Intercostal Muscles
Three layers of intercostal muscle fill each intercostal space. Their fibre directions and the neurovascular bundle position have direct clinical relevance to chest drain insertion and regional anaesthesia.
✦ The Mnemonic
"Externally Inspire, Internally Exhale, Innermost Integrates"
External (inspiration) · Internal (forced expiration) · Innermost · VAN bundle between inner layers
Clinical Breakdown
External intercostals: fibres run inferoanteriorly (hands-in-pockets). Contraction elevates ribs and expands the thorax. Anteriorly they become the external intercostal membrane.
The neurovascular bundle (VAN: Vein, Artery, Nerve — superior to inferior) runs in the costal groove on the inferior rib surface between the internal and innermost intercostal layers. Chest drain insertion must pass over the upper border of the lower rib in each space to avoid the bundle.
During quiet breathing, only external intercostals and diaphragm are active in inspiration; expiration is passive. During forced expiration (coughing, exercise), internal intercostals and abdominal muscles actively compress the thorax. The diaphragm provides ~75% of tidal volume.
⭐ Clinical Pearl
Chest drain rule: always aim over the top of the rib. The VAN bundle (Vein-Artery-Nerve) occupies the costal groove below each rib. The intercostal artery is the most dangerous structure — haemorrhage from it can be severe, particularly posteriorly where the artery is larger.