Identify the eleven pairs of external intercostal muscles [cross-section] (Figure 18.3). The fibers of these muscles run downward and forward between adjacent ribs. Each muscle begins posteriorly at the tubercles of the ribs and extends anteriorly to the costochondral junction, the junction between the costal cartilage and the sternal end of the rib. The muscle between the costal cartilages is replaced by a membranous layer called the external intercostal membrane.
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