4.1 Mediastinum & Thoracic Skeletal/Muscular Components
Key Takeaways
- The mediastinum is divided at the sternal angle (T4/T5) into superior and inferior; the inferior is subdivided into anterior, middle, and posterior compartments with distinct contents.
- The sternal angle (angle of Louis) marks the 2nd rib, the tracheal carina, the limits of the aortic arch, and the SVC entering the right atrium.
- Ribs 1–7 are true (vertebrosternal), 8–10 are false (vertebrochondral, shared costal cartilage), and 11–12 are floating; the costal groove shelters the V-A-N bundle.
- The intercostal neurovascular bundle runs V-A-N (vein, artery, nerve) in the costal groove, so thoracentesis needles are inserted above a rib to avoid lacerating it.
- The left recurrent laryngeal nerve loops under the aortic arch in the superior mediastinum; a mediastinal mass or aortic aneurysm can cause hoarseness.
The Mediastinum: Central Compartment of the Thorax
The mediastinum is the central compartment of the thoracic cavity, bounded laterally by the two pleural cavities, anteriorly by the sternum, posteriorly by the vertebral bodies (T1–T12), superiorly by the superior thoracic aperture (thoracic inlet at T1), and inferiorly by the diaphragm. It contains every thoracic viscera except the lungs themselves. Clinically, mediastinal widening on chest imaging suggests aortic injury, mediastinal mass, or tension pneumothorax.
Divisions and Contents
The classic subdivision uses the sternal angle (angle of Louis, at the T4/T5 vertebral level) as the transverse plane:
| Compartment | Landmark | Key contents |
|---|---|---|
| Superior mediastinum | Above sternal angle (T4/T5) | Thymus, SVC, brachiocephalic veins, arch of aorta and its three branches (brachiocephalic trunk, left common carotid, left subclavian), trachea, esophagus, left recurrent laryngeal nerve, vagus & phrenic nerves, thoracic duct |
| Anterior mediastinum | Anterior to pericardium | Inferior thymus, fat, internal thoracic vessels and lymph nodes; narrowest in adults |
| Middle mediastinum | Contains pericardium & heart | Pericardium, heart, roots of great vessels, phrenic nerves and pericardiacophrenic vessels |
| Posterior mediastinum | Posterior to pericardium | Descending thoracic aorta, esophagus, vagus nerves forming the esophageal plexus, thoracic duct, azygos and hemi-azygos veins, sympathetic trunks |
The left recurrent laryngeal nerve loops under the aortic arch and ascends between the trachea and esophagus — a left paratracheal mass or aortic aneurysm can cause hoarseness by compressing it. The PA-CAT Bulletin of Information, rev. 20240815, Table 3 lists "Mediastinum" and "Skeletal and Muscular Components" as testable leaves within the Anatomy content area.
Thoracic Cage
The thoracic cage consists of 12 thoracic vertebrae, 12 pairs of ribs, and the sternum. The sternum has three parts: manubrium (jugular notch superiorly, articulates with clavicles and 1st costal cartilage), body (articulates with costal cartilages 2–7), and xiphoid process (cartilaginous until roughly age 40, then ossifies; landmark for CPR compressions). The sternal angle is the manubriosternal junction — a critical PA-CAT landmark marking the level where the 2nd rib articulates, the trachea bifurcates at the carina, the aortic arch begins and ends, and the SVC enters the right atrium.
Ribs are classified as true (vertebrosternal) ribs 1–7 (each with its own costal cartilage reaching the sternum), false (vertebrochondral) ribs 8–10 (shared costal cartilage forming the costal margin), and floating (vertebral) ribs 11–12 (no anterior attachment). A typical rib (3–9) has a head (two facets articulating with adjacent vertebral bodies), a neck, a tubercle (articulates with the transverse process of the same-numbered vertebra), and a shaft with a costal groove on its inferior internal surface sheltering the intercostal neurovascular bundle. Rib 1 is atypical (single head facet, broad flat shaft, scalene attachments) and rib 2 is the sternal-angle reference.
Intercostal Muscles and Neurovascular Bundle
Three layers of intercostal muscles fill the intercostal spaces. The external intercostals (fibers run obliquely downward and forward, elevate ribs during inspiration), the internal intercostals (fibers run downward and backward, depress ribs in forced expiration), and the innermost intercostals (deepest, segmentally innervated). The transversus thoracis lies deep on the posterior sternum and completes the deepest layer.
The intercostal neurovascular bundle runs in the costal groove in the order V-A-N from superior to inferior: intercostal vein, intercostal artery, intercostal nerve. Posterior intercostal arteries arise from the descending thoracic aorta (ribs 3–11; the superior two pairs come from the supreme intercostal artery, a branch of the costocervical trunk), and anterior intercostal arteries come from the internal thoracic and musculophrenic arteries. Intercostal nerves are the ventral rami of T1–T11 (T12 is the subcostal nerve); they give lateral and anterior cutaneous branches and supply the intercostal muscles, anterior abdominal wall muscles, and the overlying skin.
For a thoracentesis, the needle is inserted above a rib (along its superior border) to avoid lacerating the V-A-N bundle traveling in the costal groove of the rib below.
Surface Landmarks
- Sternal angle (T4/T5) — 2nd rib, carina, aortic arch limits, SVC entering right atrium
- Nipple — 4th intercostal space (T4 dermatome); the 4th intercostal space is the constant clinical reference for chest tube placement in the midaxillary line
- Apex beat — 5th intercostal space, midclavicular line (left ventricle)
- Xiphoid process — T10 vertebral level (caval hiatus of the diaphragm)
These landmarks let PA-CAT candidates translate surface anatomy to internal structures without imaging.
Clinical Applications of Thoracic-Wall Anatomy
A flail chest results when two or more ribs are fractured in two or more places, producing a freely moving segment that paradoxes inward on inspiration and outward on expiration, impairing ventilation. The intercostal nerve block anesthetizes a single dermatomal band by targeting the V-A-N bundle at the angle of the rib; because the bundle runs in the costal groove, the needle is walked off the inferior border of the rib above and advanced just superior to the rib below. Thoracic outlet syndrome reflects compression of the lower trunk of the brachial plexus (C8–T1) or the subclavian artery as they pass through the cervicoaxillary canal between the anterior and middle scalenes and over the 1st rib — the same 1st rib that forms the apex of the axilla. A cervical rib can narrow this outlet and reproduce neurogenic symptoms in the medial arm and hand. Finally, the internal thoracic (internal mammary) artery runs just lateral to the sternum on the posterior surface of the anterior thoracic wall, between the internal intercostal and transversus thoracis muscles, and is the preferred conduit for coronary artery bypass grafting because its patency outperforms saphenous vein grafts.
A patient develops hoarseness after a thoracic aortic aneurysm expands. Which nerve is most likely compressed, and where does it run?
During a thoracentesis at the midaxillary line, the needle is inserted along the superior border of a rib to avoid injuring which structure?