Muscles of the Upper Extremity

V. THE MUSCLES OF THE FOREARM

The antebrachial fascia (deep fascia of the forearm), continuous above with the brachial fascia, is a dense investment which forms a general sheath for the muscles in this region; it is attached; behind, to the olecranon and posterior border of the ulna, and sends off from its deep surface numerous intermuscular septa. It gives origin to muscular fibers, especially at the upper part of the medial and lateral sides of the forearm, and also ensheathes the different muscles; transverse septa are given off both on the anterior and posterior surfaces of the forearm, separating the deep from the superficial laver of muscles. It is much thicker on the posterior than on the anterior surface, and at the lower than at the upper part of the forearm, and is strengthened above by tendinous fibers derived from the Biceps in front, and from the Triceps behind. In the region of the carpus there are two localized thickenings in the fascia, termed the flexor and extensor retinacula (transverse and dorsal carpal ligaments). The two bands retain the digital tendons in position and so increase the efficiency of their actions on the fingers. Apertures exist in the fascia, for the passage of vessels and nerves; one of these apertures, of large size and situated at the front of the elbow, transmits a communicating branch between the superficial and deep veins.

The antebrachial or forearm muscles consist of an anterior and a posterior group.

Anterolateral Muscles | Posterior Muscles
1. THE ANTERIOR ANTEBRACHIAL MUSCLES

These muscles are divided for convenience of description into two groups, superficial and deep.

(a) Superficial Group (fig. 625)

Pronator teres Palmaris longus
Flexor carpi radialis Flexor carpi ulnaris
Flexor digitorum sublimis

The muscles of this group take origin from the medial epicondyle of the humerus by a common tendon; they receive additional fibers from the antebrachial fascia near the elbow, and from the septa which pass from this fascia between the individual muscles.

The pronator teres (figs. 625, 626) has a humeral and an ulnar head of origin. The humeral head, the larger and more superficial, arises immediately above the medial epicondyle, and from the tendon common to the origin of the other muscles; also from the intermuscular septum between it and the Flexor carpi radialis and from the antebrachial fascia. The much smaller ulnar head arises from the medial side of the coronoid process of the ulna below the origin of the flexor digitorum sublimis, and joins the humeral head at an acute angle. The median nerve enters the forearm between the heads of the muscle, and is separated from the ulnar artery by the ulnar head. The muscle passes obliquely across the forearm and ends in a flat tendon, which is inserted into a rough impression on the middle of the lateral surface of the shaft of the radius. The lateral border of the muscle forms the medial boundary of the triangular hollow, which is situated in front of the elbow-joint, and contains the median nerve, brachial artery, and tendon of the Biceps.

Nerve-supply.-The Pronator teres is supplied by the median nerve (C. 6).

Actions.-The Pronator teres rotates the radius upon the ulna, turning the palm of the hand backwards; it also helps to flex the elbow-joint.

Figure 625
Superificial forarm flexor muscles anterior view - Figure 625
The Flexor carpi radialis (figs. 625, 626, 629) lies on the medial side of the Pronator teres. It arises from the medial epicondyle by the common tendon, from the antebrachial fascia, and from the intermuscular septa between it and the adjacent muscles. Its fleshy belly is fusiform in shape and, rather more than halfway down the forearm, ends in a long tendon, which passes through a. canal in the la eras part of the flexor retinaculurn (transverse carpal ligament) and occupies a groove on the trapezium (greater multangular bone); this groove is lined by a synovial sheath. The tendon is inserted into the palmar aspect of the base of the second metacarpal bone, and sends a slip to the base of the third metacarpal bone. These bony attachments are hidden by the origin of the oblique head of the adductor pollicis (fig. 638). In the lower part of the forearm the radial artery lies between the tendon of this muscle and that of the Brachioradialis.

Nerve-supply.–The Flexor carpi radialis is supplied by the median nerve (C. 6).

Actions.-Acting with the flexor carpi ulnaris, the flexor carpi radialis flexes the wrist; acting with the radial extensors of the wrist, it helps to abduct the hand.

