In human anatomy, the dorsal interossei (DI) are four muscles in the back of the hand that act to abduct (spread) the index, middle, and ring fingers away from hand's midline (ray of middle finger) and assist in flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of the index, middle and ring fingers. Structure There are four dorsal interossei in each hand. They are specified as 'dorsal' to contrast them with the palmar interossei, which are located on the anterior side of the metacarpals. The dorsal interosseous muscles are bipennate, with each muscle 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 extensor expansion of the corresponding extensor digitorum tendon. The middle digit has two dorsal interossei insert onto it while the first digit (thumb) and the fifth digit (little finger) have none. Each finger is provided with two interossei (palmar or dorsal), with the exception of the little finger, in which the abductor digiti minimi muscle takes the place of one of the dorsal interossei. The first dorsal interosseous muscle is larger than the others. Between its two heads, the radial artery passes from the back of the hand into the palm. Between the heads of dorsal interossei two, three, and four, a perforating branch from the deep palmar arch is transmitted. Origins and insertions Proximal and distal interossei With some individual variations, the interossei muscles are attached either proximally or distally on the extensor expansion. The first dorsal interosseous, the most consistent, is inserted entirely into the base of its proximal phalanx and the extensor hood there. The second, third, and fourth dorsal interossei have insertions both proximally on the base of the metacarpal and hood, and distally on the lateral bands and central tendon of the extensor mechanism. The abductor digiti minimi, effectively the "fifth dorsal interosseus" or the dorsal interosseus of the little finger, has only a proximal insertion. The palmar interossei, in contrast, have only distal insertions. The interossei can, thus, be divided into a proximal and a distal group: the proximal interossei are mainly affecting the metacarpophalangeal (MP) joints, whereas the distal interossei are mainly affecting the interphalangeal (IP) joints (but, with continued action, will also affect the MP