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Laboratory 18. Thoracic Wall
Step 3. The Internal Intercostal Muscle

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Identify next the internal intercostal muscles [cross-section] deep to the external intercostal membrane anteriorly. These eleven pairs of internal intercostal muscles extend from the border of the sternum anteriorly to the angle of the ribs posteriorly. Fibers of these muscles run downward and posteriorly at right angles to the fibers of external intercostals muscles.

Beginning at the sternum, cut the external intercostal membrane and external intercostal muscle away from inferior border of the fourth rib and reflect them inferiorly . Cut the external intercostal muscle as far laterally as the serratus anterior muscle. Observe that the thickness of external intercostal muscle increases posteriorly.

Links and References:
Grant's: 1.15, 1.20
Netter: 176
Rohen/Yokochi: 193, 194