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AN21.1-11 | Thoracic cage — Part 2

Thoracic Inlet and Outlet — Two Openings, Two Functions (AN21.4)

The thoracic cage has an opening at the top and one at the bottom:

Thoracic inlet (= superior thoracic aperture) — the gateway between the neck and the thorax. Shaped like a kidney bean, it's tilted forward (slopes downward from back to front).

Boundaries:
• Posterior: body of T1 vertebra
• Lateral: 1st rib and its costal cartilage on each side
• Anterior: superior border of the manubrium (jugular notch)

What passes through: trachea, oesophagus, major vessels (subclavian arteries, brachiocephalic veins), vagus and phrenic nerves, sympathetic trunks, thoracic duct (left side), lymphatic duct (right side). A cervical rib (an extra rib from C7) or a tight fibrous band can compress the subclavian artery and lower trunk of the brachial plexus here — causing thoracic outlet syndrome (despite the name, the compression is at the inlet).

Thoracic outlet (= inferior thoracic aperture) — the larger opening at the bottom, closed by the diaphragm.

Boundaries:
• Posterior: body of T12 vertebra
• Posterolateral: 11th and 12th ribs
• Anterolateral: costal margins (cartilages of ribs 7–10)
• Anterior: xiphoid process

The diaphragm seals this opening, with three major apertures: the aortic hiatus (T12), the oesophageal hiatus (T10), and the vena caval foramen (T8). Mnemonic for levels: "I 8 10 Eggs At 12" — IVC at T8, oEsophagus at T10, Aorta at T12.

Layers of the Intercostal Space (AN21.5)

The space between two adjacent ribs is the intercostal space. There are 11 intercostal spaces on each side (between ribs 1-2, 2-3, ... 11-12). Each space contains three layers of muscle, along with nerves and vessels.

From superficial to deep, the three muscle layers are:

• External intercostal muscle — the outermost layer. Fibres run downward and forward (like putting your hands in your pockets). It extends from the tubercle of the rib posteriorly to the costochondral junction anteriorly, where it is replaced by the external intercostal membrane (a thin aponeurosis).

• Internal intercostal muscle — the middle layer. Fibres run downward and backward (at right angles to the external). It extends from the sternum anteriorly to the angle of the rib posteriorly, where it is replaced by the internal intercostal membrane.

• Innermost intercostal muscle — the deepest layer. Fibres run in the same direction as the internal intercostal. It's an incomplete layer — separated from the internal intercostal by the neurovascular bundle (this is where the intercostal vessels and nerve lie).

Think of it as a sandwich: External muscle → neurovascular bundle → Innermost muscle, with the internal muscle on the outside of the neurovascular plane.

Action: During quiet breathing, the external intercostals are the primary muscles of inspiration (they elevate the ribs). The internal intercostals (especially the interosseous part) are muscles of expiration (they depress the ribs). During forced breathing, both layers work harder, assisted by accessory muscles.

The Intercostal Neurovascular Bundle — VAN Order (AN21.6, AN21.7)

The intercostal vessels and nerve travel together as a bundle in the costal groove on the inferior border of each rib, between the internal and innermost intercostal muscles.

The order from above downward is VAN — a mnemonic you must never forget:

• V — Intercostal Vein (most superior)
• A — Intercostal Artery (middle)
• N — Intercostal Nerve (most inferior, and therefore most exposed)

The intercostal arteries come from two sources:
• Posterior intercostal arteries — the upper two spaces (1st and 2nd) are supplied by the supreme (superior) intercostal artery (from the costocervical trunk). Spaces 3–11 are supplied directly by the thoracic aorta.
• Anterior intercostal arteries — spaces 1–6 from the internal thoracic (internal mammary) artery; spaces 7–9 from the musculophrenic artery (a terminal branch of the internal thoracic).

The anterior and posterior intercostal arteries anastomose with each other — creating a ring of blood supply around each intercostal space.

The intercostal veins drain posteriorly into the azygos system (right side) and hemiazygos system (left side).

Clinical pearl — chest drain insertion: When inserting a chest tube, you go just above the rib below (through the lower part of the intercostal space). Why? Because the VAN bundle runs in the costal groove along the inferior border of the rib above. Inserting just above the lower rib avoids the neurovascular bundle. Never insert along the upper border of an intercostal space — you'll hit the VAN.