The external layer of the intercostal muscles arise from the lower border of one rib and pass antero-medially to the upper border of the rib below, inserting between the tubercles posteriorly and costochondral junctions anteriorly.  It continues anteriorly as the external intercostal membrane.  

The internal layer arises from the lower border of one rib and extends postero-laterally to the upper border of the rib below, passing between the sternum anteriorly and angle of the rib posteriorly.  It continues as the ‘internal intercostal membrane’ posteriorly.  

The innermost layer of the intercostals muscles passes postero-laterally from the internal aspect of one rib to the internal surface of the second or third rib below.