The interossei are in two groups, palmar and dorsal. The former are small and arise from only one (their own) metacarpal bone; the latter are larger and arise from the adjacent metacarpal bones of the space in which they lie. The dorsal interossei are visible from the palmar aspect of the interosseous spaces. It is easy to recall the attachments of the interossei by appreciating their functional requirements. The formula ‘PAD and DAB’ indicates that palmar adduct and dorsal abduct the fingers relative to the axis of the palm, which is the third metacarpal bone and middle finger tin the foot the axis passes through the second digit).
The palmar interossei adduct the fingers. Thus the thumb requires no palmar interosseous, already possessing its own powerful adductor pollicis muscle. Nevertheless a few fibers are sometimes found passing from the base of the metacarpal of the thumb to the base of its proximal phalanx; when present these fibers represent the first palmar interosseous muscle. Similarly the middle finger has no palmar interosseous; it cannot be adducted towards itself. The second, third and fourth palmar interossei arise from the middle finger side of the metacarpal bone of the index, ring and little fingers and are inserted into the same side of the extensor expansion and proximal phalanx of each respective finger.
The dorsal interossei, more powerful than the palmar, abduct their own fingers away from the midline of the palm. The thumb and little finger already possess their proper abducting muscles in the thenar and hypothenar eminences. Thus there are dorsal interossei attached only to index, middle and ring fingers. In the case of index and ring fingers they are inserted into the side of the finger away from the middle finger. In the case of the middle finger itself a dorsal interosseous is present on each side. All four dorsal interossei arise by two heads, one from each bone bounding the interosseous space.
The tendons of palmar and dorsal interossei all pass on the posterior side of the deep transverse metacarpal ligament to reach their distal attachments. They are inserted chiefly into the appropriate side of the extensor expansion (like the lumbricals), but partly also into the base of the proximal phalanx (to which the lumbricals are not attached).
Nerve supply of interosseous muscles:
All the interossei are supplied by the deep branch of the ulnar nerve (T1), but an occasional variant is for the first dorsal interosseous to be supplied by the median nerve.