The Palmaris longus (figs. 621, 626, 637) is a slender, fusiform muscle, lying on the medial side of the Flexor carpi radialis. It arises from the medial epicondyle of the humerus by the common tendon, from the intermuscular septa between it and the adjacent muscles; and from the antebracbial fascia. It ends in a long slender tendon, which passes in front of the flexor retinaculum (transverse carpal ligament), and is inserted into the anterior surface of the distal one-half of this ligament and into the central part of the palmar aponeurosis, frequently sending a tendinous slip to the short muscles of the thumb. Just above the wrist the median nerve lies deep to the tendon, and projects a little beyond its lateral edge.

This muscle is often absent, and is subject to very considerable variations it may be tendinous above and. muscular below, or muscular in. the middle with a tendon above and below; it may consist of two muscular bundles with a central tendon; or may be represented solely by a tendinous band.

Nerve-supply.–The Palmaris longus is supplied by the median nerve (C. 8).

Actions.-The Palmaris longus tightens the palmar aponeurosis and flexes the wrist.

The Flexor carpi ulnaris (figs. 625, 626. 629) lies along the ulnar side of the forearm. It arises by two heads, humeral and ulnar, connected by a tendinous arch, beneath which the ulnar nerve passes downwards and the posterior ulnar recurrent artery upwards. The humeral head is very small and arises from the medial epicondyle of the humerus by the common tendon; the ulnar head arises from the medial margin of the olecranon, and from the upper two-thirds of the posterior border of the ulna by an aponeurosis common to it and the Extensor carpi ulnaris and Flexor digitorum profundus, and from the intermuscular septum between it and the Flexor digitorum sublimis. The fibers end in a tendon which is formed along the anterolateral border of the muscle in its distal one-half and is inserted into the pisiform bone, whence it is prolonged to the hamate and fifth metacarpal bones by the pisohamate and pisometacaxpal ligaments; it is also attached by a few fibers to the flexor retinaculum (transverse carpal ligament). The ulnar vessels and nerve lie on the lateral side of the tendon of insertion of this muscle.

Nerve- supply.-The Flexor carpi ulnaris is supplied by the ulnar nerve (C. 8 and T. 1).

Actions.-Acting with the flexor carpi radialis, the flexor carpi ulnaris flexes the wrist; acting with the extensor carpi ulnaris, it is a powerful adductor of the hand.

The Flexor digitorum sublimis (figs. 625, 626, 629) is deep to the preceding muscles; it is the largest of the muscles of the superficial group, and arises by two heads, humero-ulnar and radial. The humero-ulnar head arises from the medial epicondyle of the humerus by the common tendon, from the anterior portion of the medial (ulnar collateral) ligament of the elbow, from the intermuscular septa between it and the preceding muscles, and from the medial side of the coronoid process, above the ulnas origin of the Pronator teres. The radial head, a thin sheet of muscle, arises from the anterior border of the radius, extending from the radial tuberosity to the insertion of the Pronator teres. The median nerve and the ulnar artery pass downwards through the gap which intervenes between these two heads. The muscle speedily separates into two strata of muscular fibers, superficial and deep; the superficial stratum, which is joined on its lateral side by the radial head, divides into two parts, which end in tendons for the middle and ring fingers; the deep stratum gives off a muscular slip to join that part of the superficial plane which is associated with the tendon of the ring finger, and then divides into two parts, which end in tendons for the index and little fingers. As the four tendons pass behind the flexor retinaculum (transverse carpal ligament), they are arranged in pairs, the superficial pair going to the middle and ring; the deep pair to the index and little fingers. The tendons diverge from one another in the palm and opposite the bases of the proximal phalanges each divides into two slips to allow of the passage of the corresponding tendon of the Flexor digitorum profundus; the surfaces of the two slips become reversed and they then reunite, partially decussate, and so form a grooved channel for the reception of the tendon of the Flexor digitorum profundus. Finally the tendon divides and is inserted into the sides of the shaft of the middle phalanx.

Figure 626
Forarm muscles, arteries, nerves, veins, transverse section at radial tuberosity - Figure 626
Nerve-supply.-The Flexor digitorum. sublimis is supplied by the median nerve (C. 7 and 8 and T. 1).

