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The dorsal interossei are muscles found between the bones of the hand and foot. Specifically, they are situated between the metacarpals, the long bones of the palm, as well as between the metatarsals, the bones of the foot beneath the toes. In the hands and feet are two sets of interossei, the palmar/plantar interossei and the dorsal interossei, but only the dorsal interossei are sandwiched between the bones. These muscles produce the action of abduction, or spreading the fingers and toes away from a midline of the hand and foot. In the hand, that midline is the middle finger, while in the foot, it is the second toe.
Similar in shape to the body of a fish, the dorsal interossei are bipennate muscles. This means that they arise from two separate points — one head originates on the side of the metacarpal beneath one finger, for example, while the other head arises from the adjacent side of the metacarpal beneath the next finger. The two heads converge in the middle in the space between the bones, with the fibers of each angling toward the muscle’s midline. They form a single muscle that tapers as it approaches its insertion point on the proximal phalange, which is the nearest bone of the finger or toe above it.
A total of four dorsal interossei apiece occupy the space between the bones of the hands and feet. In the hand, there is one muscle between the thumb and index finger, one between the index and middle fingers, one between the middle and ring fingers, and one between the ring and pinky fingers. As such, two are found to one side of the middle finger and two to the other side, with the middle finger forming the midline. When the muscles contract simultaneously, in other words, the ring and pinky spread away from the middle finger in one direction while the thumb and index finger spread in the opposite direction, and the middle finger does not move.
Likewise, the dorsal interossei of the foot are found between the first or big toe and the second toe, between the second and third toes, between the third and fourth toe, and between the fourth and fifth or pinky toe. Here, only one is found to the inside of the midline of the foot while three are found to the other side. When these muscles contract, they pull the toes away from the second toe, though this is a much smaller movement than abduction of the fingers. They also contribute to toe flexion, or the downward curling of the toes.
It should be noted that each individual interosseous angles toward the midline of the hand or foot. Therefore, the muscles between the index and middle fingers will attach to the middle finger, while the muscles between the middle and ring fingers will also attach to the middle finger. Similarly, the muscles between the fourth and fifth toes will attach to the fourth toe, and those between the third and fourth toes will attach to the third toe, while those between the first and second toes will attach to the second toe.
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