The external intercostal muscles are the outermost of the three intercostal muscle groups. The intercostals together can be thought of as the thoracic equivalent of the three anterior abdominal muscles. The external intercostals are most active during inspiration, they increase or maintain the tone of the intercostal spaces.

External Intercostals

 

There are 11 pairs of external intercostal muscles.

The fibres run obliquely downwards from the rib above to the rib below - think ‘hand in pockets.’

 

Attachments

They are attached posteriorly to the tubercles of the ribs.

Anteriorly the muscle fibres are replaced by the external intercostal membranes and are attached to the costochondral junctions

Each muscle attached superiorly to the inferior border of the rib above and the inferiorly to the superior border of the rib below.

Along the inferior thoracic cage the external intercostals are continuous with the external oblique muscle of the anterior abdominal wall

 

Vascular supply

Supplied by intercostal arteries

Drained by intercostal veins

 

Innervation

Intercostal nerves - anterior rami of the first 11 thoracic spinal nerves

 

Action 

Elevation of the ribs during forced inspiration

The intercostals also serve to strengthen the tissues of the intercostal spaces, thereby preventing the sucking in or the blowing out of the tissues with changes in interthoracic pressure during respiration

Attachments	Vascular supply	innervation	Actions
Anterior - costochondral junctions via the external intercostals membrane	Arterial - intercostal arteries	Intercostal nerves	Elevation of ribs during forced inspiration
Posterior - tubercles of the ribs	Venous - intercostal veins		Strengthening of intercostal spaces
Superior - inferior border of rib above
Inferior - superior border of rib below

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The external intercostal runs obliquely downwards in the direction of someone putting their hands into their pockets. It is comparable to, and continuous with the external oblique muscle of the anterior abdominal wall.

 

Intuitively, the internal intercostal muscles that run in a perpendicular direction, have an opposite mode of action which will be explored in their specific section.

 

The neurovascular supply of all the intercostals is derived from the bundle in the costal groove. The origins of these vessels and nerves will be covered in the neurovascular bundle section.

Snell, R. S. (2004). Clinical Anatomy by Regions. 7th ed. United States of America: Lippincott Williand & Wilkins. p46-55.

Moore, K. L., Dalley, A. F. (2006). Clinically Orientated Anatomy. 5th ed. United States of America: Lippincott Williams & Wilkins. p77-79.