Actions.-The Flexor digitorum sublimis flexes first the middle and then the proximal phalanges. It also acts as a flexor of the wrist.

Relations.-In the forearm. The Flexor digitorum sublimis is covered by the Palmaris longus. Flexor carpi radialis, Pronator teres, Brachioradialis, the radial artery and the radial nerve. It is placed in front of the Flexor digitorum profundus, the upper part of the ulnar artery, the median nerve (which is closely bound to it by fibro-areolar tissue) and the Flexor pollicis longus. At the wrist, the tendons of the Flexor digitorum sublimis pass behind the flexor retinaculum, lying in front of the tendons of the Flexor digitorum profundus and sharing a common synovial sheath with them (fig. 634). The Flexor pollicis longus tendon and the median nerve lie to their lateral side. In the hand, the tendons lie behind the palmar aponeurosis, the superficial palmar (volar) arch and the digital branches of the median and ulnar nerves, but in front of the tendons of the Flexor digitorum profundus and the lumbrical muscles.

(b) Deep Group (fig. 627)

Flexor digitorum profundus Flexor pollicis longus
Pronator quadratus

The Flexor digitorum profundus (figs. 626; 627, 629) is situated on the ulnar side of the forearm, deep to the superficial flexors. It arises from the upper three-fourths of the anterior and medial surfaces of the shaft of the ulna embracing the insertion of the Brachialis above, and extending to within a short distance of the Pronator quadratus below. It also arises from a depression on the medial side of the coronoid process of the ulna, and from the upper three-fourths of the posterior border of the bone by an aponeurosis, in common with the Flexor and Extensor carpi ulnaris; it also springs from the anterior aspect of the ulnar half of the interosseous membrane. The muscle ends in four tendons, which run behind the flexor retinaculum (transverse carpal ligament) deep to the tendons of the Flexor digitorum sublimis. The portion of the muscle for the index finger is usually distinct throughout, but the tendons for the middle, ring and little fingers are connected together by areolar tissue and tendinous slips, as far as the palm of the hand. Opposite the proximal phalanges the tendons pass through the openings in the tendons of the Flexor digitorum sublimis, and they are inserted into the palmar aspects of the bases of the distal phalanges.

Nerve-supply.-The medial part of the Flexor digitorum profundus is supplied by the ulnar nerve and. its lateral part by the anterior interosseous branch of the median nerve (C. 7 and 8 and T. 1).

Actions.-The Flexor digitorum profundus flexes the distal phalanges, after the Flexor digitorum sublimis has bent the middle phalanges; it also assists in flexing the wrist.

Four small muscles, named the Lumbricales, are connected with the tendons of the Flexor digitorum profundus in The palm, and are described with the muscles of the band.

Fibrous sheaths of the flexor tendons.-After leaving the palm, the tendons of the Flexor digitorum sublimis et profundus lie in osseo-aponeurotic canals (figs. 635, 640), formed behind by the phalanges, and in front by fibrous bands which arch across the tendons, and are attached on each side to the margins of the phalanges and to the palmar ligaments of the interphalangeal joints. Opposite the middle of the proximal and middle phalanges the bands (digital vaginal ligaments) are very strong, and the fibers are transverse; but opposite the joints they are much thinner, and consist of annular and cruciate fibers. Each canal is lined by a synovial sheath, which is reflected on to the contained tendons,

As the flexor tendons approach their insertions they are connected to the dorsal parts of the enclosing synovial sheaths by triangular and thread-like bands of synovial membrane. These bands, termed vincula tendinum (fig. 628), convey minute vessels to the tendons, and are of two kinds, (a) vincula brevia and (b) vincula longa.

