External intercostal muscles

The external intercostal muscles are the most superficial layer of the intercostal musculature, occupying the spaces between adjacent ribs. They play a primary role in respiration, particularly during inspiration, by facilitating expansion of the thoracic cavity.

Attachments and extent

• Origin: Inferior border of the rib above

• Insertion: Superior border of the rib below

• Each muscle spans the intercostal space from the tubercle of the rib posteriorly, to the costochondral junction anteriorly. From here onwards, the muscle is replaced by the external intercostal membrane

Fibre direction

• External intercostal muscle fibres run inferiorly and anteriorly (hands-in-pockets orientation)

• This oblique arrangement is optimised for elevating the ribs during contraction

Innervation

• Intercostal nerves (anterior rami of T1 to T11 spinal nerves)

Blood supply

• Posterior intercostal arteries (from the thoracic aorta)

• Anterior intercostal arteries (branches of the internal thoracic artery)

Functions

• Elevate the ribs, increasing the transverse and anteroposterior diameters of the thoracic cavity

• Act as the primary muscles of quiet inspiration

• Contribute to stabilisation of intercostal spaces during breathing

--- Functional Mechanics

• Rib elevation produces:

  • Pump-handle movement → increases anteroposterior diameter

  • Bucket-handle movement → increases transverse diameter

Clinical Relevance

• Essential in normal respiratory mechanics; dysfunction may impair ventilation

• Intercostal nerve injury can lead to segmental weakness and altered breathing patterns

• Important landmarks in thoracic procedures (e.g., chest drain insertion placed above the rib to avoid the neurovascular bundle)