subcostalis muscle subcostalis muscle the subcostales (singular: subcostalis) (infracostales) consist of muscular and aponeurotic fasciculi, which are usually well-developed only in the lower part of the thorax; each originates from the inner surface of one rib, and is inserted into the inner surface of the second or third rib below, near its angle. subcostalis muscledetailsorigininner surface of one ribinsertioninner surface of the second or third rib below, near its anglenerveintercostal nervesactionsdepress ribs.identifierslatinmusculus subcostalista98a04.4.01.015ta22314fma71315anatomical terms of muscle their fibers run in the same direction as those of the intercostales interni. depresses the ribs to assist in expiration.