VI. THE MUSCLES OF THE HAND
1, those of the thumb, which occupy the lateral side and produce the thenar eminence;
2, those of the little finger, which occupy the medial side and give rise to the hypothenar eminence;
3, those in the middle of the palm and between the metacarpal bones.
A localized thickening in the general investing layer of the antebrachial fascia which extends laterally from the pisiform bone (fig. 637) is termed the superficial part of the flexor retinaculum (volar carpal ligament). It crosses in front of the ulnar vessels and nerves and blends with the rest of the retinaculum on their lateral side.
The synovial sheaths of the tendons on the front of the wrist.-Two synovial sheaths envelop the flexor tendons as they traverse the carpal tunnel, one for the Flexores digitorum sublimis et profundus, the other for the Flexor pollicis longus (fig. 635). These sheaths extend into the forearm for about 2.5 cm. above the flexor retinaculum, and occasionally communicate with each other behind that structure. The sheath of the Flexores digitorum tendons reaches about halfway along the metacarpal bones, where it ends in blind diverticula around the tendons to the index, middle, and ring fingers. It is prolonged on the tendons to the little finger and usually communicates with the digital synovial sheath of these tendons. A transverse section through the carpus (fig. 634) shows that the tendons have been invaginated into the sheath from the lateral side. The parietal layer lines the flexor retinaculum and the floor of the earpal tunnel and is reflected, at the lateral side, on to the tendons of the flexor digitorum sublimis from in front, and on to those of the flexor digitorum profundus from behind. On the medial side a recess of the sheath is insinuated between the two groups of tendons for a variable distance. The sheath of the tendon of the Flexor pollicis longus, which may be separate or may communicate with the common flexor sheath behind the flexor retinaculum, is continued along the thumb as far as the insertion of the tendon.
The central, portion occupies the middle of the palm, is triangular in shape, and of great strength and thickness. Its apex is continuous with the distal margin of the flexor retinaculum (transverse carpal ligament) and gives insertion to the expanded tendon of the Palmaris longus. Its base divides into four slips, one for each finger. The slips give off superficial fibers to the skin of the palm and fingers; those to the palm joining the skin at the furrow corresponding to the metacarpophalangeal joints, and those to the fingers passing into the skin at the transverse folds at the roots of the fingers. The deeper part of each slip subdivides into two processes which are inserted into the fibrous sheaths of the flexor tendons; from the sides of these processes offsets are attached to the deep transverse ligaments of the palm (transverse metacarpal ligaments). By this arrangement short channels are formed on the front of the heads of the metacarpal bones; through these the flexor tendons pass. The intervals between the four slips transmit the digital vessels and nerves, and the tendons of the Lumbrical muscles. At the points of division into the slips mentioned, numerous strong, transverse fibers bind the separate processes together. The central part of the palmar aponeurosis is intimately bound to the skin by dense fiber-areolar tissue, and the proximal part of its medial margin gives origin to the Palmaris brevis. It covers the superficial palmar arch, the tendons of the Flexores digitorum, the terminal part of the median nerve; and the superficial part of the ulnar nerve; on each side it gives off an indefinite septum, which separates the intermediate from the lateral and medial groups of muscles.
The lateral and medial portions of the palmar aponeurosis are thin fibrous layers which cover the muscles of the ball of the thumb and the short muscles of the little finger respectively; they are continuous with the central portion and with the fascia on the dorsum of the hand.
The superficial transverse ligament of the palm forms a thin band (fig. 637) which stretches across the roots of the fingers, and is attached to the shin of the clefts, and medially to the fifth metacarpal bone, forming a sort of rudimentary web. The digital vessels and nerves pass deep to these fasciculi.
Applied Anatomy.-The palmar aponeurosis is liable to undergo contraction, producing a very inconvenient deformity known as Dupuytren’s contraction.’ The ring and little fingers are most frequently implicated, but the others may also be involved. The proximal phalanx is flexed and cannot be straightened, and the middle and distal phalanges become similarly flexed as the disease advances.
Owing to their constant exposure to injury and septic influences, the fingers are very liable to become the seat of serious inflammatory mischief. In some cases, the inflammation may involve the sheath or theca of the flexor tendons and may spread rapidly upwards along it; but the extent will depend upon the particular digit involved. From the description of the flexor sheaths given above, it will be evident that inflammation of the synovial sheaths of the thumb and little finger may prove a far more formidable affection than that of the other three digits, because the sheaths of these two digits communicate with the large synovial sheath which surrounds the flexor tendons, and the inflammation may extend into the palm of the hand and behind the flexor retinaculum into the forearm.
