Layers and Structure of the Chest Wall
Under the skin of the thorax, there are several fascias and muscle layers from outside to inside: The pectoral fascia is a fascia that lies externally on the M. pectoralis major, the clavipectoral fascia envelops the M. pectoralis minor. The bony thorax itself also has its own fascias: The thoracic fascia covers the periosteum of the ribs externally as well as the muscles of the intercostal spaces, the endothoracic fascia is its internal counterpart on the periosteum of the ribs as well as the internal or innermost intercostal muscles. It is covered by the parietal pleura (visceral pleura) towards the interpleural space. All these fascias, including the intercostal musculature (see below), are penetrated during the placement of a chest drain.
The Intercostal Space
Blood vessels (intercostal vessels) and nerves (intercostal nerves) of the chest wall run in a ring-like manner along the inside of the thorax, corresponding to the original segments of the organism. Through the blood vessels, the dorsally located vessels aorta and V. azygos/hemiazygos are segmentally connected to the respective ventral A. and V. thoracica interna. This arises from the A. subclavia, runs together with identically named accompanying veins on both sides parallel to the sternum immediately outside the endothoracic fascia caudally and then divides at the level of the diaphragm (Larrey's cleft) into the A. epigastrica superior (anastomoses with the A. epigastrica inferior from the A. iliaca externa) as well as an artery to the diaphragm (A. musculophrenica). On this path, on the inside of the thorax, mainly the anterior portions of the intercostal arteries as well as branches to the mediastinum arise from it.
Arteries
The intercostal arteries thus have an anterior inflow from the A. thoracica interna and are supplied dorsally either directly from the aorta (3rd – 11th branch) or the A. subclavia (via the costocervical trunk). Below the 12th rib, the A. subcostalis runs as a counterpart. Regarding the overall course of the intercostal arteries, it is particularly important for punctures that the respective dorsal portions of the arteries run along the lower edge of the overlying rib (together with intercostal vein and nerve) up to the level of the anterior axillary line and then typically divide into an upper and a lower branch (at the lower or upper edge of the adjacent ribs). Typical accumulations of lymph nodes are found in the area of the chest wall parasternal, axillary, as well as just above and below the clavicle.
Veins
The intercostal veins run parallel to the identically named arteries and drain ventrally via the V. thoracica interna into the V. brachiocephalica, dorsally into the V. azygos/hemiazygos or in the 1st and 2nd segment also into the V. brachiocephalica.
Nerves
The intercostal nerves, after leaving the intervertebral foramen with separation of a dorsal branch, also run segmentally along the lower edge of the respective rib in the intercostal space and supply the lateral and anterior skin of the thorax. The cranial thoracic portions are still supplied from the cervical plexus, the Th1 segment runs as part of the brachial plexus (C5 to Th1) into the arm, from Th2 the segments again serve as described for skin innervation, innervation of the intercostal musculature, and cranial sections of the abdominal musculature (together with the lumbar plexus). This special arrangement of the dermatomes leads to a significant jump between segment C4 (level of clavicle) and immediately thereafter segment Th2 for the upper thoracic wall (segment Th5 runs approximately at the level of the nipple).
Musculature
The intercostal musculature (Mm. intercostales externi, interni, and intimi) form a dense closure of the lateral chest wall. During inspiration, the Mm. intercostales externi and the ventral internal intercostal muscles are activated, during expiration the posterior and lateral internal intercostal muscles.