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Thorax

First and second modules — the thoracic wall, then the cavity

The thorax is usually taught in two passes: the wall and diaphragm early (they are body-wall anatomy and the gateway to the cavity), then the pleura, lungs, mediastinum, and heart once the chest is opened. It is the highest-yield region in the course for the boards — cardiovascular and pulmonary are 20% of the PANCE between them — and the two procedures every first-year learns the landmarks for, chest-tube placement and thoracentesis, are pure intercostal anatomy.

On the PANCE this region feeds:Cardiovascular · 11%Pulmonary · 9%
High-Yield Pearl

Three landmarks carry the thorax: the sternal angle (rib 2, T4–T5, the arch, the carina, the mediastinal boundary), the costal groove (VAN under the rib — go over the top), and the lung root (phrenic in front, vagus behind, right artery in front of the bronchus, left artery above it).

What the written exam asks
Relationships, supply, levels, and what happens when each fails.
  • Ribs and sternum. 12 pairs: true ribs 1–7 reach the sternum by their own cartilage, false ribs 8–10 join the cartilage above, floating ribs 11–12 end in the muscle. A typical rib has a head (two facets), neck, tubercle (articulates with the transverse process), angle, and costal groove on the inferior inner edge. The sternal angle (of Louis) marks rib 2, the T4–T5 disc, the start and end of the aortic arch, the tracheal bifurcation, and the boundary between superior and inferior mediastinum.
  • Intercostal space. Three muscle layers — external (fibers down and forward, inspiration), internal (down and back, forced expiration), innermost — with the neurovascular bundle between the internal and innermost layers, tucked under the rib in the costal groove in the order Vein, Artery, Nerve from top to bottom. Needles and chest tubes go over the TOP of the rib below the space to spare the bundle. Intercostal nerves are the ventral rami of T1–T11; T12 is the subcostal nerve.
  • Diaphragm. Motor and most sensory supply from the phrenic nerve (C3, C4, C5 keeps the diaphragm alive); the periphery gets sensory fibers from the lower intercostals. Openings: caval hiatus at T8 (IVC, right phrenic nerve), esophageal hiatus at T10 (esophagus, both vagal trunks), aortic hiatus at T12 (aorta, thoracic duct, azygos vein). Irritation of the central diaphragm refers pain to the shoulder (C3–C5 dermatomes).
  • Breast. Lies on pectoralis major in the superficial fascia, from rib 2 to 6, with the axillary tail. About 75% of lymph drains to the axillary nodes (the first node is the sentinel), the rest to the parasternal nodes — the anatomy behind sentinel-node biopsy and the medial-quadrant tumors that cross the midline. Suspensory ligaments (Cooper) tether it to the skin: dimpling when a tumor shortens them.
  • Pleura and recesses. Parietal pleura (costal, diaphragmatic, mediastinal, cervical) is somatically innervated and hurts; visceral pleura is not. The potential pleural cavity has a few milliliters of fluid. The costodiaphragmatic recess is where fluid collects upright and where thoracentesis is done — posteriorly in the 8th–10th intercostal space, over the top of the rib, below the lung's inferior border (rib 8 midclavicular, 10 midaxillary, 12 paravertebral for the pleura; the lung is two ribs higher).
  • Lungs. Right: three lobes, two fissures (oblique and horizontal); left: two lobes, one fissure, a lingula, and the cardiac notch. The right main bronchus is wider, shorter, and more vertical — aspirated objects go right, usually to the lower lobe (or the posterior segment of the upper lobe when supine). At the hilum: pulmonary artery, two pulmonary veins, main bronchus, bronchial vessels, lymph nodes. Ten bronchopulmonary segments per lung, each with its own segmental bronchus and artery — the surgical unit.
  • Mediastinum. Superior (above the sternal angle): thymus remnant, brachiocephalic veins and SVC, aortic arch and its three branches, trachea, esophagus, vagus and phrenic nerves, left recurrent laryngeal nerve, thoracic duct. Inferior: anterior (fat, thymus remnant), middle (heart, pericardium, roots of the great vessels, phrenic nerves), posterior (descending aorta, esophagus, azygos system, thoracic duct, sympathetic trunks, splanchnic nerves).
  • Heart and pericardium. Fibrous pericardium outside, serous pericardium (parietal and visceral/epicardium) inside, with the transverse sinus behind the great-vessel roots and the oblique sinus behind the left atrium. The right border is the right atrium, the inferior border mostly the right ventricle, the left border the left ventricle, the apex at the 5th intercostal space in the midclavicular line. Auscultation: aortic right 2nd ICS, pulmonic left 2nd, tricuspid left lower sternal border, mitral at the apex.
  • Coronary arteries. Right coronary: SA nodal branch (about 60% of people), right marginal, and — in the 85% who are right-dominant — the posterior descending (PDA), which supplies the AV node and the inferior wall. Left coronary: LAD (anterior wall, anterior two-thirds of the septum, the apex — the "widowmaker") and circumflex (lateral wall; the PDA in left-dominant hearts). Venous return via the coronary sinus into the right atrium.