Chronic inflammation of the common flexor sheath is occasionally met with, constituting a disease known as compound palmar ganglion; it presents an hour-glass outline, with a swelling in front of the wrist and another in the palm of the hand, and a constriction, corresponding to each flexor retinaculum, between the two. The fluid can the forced from the one lining to the other under the retinaculum, and when this is done a creaking sensation is sometimes perceived, from the presence of ‘melon-seed’ bodies in the interior of the ganglion.
1. THE LATERAL MUSCLES OF THE PALM. (figs. 638, 639, 640)
| Abductor pollicis brevis | Flexor pollicis brevis |
| Opponens pollicis | Adductor pollicis |
The Abductor pollicis brevis (fig. 640) is a thin, subcutaneous muscle which occupies the radial part of the thenar eminence; its chief origin is from the flexor retinaculum (transverse carpal ligament), but a few fibers spring from the tubercle of the scaphoid bone and the crest of the trapezium and frequently some arise from the tendon of the Abductor pollicis longus. It is inserted by a thin, flat tendon into the radial side of the base of the proximal phalanx of the thumb; it also sends a slip to the tendon of the Extensor pollicis longus.
Nerve-supply-The Abductor pollicis brevis is supplied by the lateral ter. urinal branch of the median nerve.
Actions.-The Abductor pollicis brevis draws the thumb forwards in a plane at right angles to the palm of the hand.
The Opponens pollicis (figs. 638, 631) is placed under cover of the Abductor pollicis brevis. It arises from the crest on the trapezium and from the flexor retinaculum, and is inserted into the whole length of the lateral border, and the lateral one-half of the palmar surface; of the metacarpal bone of the thumb.
Nerve-supply.-The Opponens pollicis is supplied by the lateral terminal branch of the median nerve.
The Flexor pollicis brevis (fig. 640) lies on the ulnar side of the Abductor pollicis brevis. It arises from the lower border of the flexor retinaculum and the lower part of the crest on the trapezium; it passes along the radial side of the tendon of the Flexor pollicis longus, and is inserted into the radial side of the base of the proximal phalanx of the thumb; in its tendon of insertion there is a sesamoid bone. It is frequently more or less blended with the medial border of the opponens pollicis.
In the B.N.A. the Flexor pollicis brevis has a deep head, which is now described as the first palmar interosseous muscle.
Nerve-supply.-The Flexor pollicis brevis is supplied by the lateral terminal branch of the median nerve.
Actions.-The Flexor pollicis brevis flexes the proximal phalanx of the thumb.
The Adductor pollicis (fig. 639) arises by two heads, an oblique and a transverse. The oblique head arises from the capitate and trapezoid (lesser multangular) bones, the bases of the second and third metacarpal bones, the palmar ligaments of the corpus and the sheath of the tendon of the Flexor carpi radialis. Most of its fibers converge to a tendon, which, uniting with the tendons of the first palmar interosseous muscle (deep portion of the Flexor pollicis brevis) and the transverse head of the Adductor, is inserted into the ulnar side of the base of the proximal phalanx of the thumb, a sesamoid bone being present in the tendon. A considerable fasciculus, however, passes deep to the tendon of the Flexor pollicis longus and joins the Flexor pollicis brevis and the Abductor pollicis brevis. The transverse head (fig. 639) is the most deeply seated of this group of muscles. It is of a triangular form, and arises from the distal two-thirds of the palmar surface of the third metacarpal bone; the fibers converge to be inserted, with the oblique head of the muscle and with the first palmar interosseous muscle, into the ulnar side of the base of the proximal phalanx of the thumb.
Action-The Adductor pollicis approximates the thumb to the palm of the hand.
2. THE MEDIAL MUSCLES OF THE PALM (figs. 639, 640)
| Palmaris brevis | Flexor digiti minimi |
| Abductor digiti minimi | Opponens digiti minimi |
The Palmaris brevis (fig. 637) is a thin, quadrilateral muscle, placed beneath the skin of the ulnar side of the hand. It arises from the flexor retinaculum (transverse carpal ligament) and from the medial border of the central part of the palmar aponeurosis; it is inserted into the skin on the ulnar border of the hand.
Nerve-supply.-The Palmaris brevis is supplied by the superficial branch of the ulnar nerve.
Action.-The Palmaris brevis wrinkles the skin on the ulnar side of the palm of the hand.
Nerve-supply.-The Abductor digiti minimi is supplied by the deep branch of the ulnar nerve.
Action.-The Abductor digiti minimi abducts the proximal phalanx of the little finger.
