Introduction: the interossei of the hand are muscles formed between the bones of the hand. They are in two groups: palmar and dorsal
1. Palmar interossei: they are small and they arise from their own metacarpal bones only. They are seen only from the palmar aspect of the interosseous spaces. The thumb and middle finger do not have palmar interossei.
Origin: the 2nd, 3rd and 4th palmar interossei arise from the middle finger side of the carpal bones of the index, ring and little fingers.
Insertion: same side of the extensor expansion and proximal phalanx of each respective finger.
2. Dorsal interossei: they are larger and more powerful than the palmar.
Origin: they arise from the adjacent metacarpal bones of the space in which they lie and they can be seen in both dorsal and palmar aspects of the interosseous spaces. The thumb and little finger do not have dorsal interosseous muscle attached to them. All the four dorsal interossei arise by two heads one from each bone bounding the interosseous space.
Insertion: in the middle and ring finger they are inserted into the side of the finger away from the middle finger. In the middle finger a dorsal interosseous muscle is present on each side.
Nerve supply: all the interossei are supplied by the deep branch of the ulnar nerve (C8, T1).
Action: the palmar interossei adduct (PAD) and the dorsal interossei abduct (DAB) the fingers relative to the axis of the palm.