  • Great vessels and nerves of the arch. Arch branches right to left: brachiocephalic trunk, left common carotid, left subclavian. The ligamentum arteriosum (ductus remnant) tethers the arch to the pulmonary trunk, and the LEFT recurrent laryngeal nerve hooks under it — the right one hooks under the right subclavian. The phrenic nerves pass ANTERIOR to the lung roots; the vagi pass POSTERIOR.
  • Azygos system and thoracic duct. Azygos (right) drains the posterior intercostals into the SVC, arching over the right lung root; hemiazygos and accessory hemiazygos (left) cross to join it. The thoracic duct rises between the aorta and azygos, crosses to the left at T4–T6, and empties at the left venous angle (internal jugular meets subclavian) — a chylothorax follows injury to it.
What the practical tags · 41 structures
In the order a dissection meets them — each with the one relationship that identifies it. Your lab's study list is the authority; use this to check yours against.
  1. 1Manubrium, sternal body, xiphoid — and the sternal angle between the first two
  2. 2Clavicle and its sternoclavicular joint — the only bony link of the upper limb to the trunk
  3. 3Rib: head, neck, tubercle, angle, costal groove — identify a numbered rib by its cartilage
  4. 4Costal cartilage and costal margin — hyaline bars joining ribs to the sternum; the margin is the palpable lower edge of ribs 7–10
  5. 5External intercostal muscle — fibers run down and forward (hands-in-pockets)
  6. 6Internal intercostal — fibers down and back; visible anteriorly where the external becomes membrane
  7. 7Innermost intercostal — the deepest layer, with the bundle between it and the internal
  8. 8Intercostal vein, artery, nerve — in the costal groove, VAN from above down
  9. 9Internal thoracic artery and vein — a finger's breadth lateral to the sternum
  10. 10Transversus thoracis — the fan on the inner anterior wall
  11. 11Pectoralis major and minor, serratus anterior — the muscles removed to reach the wall
  12. 12Breast, nipple, axillary tail, suspensory ligaments — on the pectoral fascia, ribs 2–6, with the tail reaching the axilla (female cadaver)
  13. 13Diaphragm: right and left domes, central tendon, right and left crura, median arcuate ligament
  14. 14Caval opening (T8), esophageal hiatus (T10), aortic hiatus (T12)
  15. 15Phrenic nerve — descends anterior to the lung root, on the pericardium
  16. 16Vagus nerve — posterior to the lung root, forming the esophageal plexus
  17. 17Left recurrent laryngeal nerve — hooks under the arch at the ligamentum arteriosum
  18. 18Sympathetic trunk and greater splanchnic nerve — on the rib heads
  19. 19Azygos vein — arching over the right lung root into the SVC
  20. 20Hemiazygos and accessory hemiazygos — left side, crossing behind the aorta
  21. 21Thoracic duct — thin, between the azygos and the aorta
  22. 22Esophagus and trachea — trachea anterior; carina at the sternal angle
  23. 23Right and left main bronchi — the right is wider and more vertical
  24. 24Right lung: superior, middle, inferior lobes; oblique and horizontal fissures
  25. 25Left lung: superior and inferior lobes, oblique fissure, lingula, cardiac notch
  26. 26Hilum: pulmonary artery, superior and inferior pulmonary veins, bronchus — know the arrangement on each side
  27. 27Parietal and visceral pleura; the costodiaphragmatic recess
  28. 28Pulmonary ligament — the pleural fold below the hilum
  29. 29Fibrous pericardium; parietal and visceral serous pericardium
  30. 30Transverse and oblique pericardial sinuses — a finger passes behind the aorta and pulmonary trunk in the transverse sinus
  31. 31Right atrium and auricle, crista terminalis, fossa ovalis, coronary sinus opening
  32. 32Right ventricle: trabeculae carneae, papillary muscles, chordae tendineae, septomarginal trabecula (moderator band), conus arteriosus
  33. 33Tricuspid and pulmonary valves; mitral and aortic valves with the aortic sinuses
  34. 34Left atrium with four pulmonary vein openings; left ventricle
  35. 35Right coronary artery, right marginal branch, posterior descending (interventricular) artery
  36. 36Left coronary artery, anterior descending (interventricular) artery, circumflex artery
  37. 37Great cardiac vein and coronary sinus
  38. 38Superior and inferior vena cava; ascending aorta, arch, descending aorta
  39. 39Brachiocephalic trunk, left common carotid, left subclavian arteries
  40. 40Pulmonary trunk and arteries; ligamentum arteriosum
  41. 41Brachiocephalic veins; thymus remnant in the anterior mediastinum
What everyone misses
The distinctions that lose points on the practical, year after year.
  • Right vs left hilum. On the right the pulmonary artery lies ANTERIOR to the bronchus; on the left it lies SUPERIOR ("RALS"). The eparterial bronchus to the right upper lobe is the only bronchus above its artery. A hilum tagged "which side?" is a standard practical item.
  • Ligamentum arteriosum and the left recurrent laryngeal nerve. The nerve hooks under the arch exactly at the ligamentum; in the lab it is a thread that is easy to cut. Hoarseness after anything in the left chest (aortic aneurysm, left atrial enlargement, hilar tumor) is this nerve.