The Flexor digiti minimi (Flexor digiti quinti brevis) (fig. 640) lies on the radial side of the preceding muscle. It arises from the convex surface of the hook of the hamate bone and the palmar surface of the flexor retinaculum, and is inserted into the ulnar side of the base of the proximal phalanx of the little finger with the Abductor digiti minimi. Its origin is separated from that of the Abductor by the deep branches of the ulnar artery and nerve. This muscle may be wanting, or may be fused with the Abductor.
Nerve-supply.—The Flexor digiti minimi is supplied by the deep branch of the ulnar nerve.
Actions.–The Flexor digiti minimi flexes the proximal phalanx of the little finger.
The Opponens digiti minimi (fig. 639) is of a triangular form; and placed under cover of the Flexor and Abductor. It arises from the convexity of the hoof of the hamate bone, and contiguous portion of the flexor retinaculum; it is inserted into the whole length of the ulnar margin of the fifth metacarpal bone.
Nerve-supply.-The Opponens digiti minimi is supplied by the deep branch of the ulnar nerve.
Action.-The Opponens digiti minimi draws the fifth metacarpal bone forwards and laterally, so as to deepen the hollow of the palm.
3. THE INTERMEDIATE MUSCLES
| Lumbricals | Interossei |
The Lumbricals (fig. 640) are four small fleshy fasciculi which take origin from the tendons of the Flexor digitorum profundus. The first and second arise from the radial sides and palmar surfaces of the tendons of the index and middle fingers respectively; the third, from the contiguous sides of the tendons of the middle and ring fingers; and the fourth, from the contiguous sides of the tendons of the ring and little fingers. Each passes to the radial side of the corresponding finger, and is inserted partly into the base of the proximal phalanx but mainly into the lateral margin of the tendinous expansion of the Extensor digitorum covering the dorsal surface of the finger.
Nerve-supply.-The first and second Lumbricals are supplied by the median nerve : the third and fourth Lumbricals by the deep terminal branch of the ulnar nerve. The third Lumbrical frequently receives a twig from the median nerve. ‘
Actions.—The Lumbricals flex the proximal and extend the middle and distal phalanges, a combination of movements which is called for in the upstroke in writing, etc.
The Interossei occupy the intervals between the metacarpal bones, and are divided into a dorsal and a palmar set.
The Interossei dorsales (fig. 641), four in number, are bipennate muscles, each arising by two heads from the adjacent sides of the metacarpal bones, but more extensively from the metacarpal bone of the finger into which the muscle is inserted. They are inserted into the bases of the proximal phalanges and into the aponeuroses of the tendons of the Extensor digitorum. Between the double origin of each of these muscles there is a narrow triangular interval; through the first of these intervals the radial artery passes; through each of the otlhers a perforating branch from the deep palmar arch is transmitted. The first, and largest; is sometimes named the Abductor indicis; it is inserted into the radial side of the index finger. The second and third are inserted into the middle finger, the former into its radial, the latter into its ulnar side. The fourth is inserted into the ulnar side of the ring finger.
The Interossei palmares (fig. 642), four in number, are smaller than the dorsal interossei, and are placed upon the palmar surfaces of the metacarpal bones, rather than between them. With the exception of the first, each arises from the entire length of the metacarpal bone of one finger, and is inserted into the side of the base of the proximal phalanx of the same finger, and into the aponeurosis of the Extensor digitorum tendon.
The first (formerly known as the ‘deep head of the Flexor pollicis brevis’) arises from the ulnar side of the palmar aspect of the base of the first metacarpal bone (fig. 638), and is inserted into the ulnar side of the base of the proximal phalanx of the thumb; in common with the principal part of the oblique bead of the Adductor pollicis. It lies in front of the lateral head of the first Dorsal interosseous muscle, and is covered by the oblique head of the adductor pollicis. The second arises from the ulnar side of the second metacarpal bone, and is inserted into the same side of the proximal phalanx of the index finger. The third arises from the radial side of the fourth metacarpal bone, and is inserted into the same side of the ring finger. The fourth arises from the radial side of the fifth metacarpal bone, and is inserted into the same side of the little finger. From this account it may be seen that each finger is provided with a pair of Interosseous muscles, with the exception of the little finger, in which the Abductor digiti minimi takes the place of one of the pair.
Actions.—The dorsal interossei abduct the fingers from an imaginary line drawn longitudinally through the center of the middle finger; and the palmar interossei adduct the fingers to that line. The Interossei, in conjunction with the Lumbricals, flex the proximal phalanges, and, in consequence of their insertions into the expansions of the Extensor tendons, extend the middle and distal phalanges. The first Palmar interosseous flexes the proximal phalanx of the thumb.
SEGMENTAL INNERVATTON OF THE MUSCLES OF THE HAND
Clinical observations indicate that all the small muscles of the hand receive their motor innervation from T. 1.
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