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.

The external intercostal muscles are the outermost muscle of the three intercostal muscles and arise from the lower border of the rib above the space. The fibres 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 continues 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.

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.

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.

Thoracic anatomy
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Article Information

rID: 26606
Section: Anatomy
Synonyms or Alternate Spellings:
  • Intercostales externi
  • External intercostal muscles
  • Outermost intercostal muscle
  • Outermost intercostal muscles

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    Figure 1: Intercostal space
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