1) Any of four small muscles of the hand each of which arises by two heads from the dorsal aspect of two adjacent metacarpals, extends along the interval between them to insert into the base of the proximal phalanx of the thumb side of the index finger, the little-finger or thumb side of the middle finger, or the little-finger side of the fourth finger, and acts to draw the fingers away from the long axis of the middle finger, flex the fingers at the metacarpophalangeal joints, and extend their distal two phalanges.

2) Any of four small muscles of the foot each of which arises by two heads from the dorsal aspects of the adjacent sides of two metatarsals, extends along the interval between them to insert into the base of the first phalanx of the medial side of the second toe or the lateral side of the second, third, or fourth toe, and acts to draw the toes away from the long axis of the second toe, flex their proximal phalanges, and extend the distal phalanges.