The vincula brevia, two in number in each finger, are triangular hands attached to the deep surfaces of the tendons close to their insertions; one connects the tendon of the Flexor digitorum sublimis to the front of the proximal interphalangeal joint and adjacent part of the proximal phalanx, and. the other the tendon of the Flexor digitorum profundus to the front of the distal interphalangeal joint and adjacent part of the middle phalanx. The vincula longa are thread-like slips, of which two are usually attached to each tendon of the Flexor digitorum sublimis, and one to each tendon of the Flexor digitorum profundus. Those of the Flexor digitorum sublimis are connected to the slips of that tendon where these fold over the tendon of the Flexor digitorum profundus, and, passing one on each side of the latter tendon, are attached to the sheath at the lateral margins of the proximal end of the proximal phalanx. That of the tendon of the Flexor digitorum profundus is fixed to its tendon shortly after the latter has pierced the tendon of the Flexor digitorum sublimis. It runs upwards and backwards, perforates one of the two slips of the latter tendon, or passes between the two slips; thereafter it blends with the vincula breva of the Flexor digitorum sublimis, and is attached to the dorsal wall of the synovial sheath at the distal end of the proximal phalanx.

Figure 627
Deep forarm flexor muscles anterior view - Figure 627
The Flexor pollicis longus (figs. 627, 629) is situated on the radial side of the forearm in the same plane as the Flexor digitorum profundus. It arises from the grooved, anterior surface of the shaft of the radius, extending from immediately below the tuberosity to within a short distance of the Pronator quadratus. It arises also from the adjacent part of the interosseous membrane and generally by a fusiform, fleshy slip from the medial border of the coronoid process, distal to the Flexor digitorum sublimis and Pronator teres, or from the medial epicondyle of the humerus. The fibers end in a flattened tendon, which passes behind the flexor retinaculum (transverse carpal ligament), is then lodged between the opponens pollicis and the oblique head of the Adductor pollicis, and entering an osseo-aponeurosic canal similar to those for the flexor tendon of the fingers, is inserted into the palmar aspect of the base of the distal phalanx of the thumb. The anterior interosseous nerve and vessels descend on the front of the interosseous membrane between the Flexor pollicis longus and Flexor digitorum profundus.

Figure 628
Hand muscle web of thumb and index finger laterla view - Figure 628
Nerve-supply.-The Flexor pollicis longus is supplied by the anterior interosseous branch of the median nerve (C. 8 and T. 1).

Actions.- The Flexor pollicis longus is a flexor of the phalanges of the thumb; it also acts as a flexor of the wrist.

The Pronator quadratus (fags. 627, 633) is a flat, quadrilateral muscle, extending across the front of the lower parts of the radius and ulna. It arises from the oblique ridge on the lower pat of the anterior surface of the shaft of the ulna (fig. 415); from the medial part of the anterior surface of the lower one-fourth of the ulna; and from a strong aponeurosis which covers the medial one-third of the muscle. The fibers pass laterally and slightly downwards, to be inserted into the lower one-fourth of the anterior border and surface of the shaft of the radius; the deeper fibers are inserted into the triangular area above the ulnar notch of the radius.

Nerve-supply.-The Pronator quadratus is supplied by the anterior interosseous branch of the median nerve (C. 6 and 7).

Action.-The Pronator quadratus pronates the forearm, i.e. turns it so that the palm of the hand is directed backwards.

2. THE POSTERIOR ANTEBRACHIAL MUSCLES

These muscles are divided for convenience of description into two groups; superficial and deep.

(a.) Superficial Group (fig. 630)

 

Brachioradialis Extensor digitorum
Extensor carpi radialis longus Extensor digiti minimi
Extensor carpi radialis brevis Extensor carpi ulnaris
Anconeus

The Brachioradialis (figs. 625, 626, 630) is the most superficial muscle on the radial side of the forearm. It arises from the upper two-thirds of the lateral supracondylar ridge of the humerus, and from the front of the lateral intermuscular septum. The radial nerve and the anastomosis between the arteria profunda brachii and the radial recurrent artery are interposed between it and the Brachialis. The fibers end above the middle of the forearm in a fiat tendon which is inserted into the lateral side of the lower end of the radius, immediately above the styloid process. The tendon is crossed at its insertion by the tendons of the Abductor pollicis longus and Extensor pollicis brevis; the radial artery is on its ulnar side.

Figure 629
Forarm muscles, arteries, nerves, veins, transverse section at middle of forarm - Figure 629
Nerve-supply.-The Brachioradialis is supplied by the radial nerve (C. 5 and 6).

