Internal intercostal muscles

Dr Henry Knipe and Dr Craig Hacking et al.

The internal intercostal muscles are an important muscle of respiration. They number eleven on each side and are located in the intercostal spaces, which reduces the transverse dimension of the thoracic cavity during expiration.

The internal intercostal muscles are the middle muscle of the three intercostal muscles and arise from the costal groove of the rib above. The fibres run in a downwards, backwards and lateral direction (perpendicular to the external intercostal muscles) and insert into the superior border of the rib below. As their name indicates, they are internal to the external intercostal muscles. Anteriorly, the lower muscles become continuous with the internal oblique muscles in the anterolateral abdominal wall.

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.

Internal intercostal muscle contraction causes reduction in volume of the thoracic cavity in the transverse dimension, expelling air from the lungs during inspiration. They are weaker than the external intercostal muscles.

Thoracic anatomy
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Article information

rID: 37684
Section: Anatomy
Synonyms or Alternate Spellings:

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    Figure 1: Intercostal space
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