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Layers and structure of the chest wall
The soft tissues of the chest between the skin and rib cage comprise several fascias and muscle layers.: The pectoral fascia is a fascia overlaying the pectoralis major muscle, while the clavipectoral fascia invests the pectoralis minor muscle. The rib cage itself also has its own fascias: On the outside, the thoracic fascia covers the periosteum of the ribs as well as the muscles in the intercostal spaces. The endothoracic fascia is its inner counterpart on the periosteum of the ribs and the inner and innermost intercostal muscles. Across the interpleural space, it is invested by the parietal pleura (costal pleura). Thoracotomy will perforate all these fasciae together with the intercostal muscles (see below).
The intercostal space
Since the blood vessels (intercostal vessels) and nerves (intercostal nerves) of the chest wall follow the original segments of the body, they course on the inside of the rib cage in annular fashion. Segment by segment, these intercostal blood vessels connect the posterior aorta and azygos/hemiazygos vein with the internal thoracic arteries and internal thoracic veins)anteriad. The former arises from the subclavian artery, courses caudad together with the homonymous veins paralleling the sternum on both sides immediately outside the endothoracic fascia and then divides at the level of the diaphragm (Larrey cleft or sternocostal trigonum) into the superior epigastric artery (anastomoses with the inferior epigastric artery from the external iliac artery) and an artery to the diaphragm (musculophrenic artery). Along its course, on the inner surface of the chest, it gives rise primarily to the anterior segments of the intercostal arteries and to mediastinal branches.
The intercostal arteries thus are fed anteriad from the internal thoracic artery and posteriad either directly from the aorta (branches 3-11) or the subclavian artery (via the costocervical trunk). The subcostal artery coursing inferior to the 12th rib is its counterpart. One aspect in the overall course of the intercostal arteries is important: Up to the level of the anterior axillary line the posterior segment of the intercostal artery (together with the corresponding intercostal vein and nerve) run along the inferior aspect of the rib immediately superior to the artery and then typically divides into a superior and inferior branch (at the inferior and superior aspect respectively of the adjacent ribs). In the chest wall region, typical collections of lymph nodes are found in the parasternal and axillary areas as well as just superior and inferior to the clavicle.
The intercostal veins parallel the homonymous arteries and empty anteriorly via the internal thoracic vein into the brachiocephalic vein and posteriorly into the azygos/hemiazygos veins or, in the first and second segments, also into the brachiocephalic vein.
After they leave the intervertebral foramen, the intercostal nerves, give off a posterior branch and course segmentally in the intercostal space along the inferior aspect of the corresponding rib and supply the skin of the lateral and anterior chest. The superior aspects of the chest are still supplied from the cervical plexus; as part of the brachial plexus (C5 to T1) segment T1 runs into the arm; starting at T2, these segments again innervate the skin, as described, and the intercostal muscles and superior aspects of the abdominal musculature (along with the lumbar plexus). This particular layout of the dermatomes results in a big discontinuity between segment C4 (level of the clavicle) and then immediately segment T2 for the superior chest wall (segment T5 runs at roughly the level of the nipples
The intercostal musculature (external, internal, and innermost intercostal muscles) tightly encircle the lateral chest wall. Inhalation activates the external and anterior internal intercostal muscles, while exhalation triggers the posterior and lateral internal intercostal muscles.