Action.-The Brachioradialis is a flexor of the elbow-joint, but is supplied by the nerve of the extensor muscles, i.e. the radial nerve. It acts to best advantage when the forearm is in the midprone (neutral) position.

The Extensor carpi radialis longus (figs. 626, 630) is partly covered by the Brachioradialis. It arises mainly from the lower one-third of the lateral supracondylar ridge of the humerus and from the front of the lateral intermuscular septum; but it receives a few fibers from the common tendon of origin of the extensor muscles of the forearm. The muscle ends at the junction of the upper and middle thirds of the forearm in a flat tendon, which runs along the lateral border of the radius, deep to the abductor pollicis longus and Extensor pollicis brevis; it then passes under cover of the extensor retinaculum (dorsal carpal ligament), where it lies on the bath of the radius in a groove immediately behind the styloid process. It is inserted into the radial side of the dorsal surface of the base of the second metacarpal bone.

Nerve-supply.-The Extensor rarpi radialis longus is supplied by the radial nerve (C. 6 and 7).

The Extensor carpi radialis brevis (figs. 626, 629, 630) is shorter than the preceding muscle and is covered by it. It arises from the lateral epicondyle of the humerus, lay a tendon common to it and the next three muscles: from the lateral ligament of the elbow-joint; from a strong aponeurosis which covers its surface: and from the intermuscular septa between it and the adjacent muscles. The fibers end about the middle of the forearm in a flat tendon which closely accompanies that of the preceding muscle to the wrist: it passes deep to the Abductor pollicis lorigus and Extensor pollicis brevis, then under cover of the extensor retinaculum (dorsal carpal ligament), and is inserted into the dorsal surface of the base of the third metaearpal bone, on its radial side and distal to the styloid process, and into the adjoining part of the base of the second metacarpal bone. Under the extensor retinaculum the tendon lies on the back of the radius in a shallow groove, on the ulnar side of that which lodges the tendon of the Extensor carpi radiallis longus, and separated from it by a faint ridge.

The tendons of the two preceding muscles pass through the same compartment of the extensor retinaculum in a single synovial sheath.

Nerve-supply. The Extensor carpi radialis brevis is supplied by the posterior Interosseous nerve (deep branch of the radial nerve) (C. 6 and 7.).

Actions. – Both the preceding muscles, working with the Extensor carpi ulnaris extend the wrist; working with the Flexor carpi radialis; they abduct the hand.

The Extensor digitorum (Extensor digitorum communis) (figs. 626. 629, 6311) arises from the lateral epicondyle of the humerus by the common tendon from the intermuscular septa between it and the adjacent muscles, and from the antebrachial fascia. It divides below into four tendons, which pass together with that of the Extensor indicis, through a compartment of the extensor retinaculum (dorsal carpal ligament), within a synovial sheath. The tendons then diverge on the back of the hand, and are inserted into the middle- and distal phalanges of the fingers in the following manner. Opposite the metacarpophalangeal joint each tendon is bound by fasciculi to the collateral ligaments, and serves as the dorsal ligament of this joint; after crossing the joint, it spreads into an aponeurosis, which covers the dorsal surface of the proximal phalanx and is there reinforced by the corresponding tendons of the Interosseous and Lumbrical muscles. Opposite the proximal interphalangeal joint this aponeurosis divides into three slips, an intermediate and two collateral: the intermediate is inserted into the base of the middle phalanx; the two collateral are continued onwards along the sides of the middle phalanx, and uniting by their contiguous margins, are inserted into the dorsal surface of the base of the distal phalanx. As the tendons cross the interphalangeal joints, they serve as their dorsal ligaments. The tendon to the index finger is accompanied by the Extensor indicis, which lies on its ulnar side. On the back of the hand, the tendons to the middle, ring and little fingers are connected by two obliquely placed bands, one from the third tendon passing downwards and laterally to the second tendon, and the other passing from the fourth tendon to the third. Occasionally the second tendon is connected to the first by a thin, oblique band.

Nerve-supply.-The Extensor digitorum is supplied by the posterior interosseous nerve (deep branch of radial nerve) (C. 7).

