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The interosseous muscles occupy the intermetacarpal intervals and are of two types: dorsal and palmar. Each intermetacarpal space contains one palmar and one dorsal interosseous muscle. The four dorsal interosseous muscles are abductors of the digits and are bipennate; the three palmar interosseous muscles are adductors and are unipennate. The plane of reference for abduction and adduction of the fingers is the midplane of the third digit. This is evident on simultaneously spreading and then approximating the extended digits. The placement of these muscles follows from the above considerations of actions and reference plane for abduction and adduction.


A dorsal interosseous muscle lies on either side of the third metacarpal, since any movement of the third digit away from its plane of reference is abduction. The other two dorsal interosseous muscles occupy the space between the first second metacarpals for the first dorsal muscle and between the fourth and fifth metacarpals for the fourth dorsal muscle. These latter two muscles abduct the second and fourth digits. The bipennate dorsal interosseous muscles arise by two heads from the adjacent sides of the metacarpals between which they lie. The first dorsal interosseous muscle is considerably larger than the others; the radial artery also passes into the palm between its heads. Dorsal perforating arteries pass between the heads of the other muscles.
The smaller palmar interosseous muscles adduct the same digit from whose metacarpal bone they arise and thus take origin from the palmar surfaces of the second, fourth, and fifth metacarpals. The tendons of both the dorsal and the palmar interosseous muscles pass dorsal to the deep transverse metacarpal ligaments between the heads of the metacarpals, and they have two insertions. The first insertion is to the base of the proximal phalanx; it is concerned with the abduction-adduction function. The second insertion is into the extensor expansion of the tendon of the extensor digitorum muscle; it produces flexion at the metacarpophalangeal joints and extension of the middle and distal phalanges at the interphalangeal joints. All the interosseous muscles are innervated by the deep branch of the ulnar nerve.

Free movement of the thumb is most important in the more precise activities of the hand. The flexor pollicis longus muscle flexes the thumb, and the extensor pollicis longus and extensor pollicis brevis muscles extend it. The abductor pollicis longus muscle is an accessory flexor of the wrist; it abducts and extends its metacarpal. The short muscles of the thumb provide flexion, abduction, adduction, and opposition. Abduction of the thumb carries it anteriorly out of the plane of the palm because of the rotated position of the first metacarpal, which directs its palmar surface medially. The abductor pollicis brevis muscle also assists in flexion. The opponens pollicis muscle acts solely on the metacarpal of the thumb, drawing the digit across the palm and rotating it medially.
The components of opposition are abduction, flexion, and medial rotation, the tip of the thumb reaching contact with the pads of the other slightly flexed digits. In firm grasp, the flexor pollicis brevis muscle is especially active. The motor, or recurrent, branch of the median nerve innervates the three muscles involved. The adductor pollicis muscle adducts the thumb. The abductor digiti minimi and the flexor digiti minimi brevis muscles produce their characteristic movements. The opponens digiti minimi muscle rotates the fifth metacarpal medially and deepens the hollow of the hand.
The intrinsic muscles of the hand are palmar and are therefore innervated by either the median or the ulnar nerve. Specific sets of muscles of the thumb and little finger, respectively, occupy the thenar and hypothenar compartments.

Each compartment contains an abductor, an opponens, and a flexor muscle for its specific digit (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis, abductor digiti minimi, flexor digiti minimi brevis, and opponens digiti minimi muscles). In each compartment, the positions and attachments of these muscles are similar. The flexor retinaculum and the bones to which it attaches (the scaphoid and trapezium radially and the hamate and pisiform on the ulnar side) provide the sites of origin for these muscles. The insertions of comparable muscles on the two sides are also the same: the base of the proximal phalanx for the abductor and flexor muscles and the shaft of the metacarpal for the opponens muscles.
The central compartment contains four slender lumbrical muscles associated with the flexor digitorum profundus tendon. The interosseous muscles located in the intervals between the metacarpals occupy, with the adductor pollicis muscle, a deeply placed interosseousadductor compartment that is bound by the dorsal and palmar interosseous fasciae. To complete these generalizations, the rule of nervous innervation may also be stated: the median nerve supplies the abductor pollicis brevis, opponens pollicis, flexor pollicis brevis, and the most radial two lumbrical muscles; the ulnar nerve supplies all the other intrinsic muscles of the hand.
The adductor pollicis muscle has two heads of origin, separated by a gap through which the radial artery enters the palm. The oblique head arises from the capitate and from the bases of the second and third meta-carpals. The transverse head arises from the palmar ridge (shaft) of the third metacarpal. The two heads insert together by a tendon that ends in the ulnar side of the base of the proximal phalanx of the thumb.
This tendon usually contains a sesamoid that together with the sesamoid in the tendon of the flexor pollicis brevis muscle forms a pair of small sesamoids on either side of the tendon of the flexor pollicis longus muscle. The adductor pollicis muscle overlies the interosseous muscles on the radial side of the third metacarpal. This muscle is supplied by the deep branch of the ulnar nerve.

Friction between tendons and compartments or bony surfaces is reduced by synovial sheaths. A sheath is formed like a double-walled tube: the delicate inner wall is closely applied to the tendon, and the outer wall is the lining of the compartment in which the tendon lies. The layers are continuous with each other at the ends of the tube (as elsewhere); their facing surfaces are smooth and separated by a small amount of synovial fluid.
In the hand, the flexor tendons have significant excursion, which causes friction between the tendons and the carpal ligament and in each finger against the fibro-osseous pulley system during gripping activities. Accordingly, the tendons to the thumb and fingers are protected and lubricated for optimal movement.
Variations in the anatomy of the synovial sheaths (bursae) exist and have an impact on the pattern of presentation of infections when they occur (see Plates 4-37 and 4-38). Similarly, several potential spaces exist in the palm and can become sites of infection. The thenar space exists just anterior to the adductor pollicis muscle. The midpalmar space exists posterior (deep) to the central compartment that c ntains the long flexor tendons and lumbrical muscles.