inner fingers can only be flexed and extended together, on account of the connecting bands joining the extensor tendons together on the back of the hand. 5. Movements of the Thumb.—The movements of which the thumb is capable are flexion