Actions.-The Extensor digitorum extends the phalanges and then the wrist. Owing to its attachments to the collateral ligaments of the metacarpophalangeal joints it acts principally on the proximal phalanges, the middle and distal phalanges being extended mainly by the Interosseous and Lumbrical muscles it tends to separate the fingers as it extends them.

The Extensor digiti minimi (Extensor digiti qunti proprius) (fig. 630) is a slender muscle medial to, and usually connected with, the Extensor digitorum. It arises from the common extensor tendon by a thin tendinous slip, and from the intermuscular septa between it and the adjacent muscles. Its tendon runs through a compartment of the extensor retinaculum behind the inferior radioulnar joint, then divides into two as it crosses the hand, and finally joins the expansion of the Extensor digitorum tendon on the dorsum of the proximal phalanx of the fifth digit.

Nerve-supply.–The Extensor digiti minimi is supplied by the posterior interosseous nerve (C. 7).

Actions. – The Extensor digiti minimi extends the little finger and by its continued action extends the wrist.

Figure 630
Superficial forarm extensors posterior view - Figure 630
The Extensor carpi ulnaris (figs. 629, 630) arises from the lateral epicondyle of the humerus, by the common extensor, tendon; from the posterior border of the u1na by an aponeurosis in common with the Flexor carpi ulnaris and the Flexor digitorum profundus; and from the antebrachial fascia. It ends in a tendon, which runs in the groove between the head and the styloid process of the ulna, passing through a separate compartment of the extensor retinaculum (dorsal carpal ligament), and is inserted into the tubercle on the ulnar side of the base of the fifth metacarpal bone.

Nerve-supply.-The Extensor carpi ulnaris is supplied by the posterior interosseous nerve (C. 7).

Actions. – The Extensor carpi ulnaris; acting with the Extensor carpi radialis, extends the wrist; acting with the Flexor carpi ulnaris it adducts the hand.

The Anconeus (figs. 630; 631) is a, small, triangular muscle on the back of the elbow joint, and appears to be a continuation of the Triceps. It arises by a separate tendon from the posterior part of the lateral epicondyle of the humerus; its fibers diverge as they pass medially to reach the ulna, covering the posterior aspect of the annular ligament. They are inserted into the lateral side of the olecranon, and upper one-fourth of the posterior surface of the shaft of the ulna.

Nerve-supply.–The Anconeus is supplied by the radial nerve (C. 7 and 8).

Action.-The Anconeus assists the Triceps in extending the elbow-joint.

(b) Deep Group (fig. 631)

Supinator Extensor pollicis brevis
Abductor pollieis longus Extensor pollicis longus
Extensor indicis

The Supinator (figs. 626, 631, 632) surrounds the upper one-third of the radius, and consists of a superficial and a deep part; between which the posterior interosseous nerve (deep branch of the radial nerve) passes. The two parts arise in common—the superficial one by tendinous and the deeper by muscular fibers from the lateral epicondyle of the humerus; from the lateral ligament of the elbow joint, and the annular ligament of the superior radio-ulnar joint, from the supinator crest of the ulna and from the posterior part of the triangular depression in front of it; and from a tendinous expansion which covers the surface of the muscle. The muscle is inserted into the lateral surface of the proximal one-third of the radius, reaching as far as the insertion of the Pronator teres. The insertion extends onto the anterior and posterior aspects of the radius, for the anterior and posterior borders of the bone incline medially at their upper ends (fig. 416).

Figure 631
Deep forarm extensors posterior view - Figure 631
Nerve-supply.-The Supinator is supplied by the posterior interosseous nerve (C. 5 and 6).

Actions.–The Supinator rotates the radius so as to turn the palm of the hand forwards.

The Abductor pollicis longus (figs. 629, 630, 631) lies distal to the Supinator and is closely related to the Extensor pollicis brevis. It arises from the lateral part of the posterior surface of the shaft of the ulna below the insertion of the Anconeus, from the interosseous membrane, and. from the middle one-third of the posterior surface of the shaft of the radius immediately adjoining the insertion of the supinator. Passing obliquely downwards and laterally, it ends in a tendon, (frequently two tendons), which runs in a groove on the lateral side of the lower end of the radius, accompanied by the tendon of the Extensor pollicis brevis, and is inserted into the radial side of the base of the first metacarpal bone. It occasionally gives off two slips near its insertion : one to the trapezium (greater multangular bone), and the other to blend with the origin of the Abductor pollicis brevis.

