External intercostal muscle

The external (or outermost) intercostal muscles are important muscles of respiration. They number eleven on each side and are located in the intercostal space, expanding the transverse dimension of the thoracic cavity during inspiration.

Gross anatomy

The external intercostal muscles are the outermost muscle of the three intercostal muscles and arise from the lower border of the rib above the respective intercostal space. The fibers run in a downwards, forwards and medial direction and insert into the outer lip of the superior border of the rib below. At their insertion they end in thin anterior intercostal membranes that continue towards the sternum. Anteriorly the lower muscles become continuous with the external oblique muscles in the anterolateral abdominal wall. As their name indicates, they are external to the internal intercostal muscles.

Blood supply

• arterial supply: anterior and posterior intercostal arteries
• venous drainage: internal thoracic vein and intercostal veins

Innervation

Muscular branches from the intercostal nerves of the respective intercostal space (T1-T11), which run with the intercostal vessels under the costal groove in between the internal and innermost intercostal muscles.

Action

External intercostal muscle contraction causes expansion of the thoracic cavity in the transverse dimension and causes an influx of air into the lungs during inspiration. They are stronger than the internal intercostal muscles.