  • Azygos vein vs thoracic duct. Both run up the posterior mediastinum on the right of the midline. The azygos is blue-walled and arches over the lung root into the SVC; the duct is a thin, beaded, translucent tube between the azygos and the aorta that crosses left at T4–T6.
  • Phrenic anterior, vagus posterior. Both are tagged beside the lung root. The phrenic runs down on the pericardium in front of the root; the vagus passes behind the root and breaks up on the esophagus.
  • Horizontal fissure is right-only. Finding a horizontal fissure identifies the right lung; the left has the lingula and cardiac notch instead. Many students tag a left lung as "right middle lobe" on a deflated specimen.
  • Costodiaphragmatic recess. The lung does not fill the pleural cavity; the slit below the inferior lung border is the recess. On a practical it is often tagged with a probe, and students name the lung instead.
  • Moderator band. The septomarginal trabecula in the right ventricle carries the right bundle branch to the anterior papillary muscle — it is the one named trabecula, and the one always tagged.
  • Internal thoracic artery. Runs a finger's breadth lateral to the sternum on the inner wall, under the transversus thoracis; it is the LIMA of coronary bypass and the reason a sternotomy bleeds.
Clinical correlations
Where this region shows up again — in clinical medicine, on rotations, and on the boards.
  • Pneumothorax and chest tube. Air in the pleural space collapses the lung; a tension pneumothorax shifts the mediastinum and must be decompressed. Needle decompression at the 2nd intercostal space midclavicular (or 4th–5th anterior axillary); chest tube at the 4th–5th space between the anterior and mid-axillary lines, over the top of the rib — the "safe triangle" bounded by pectoralis major, latissimus, and the nipple line.
  • Pleural effusion and thoracentesis. Fluid collects in the costodiaphragmatic recess when upright and blunts the costophrenic angle on X-ray. Thoracentesis: posterior, 8th–10th intercostal space in the midaxillary-to-posterior line, over the top of the rib, one or two spaces below the top of the effusion — never below rib 9 midaxillary or you enter the abdomen.
  • Aspiration. The right main bronchus is wider, shorter, and more vertical, so foreign bodies and aspirated fluid go right — to the right lower lobe sitting up, the posterior segment of the upper lobe or superior segment of the lower lobe lying down.
  • Myocardial infarction by territory. LAD occlusion → anterior/septal MI (V1–V4); circumflex → lateral (I, aVL, V5–V6); RCA → inferior (II, III, aVF) with bradycardia and AV block from the nodal branches, and right-ventricular involvement. The PDA's origin (right-dominant in 85%) decides who owns the inferior wall.
  • Cardiac tamponade and pericardiocentesis. Fluid in the unyielding fibrous pericardium compresses the heart — Beck's triad (hypotension, distended neck veins, muffled sounds), pulsus paradoxus. The needle enters the left xiphocostal angle, aimed at the left shoulder; the left lung's cardiac notch leaves the pericardium bare there.
  • Rib fractures and flail chest. Ribs break at the angle (the weakest point); lower rib fractures threaten the liver, spleen, and kidneys; an upper rib fracture (1–2) implies a major mechanism and vascular injury. Three or more adjacent ribs broken in two places = a flail segment that moves paradoxically.
  • Aortic dissection, aneurysm, and coarctation. Tearing chest pain radiating to the back, a pulse or pressure difference between arms, a widened mediastinum. An arch aneurysm can stretch the left recurrent laryngeal nerve (hoarseness). Coarctation distal to the left subclavian causes upper-limb hypertension and the rib notching of enlarged intercostal collaterals.
  • Pancoast tumor and Horner syndrome. An apical lung tumor invades the cervical sympathetic trunk (ptosis, miosis, anhidrosis), the lower brachial plexus (C8–T1 hand weakness), and the subclavian vessels — apical anatomy on a chest film.
  • Referred diaphragmatic pain. Blood or pus under the diaphragm (a ruptured spleen, a subphrenic abscess, a ruptured ectopic pregnancy) irritates the phrenic nerve and is felt at the shoulder tip (Kehr's sign) — C3–C5 dermatomes.
  • Diaphragmatic hernias. Sliding hiatal hernia (GE junction slides up — reflux) vs paraesophageal (the fundus rolls beside a fixed junction — strangulation risk). Congenital Bochdalek hernia is posterolateral and left, with pulmonary hypoplasia.

Frequently asked

This guide is the map. Drop this region's lecture slides on the dashboard to get flashcards and questions from your lecture, group them into a module, and run a timed module exam the week before. The gross anatomy playbook has the weekly loop.

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AI-authored study notes, not faculty-reviewed and not medical advice. Verify every structure against your atlas, your dissector, and your lab instructors; programs differ in what they tag.

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