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The interossei are intrinsic muscles of the hand found on both sides of the metacarpals, the long bones situated beneath each finger. They fill the gap between each metacarpal bone, with three on the palm side and four on the back of the hand. Like unseen webbing between the fingers, these muscles are responsible for spreading the fingers and bringing them together. The palmar interossei move the fingers together in the direction of the middle finger, while the dorsal interossei move the fingers away from the middle finger, thereby spreading the hand.
Situated nearer to the palm side of each metacarpal rather than between these bones, each palmar interosseous is a unipennate muscle. Unipennate refers to the fact that it originates along the entire length of the metacarpal bone with fibers extending outward and downward like bristles on a toothbrush rather than from a single point of origin. These fibers eventually converge at the far end of the metacarpal, forming a small tendon that crosses the metacarpophalangeal joint to insert at the proximal end of the phalanx or finger bone just above the muscle’s respective metacarpal. In other words, the interosseous that begins on the second metacarpal, the one just below the index finger, attaches to the same side of the second phalanx. Moreover, the palmar interossei are found on the side of the finger nearest the middle finger in order to draw them inward, so that on the index finger is found on the medial side of the metacarpal, and those on the ring and little finger are situated on the lateral side of their respective metacarpals.
The dorsal interossei, on the other hand, are located between the metacarpal bones. Each dorsal interosseous muscle has two heads — one attaches to the metacarpal on one side of the muscle and the other attaches to the metacarpal on the other side — and as such is considered to be bipennate, with a tendon running down the middle. This tendon is where the muscle fibers from either side converge. Like that of each palmar interosseous, it crosses the metacarpophalangeal joint and inserts on the proximal end of the adjacent phalanx that is nearest the middle finger. For instance, the dorsal interosseous muscle between the thumb and index finger inserts on the phalanx of the index finger, the second phalanx, whereas the interosseous muscle between the pinky and ring fingers inserts on the phalanx of the ring finger, the fourth phalanx.