The interosseous muscles are small bipinnate (two-headed) muscles that connect the metacarpals and metatarsals with the phalanges. They originate on the metacarpals and metatarsals and insert on the first phalange. Their function is to flex the fingers and toes. There is a dorsal and palmar (plantar in the foot group).

There are four dorsal interosseous muscles of the hand. The first, the abductor indicis, connects to the thumb and index finger. It provides a flat pad in between the two fingers. The second and third interossei insert onto the third or middle finger. The fourth inserts on the ulnar side of the ring finger.

The palmar muscles are smaller and cover the palm surface of the hand, There are only three of them. They are attached along the entire surface of a corresponding metacarpal and insert on the first phalanges of the fingers. Contraction (abduction) of these muscles brings the fingers in toward the hand while contraction (adduction) of the dorsal ones flexes the hand back into a flat position.

The foot arrangement of the interossei is quite similar to the hand. Like the hand, the there are four dorsal interossei. Like the hand, the first interossei inserts on the second toe. The other dorsal interossei insert on the outer sides of the second, third, and fourth toes.

The three plantar interossei are beneath rather than between the metatarsals. The third, fourth, and fifth metatarsals provide the base for attachment. Their insertion is on the base of the phalanges of the same toes. The dorsal interossei are the abductors and the plantar are the adductors.