15.1 Thorax: Thoracic Wall, Breast, Pleura & Lungs, Mediastinum, Heart & Great Vessels

Why this area matters

The 2025 Anatomy TOS lists "Thorax: thoracic wall (fascia, skeleton, joints, movements), breasts, thoracic cavity and viscera, heart and great vessels and pleura". The competencies tell you how it will be tested:

• localize the anatomic part described in a clinical scenario;
• correlate symptoms with the position of the organs involved;
• choose the work-up;
• plan management.

Expect procedure-based questions (chest tube, thoracentesis, pericardiocentesis) and disease localization (breast cancer, aspiration, myocardial infarction).

Thoracic wall: skeleton, joints and movements

• Skeleton: 12 thoracic vertebrae, 12 pairs of ribs and the sternum (manubrium, body, xiphoid).
  • True ribs (1 to 7) join the sternum directly.
  • False ribs (8 to 10) join the costal margin.
  • Floating ribs (11 and 12) end free.
  • The first rib is short and flat and carries the subclavian vessels; a cervical rib can compress the lower trunk of the brachial plexus (thoracic outlet syndrome).
• Joints: costovertebral and costotransverse joints posteriorly, sternocostal joints anteriorly, and the manubriosternal joint at the sternal angle.
• Movements of breathing:
  • The diaphragm increases the vertical dimension.
  • "Pump-handle" movement of the upper ribs increases the anteroposterior diameter.
  • "Bucket-handle" movement of the lower ribs increases the transverse diameter.
  • Accessory muscles of inspiration are the scalenes and sternocleidomastoid; forced expiration uses the abdominal muscles and internal intercostals.
• Fascia: endothoracic fascia lines the cavity. The suprapleural membrane (Sibson fascia) roofs the cervical pleura.

The breast

• The breast lies on the pectoralis major fascia (about rib 2 to rib 6), with an axillary tail (of Spence) into the axilla.
• Its 15 to 20 lobes drain through lactiferous ducts to the nipple.
• Suspensory ligaments (of Cooper) support it. When cancer shortens them, the skin dimples. When lymphatics are blocked, edema of the skin produces peau d'orange.
• Blood supply: internal thoracic (mammary) artery perforators, lateral thoracic artery and intercostal arteries.
• Lymphatic drainage:
  • about 75% to the axillary nodes (levels I to III defined by pectoralis minor);
  • the rest to the parasternal (internal thoracic) nodes, especially from medial quadrants;
  • some to the opposite breast and the abdomen.
  • The sentinel node is the first axillary node to receive drainage.

Thoracic Wall, Intercostal Spaces & Critical Thoracic Procedures

The Intercostal Neurovascular Bundle

The intercostal spaces are spanned by three muscular layers: the external intercostal (fibers run obliquely down and forward, "hands in pockets", mediating inspiration), the internal intercostal (fibers run down and backward, mediating forced expiration), and the innermost intercostal muscle layer.

• Between the internal intercostal and innermost intercostal muscle layers lies the neurovascular plane.
• The intercostal neurovascular bundle courses within the subcostal groove sheltered along the inferior, internal margin of the overlying rib.
• Structural arrangement within the subcostal groove from superior to inferior: Vein, Artery, Nerve (VAN).
• Collateral branches run along the superior border of the rib below, but they are significantly smaller.

Landmarks for Thoracentesis & Chest Tube Insertion

• Thoracentesis (Pleural Tap): To sample or drain pleural fluid while avoiding visceral injury:
  • The needle must always be inserted immediately above the superior margin of the lower rib to avoid the large intercostal VAN bundle sheltered in the costal groove of the rib above.
  • Safe puncture site: 7th, 8th, or 9th intercostal space along the posterior axillary or mid-scapular line, performed during suspended expiration.
  • Insertion must never occur below the 9th rib to prevent accidental penetration of the diaphragm and laceration of the spleen (left) or liver (right).
• Tube Thoracostomy (Chest Tube Insertion):
  • Indicated for pneumothorax, hemothorax, or empyema. Safe insertion is executed strictly within the Triangle of Safety:
    • Anterior border: Lateral border of the pectoralis major muscle.
    • Posterior border: Anterior border of the latissimus dorsi muscle.
    • Inferior border: Horizontal line at the level of the 5th intercostal space (corresponding to the nipple line in males or the inframammary fold).
    • Apex: High axillary apex.
  • The incision and blunt dissection track through the 4th or 5th intercostal space in the anterior-to-midaxillary line, sweeping over the superior border of the 5th rib.

