Thorax Overview

Superior: Thoracic inlet, bounded by the manubrium (anteriorly), 1st ribs and costal cartilages (laterally), and T1 vertebral body (posteriorly).

• Inferior: Diaphragm, separating the thoracic cavity from the abdominal cavity.
• Anterior: Sternum and costal cartilages.
• Posterior: Thoracic vertebrae and posterior ribs.
• Lateral: Ribs and intercostal spaces.

• The superior thoracic aperture; a relatively small, kidney-shaped opening.
• Transmits structures moving between the neck and thorax: trachea, oesophagus, great vessels (subclavian vessels, brachiocephalic veins, and the origin of the great arteries), vagus and phrenic nerves, thoracic duct, sympathetic trunks.
• Clinically important as the site of thoracic outlet syndrome (compression of the brachial plexus/subclavian vessels, often related to a cervical rib narrowing this space; see Upper Limb content for detail).

• Often used interchangeably with "thoracic inlet" in clinical contexts (the anatomical superior thoracic aperture), though strictly the term can also refer to the inferior thoracic aperture (bounded by the xiphisternal joint, costal margins, and T12 vertebra) through which the diaphragm separates the thorax from the abdomen, terminology varies by source, and this is a common point of confusion worth clarifying explicitly with students.

• Composed of the sternum, costal cartilages, ribs, thoracic vertebrae, and intercostal muscles, with an outer covering of skin, fascia, and muscle (pectoral, and back muscles overlying the posterior wall).
• Provides protection for thoracic viscera and forms a mechanically efficient structure for respiration (the "bucket-handle" and "pump-handle" rib movements).

• Divided into three main compartments: the two pleural cavities (each containing a lung) and the mediastinum (central compartment between them, containing the heart and other structures).
• Lined by endothoracic fascia deep to the thoracic wall.

• Potential spaces between the visceral and parietal pleura surrounding each lung, containing a thin film of pleural fluid.
• Facilitate frictionless lung movement during respiration and maintain the negative intrapleural pressure essential for lung inflation.

• Central compartment of the thoracic cavity, between the two pleural cavities.
• Divided into superior and inferior mediastinum (by an imaginary plane at the sternal angle, T4/T5 level); the inferior mediastinum is further divided into anterior, middle, and posterior parts.
• Contains the heart, great vessels, trachea, oesophagus, thymus, thoracic duct, and major nerves; full detail in the Mediastinum section.

• Lungs (paired, within the pleural cavities).
• Heart (within the middle mediastinum, enclosed by the pericardium).
• Trachea and bronchi (conducting airway).
• Oesophagus (posterior mediastinum, food conduit).
• Thymus (superior/anterior mediastinum, more prominent in childhood, involutes with age).

• Aorta (ascending, arch, descending thoracic), the great systemic outflow vessel.
• Pulmonary trunk and arteries: carry deoxygenated blood to the lungs.
• Superior vena cava: drains venous blood from the head, neck, and upper limbs to the right atrium.
• Azygos system (azygos, hemiazygos, accessory hemiazygos veins), drains the posterior thoracic and abdominal wall, providing a collateral pathway between the SVC and IVC systems.
• Pulmonary veins: carry oxygenated blood from the lungs to the left atrium.

• Vagus nerves (right and left), parasympathetic supply to thoracic and abdominal viscera; give rise to the recurrent laryngeal nerves.
• Phrenic nerves (right and left), sole motor supply to the diaphragm; also carry diaphragmatic sensation.
• Sympathetic trunks: paravertebral chains running the length of the thorax, giving rise to the splanchnic nerves.
• Intercostal nerves: segmental sensory and motor supply to the thoracic wall.

Sternal angle (Angle of Louis), jugular (suprasternal) notch, clavicles, xiphisternal joint, costal margins, intercostal spaces, apex beat location. Full detail in the Surface and Applied Anatomy section.

• Transpyloric plane: abdominal reference, not thoracic, but often taught alongside for orientation.
• Sternal angle plane (T4/T5 transverse plane): A critical thoracic reference, corresponds to the junction of the manubrium and body of the sternum; marks the level of the tracheal bifurcation, aortic arch (beginning and end), 2nd costal cartilage (rib counting landmark), superior mediastinum/inferior mediastinum boundary, and the azygos vein's termination into the SVC.

• Inspiration: Active process, diaphragmatic contraction increases the vertical (superoinferior) dimension of the thoracic cavity; external intercostal contraction elevates the ribs, increasing the anteroposterior and transverse dimensions via the "pump-handle" (upper ribs, AP diameter) and "bucket-handle" (lower ribs, transverse diameter) movements. This expansion reduces intrapleural and intrapulmonary pressure below atmospheric, drawing air in.
• Expiration: Passive at rest (elastic recoil of the lungs and thoracic wall); becomes active in forced expiration (exercise, coughing), recruiting internal intercostals and abdominal wall muscles to actively reduce thoracic volume.
• Accessory muscles of respiration (sternocleidomastoid, scalenes) assist inspiration in states of respiratory distress, elevating the sternum and first two ribs further.