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AN21.1-11 | Thoracic cage — SDL Guide (Part 2)

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.

Figure: Layers of the Intercostal Space (AN21.5)

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.

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

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.

Typical and Atypical Intercostal Nerves (AN21.6)

A typical intercostal nerve (nerves of spaces 3–6) is the ventral ramus of its corresponding thoracic spinal nerve. It travels along the intercostal space with the VAN bundle and has the following branches:

• Muscular branches — supply the three intercostal muscles
• Collateral branch — runs along the upper border of the rib below (unlike the main nerve which runs along the lower border of the rib above)
• Lateral cutaneous branch — pierces the intercostal muscles near the mid-axillary line, divides into anterior and posterior branches to supply skin
• Anterior cutaneous branch — emerges near the sternum to supply the skin of the anterior chest wall

Atypical intercostal nerves:

• 1st intercostal nerve — most of its fibres join the brachial plexus (T1). The remaining small 1st intercostal nerve has no lateral cutaneous branch and no anterior cutaneous branch.

• 2nd intercostal nerve — gives off the intercostobrachial nerve (the lateral cutaneous branch of T2), which crosses the axilla to supply the skin of the medial aspect of the upper arm. This is the nerve that causes referred pain from the heart to the inner arm during a heart attack.

• 7th–11th intercostal nerves — leave the intercostal space anteriorly and continue into the anterior abdominal wall between the internal oblique and transversus abdominis muscles. They are called thoraco-abdominal nerves because they supply both thoracic and abdominal wall.

• Subcostal nerve (T12) — runs below the 12th rib (not between two ribs, so technically not 'intercostal'). It enters the abdominal wall.

Dermatome landmarks: T4 = nipple, T6 = xiphoid, T10 = umbilicus. These are essential for testing sensory levels in spinal injuries.