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Bronchopulmonary Segments, Mediastinum & Cardiac Coronary Anatomy

Lung Microarchitecture & Tracheobronchial Topography

• Right Lung: Possesses 3 lobes (superior, middle, inferior) divided by the horizontal and oblique fissures; 10 bronchopulmonary segments. At the right hilum, the eparterial bronchus (superior lobar bronchus) resides most superiorly, positioned directly above the pulmonary artery; the pulmonary veins are most anterior and inferior.
• Left Lung: Possesses 2 lobes (superior, inferior) divided by a single oblique fissure; features the cardiac notch and the lingula (anatomical homologue of the right middle lobe, supplied by the superior lobar bronchus); 8 to 10 bronchopulmonary segments. At the left hilum, the pulmonary artery sits most superiorly, the main bronchus is central/posterior, and the pulmonary veins are anterior and inferior.
• Foreign Body Aspiration Topography: The right mainstem bronchus is wider, shorter (approx. $2.5\text{ cm}$ vs $5.0\text{ cm}$), and courses more vertically (branching at roughly $25^\circ$ from the tracheal vertical axis, compared to $45^\circ$ for the left mainstem bronchus). Consequently, aspirated foreign bodies preferentially enter the right tracheobronchial tree:
  • In a supine patient: aspirated contents settle into the superior segment of the right lower lobe or the posterior segment of the right upper lobe.
  • In an erect/standing patient: aspirated contents settle into the posterior basal segment of the right lower lobe.

Mediastinal Compartments

A horizontal transverse thoracic plane passing through the sternal angle of Louis anteriorly (junction of manubrium and sternal body at the 2nd costal cartilage) to the intervertebral disc between $T_4$ and $T_5$ posteriorly divides the mediastinum into superior and inferior subdivisions:

Coronary Artery Topography & Conduction System

• Coronary Dominance: Determined strictly by which coronary artery gives origin to the Posterior Descending Artery (PDA / posterior interventricular artery), which perfuses the posterior third of the interventricular septum and diaphragmatic ventricular walls:
  • Right-dominant (approx. 85%): PDA arises from the Right Coronary Artery (RCA).
  • Left-dominant (approx. 8-10%): PDA arises from the Left Circumflex Artery (LCx).
  • Codominant (approx. 7%): Branches from both the RCA and LCx contribute to the posterior interventricular groove.
• Sinoatrial (SA) Node: Located subepicardially in the right atrium at the junction of the superior vena cava and the sulcus terminalis. Supplied by the sinus nodal artery (originates from the RCA in 60%, LCx in 40%).
• Atrioventricular (AV) Node: Located subendocardially within the Triangle of Koch in the right atrium (bounded by the tendon of Todaro, the coronary sinus ostium, and the septal leaflet of the tricuspid valve). Perfused by the AV nodal artery, which arises from the dominant coronary artery at the crux (RCA in 85-90%). Therefore, acute inferior myocardial infarctions resulting from RCA thrombosis frequently precipitate severe AV nodal blocks.
• Bundle of His & Branches: Crosses the central fibrous body; the right bundle branch sweeps through the septomarginal trabecula (moderator band) to supply the anterior papillary muscle of the right ventricle.

Pleura and pleural reflections

• The parietal pleura is sensitive to pain through the intercostal and phrenic nerves; diaphragmatic pleurisy can refer pain to the shoulder (C3 to C5). The visceral pleura is insensitive.
• Lines of reflection (the "2, 4, 6, 8, 10, 12" rule): the pleura reaches the 6th costal cartilage at the midclavicular line, the 8th rib at the midclavicular line, the 10th rib at the midaxillary line and the 12th rib posteriorly. The lungs stop about two ribs higher (6, 8, 10). The gap is the costodiaphragmatic recess, where fluid collects first and which is the target of thoracentesis.
• The cervical pleura and lung apex rise above the medial clavicle, so they can be punctured during subclavian cannulation or by deep neck stab wounds, causing pneumothorax.

Pericardium and great vessels

• Fibrous pericardium and the serous pericardium (parietal and visceral layers) enclose the heart.
  • The transverse sinus lies behind the aorta and pulmonary trunk; a surgeon's finger passes through it to clamp these vessels in bypass surgery.
  • The oblique sinus lies behind the left atrium.
  • Pericardiocentesis is done through the left xiphocostal angle (subxiphoid approach), directing the needle toward the left shoulder.
• Phrenic nerves run anterior to the lung roots on the pericardium; vagus nerves run posterior to the lung roots.
• Arch of the aorta branches (left to right): left subclavian, left common carotid and the brachiocephalic trunk, which divides into the right subclavian and right common carotid arteries.
• The ligamentum arteriosum (the remnant of the ductus) is hooked by the left recurrent laryngeal nerve, so aortic aneurysms or left hilar tumors cause hoarseness.
• The thoracic duct ascends between the aorta and azygos vein and enters the junction of the left internal jugular and subclavian veins.