Medical Definition of intercostal muscle
: any of the short muscles that extend between the ribs filling in most of the intervals between them and serving to move the ribs in respiration:a: any of 11 muscles on each side between the vertebrae and the junction of the ribs and their cartilages of which each arises from the caudal margin of one rib and is inserted into the cranial margin of the rib below—called also external intercostal muscleb: any of 11 muscles on each side between the sternum and the line on a rib marking an insertion of the iliocostalis of which each arises from the inner surface of a rib and its corresponding costal cartilage and is inserted into the cranial margin of the rib below—called also internal intercostal muscle
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