Nerve-supply.-The Abductor pollicis longus is supplied by the posterior interosseous nerve. (C. 6 and 7).

Actions.–The Abductor pollicis longus, acting with the abductor pollicis brevis, abducts the thumb; acting with the Extensor pollicis; it extends the thumb at the carpometacarpal joint; acting with the Flexor carpi radialis and the corresponding extensors, it assists in abducting the hand, but it can only do so indirectly through the extended thumb.

The Extensor pollicis brevis (figs. 630, 631) lies on the medial side of, and is closely connected with, the Abductor pollicis longus. It arises from the posterior surface of the shaft of the radius below that muscle, and from the interosseous membrane. Its direction is similar to that of the Abductor pollicis longus, its tendon passing through the same groove on the lateral side of the lower end of the radius, to be inserted into the dorsal surface of the base of the proximal phalanx of the thumb.

Figure 632
Elbow supinator muscle anterior view - Figure 632
Nerve-supply.-The Extensor pollicis brevis is supplied by the posterior interosseous nerve (C. 7).

Actions.–The Extensor pollicis brevis extends the proximal phalanx of the thumb; by its continued action it extends the wrist, and abducts the hand. In the lower one-third of the forearm the Abductor pollicis longus and the Extensor pollicis brevis become superficial by emerging between the Extensor carpi radialis brevis and the Extensor digitorum. They then run obliquely across the tendons of the radial extensors of the wrist, cover the insertion of the Brachioradialis, and, passing through the most lateral compartment of the extensor retinaculum (dorsal carpal ligament), cross superficial to the styloid process of the radius and the radial artery.

The Extensor pollicis longus (figs. 630, 631) is larger than the Extensor pollicis brevis, the origin of which it partly covers, It arises from the lateral part of the middle one-third of the posterior surface of the shaft of the ulna below the origin of the Abductor pollicis longus, and from the interosseous membrane. It ends in a. tendon, which passes through a, compartment of the extensor retinaculum, lying in a narrow, oblique groove on the back of the lower end of the radius. It then crosses obliquely the tendons of the Extensor carpi radialis longus et brevis, and is separated from the Extensor pollicis brevis by a triangular interval, in which the radial artery lies; it is inserted into the base of the distal phalanx of the thumb.

Nerve-supply.-The Extensor pollicis longus is supplied by the posterior interosseous nerve (C. 7),

Actions.-The Extensor pollicis longus extends the distal phalanx of the thumb: by its continued action it extends the wrist, and abducts the hand. The Extensor indicis (Extensor indicis proprius) (fig. 631) is a narrow elongated muscle, medial to, and parallel with, the preceding. It arises from the posterior surface of the shaft of the ulna below the origin of the Extensor pollicis longus, and from the interosseous membrane. Its tendon passes under cover of the extensor retinaculum in the compartment which transmits the. tendons of the Extensor digitorum; opposite the head of the second metacarpal hone it joins the ulnar side of the tendon of the Extensor digitorum which runs to the index finger.

Nerve- supply.–The Extensor indicis is supplied by the posterior interosseous nerve (C. 7).

Actions.-The Extensor indicis extends the index finger, and assists in extending the wrist.

Applied Anatomy-The tendons of the Abductor longus and extensors of the thumb are liable to become strained, and their sheaths inflamed, (tenosynovitis) after excessive exercise producing a sausage-shaped swelling along the course of the tendons and giving a peculiar grating sensation to the touch when the muscles are put in action.

Paralysis of the extensor muscles of the wrists and fingers resulting in ‘wrist-drop’ is common in lead poisoning in painters. The different extensor muscles are affected unequally as a rule. Thus the thumb, or index, or little finger may be but slightly implicated, and recover rapidly while the extensors of the other fingers or wrist retrain power less; and some of the flexor muscles of the fingers may become paretic. This apparently selective action of the lead in cases of lead poisoning depends in reality upon occupational over-use of the affected muscles or groups of muscles.

 


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