The dorsal interossei of the hand are four muscles which are located between each of the metacarpals. Each muscle is attached at two points to the metacarpals, giving them added strength. They are attached to the fingers (phalanges) at the base, commonly known as the second knuckle. They are named dorsal, because they are on the back of the hand, and thus are opposite to the palmar interossei, located on the lower side of the hand, the palm. The first dorsal interosseous muscle, located between the thumb and index finger, is the largest of the four. The primary function of these muscles is to flex the metacarpophalangeal joint, commonly known as the first knuckle. This is the place where it appears from an external view that the fingers attach to the palm of the hand. The muscle group also extends and abducts the fingers. Abduction of the fingers is defined as moving them away from the middle finger. While the actual cause is not known, the dorsal interossei (hand) muscles may play a role in generating the sound that results when the joints are moved quickly to snap or make a cracking sound.