External intercostal muscles
The external intercostal muscles, or external intercostals (Intercostales externi) are eleven in number on both sides.
|External intercostal muscles
Position of the external intercostal muscles (shown in red) seen from the back.
|Lower border of ribs
|Upper border of rib below
|intercostales interni muscles
|musculi intercostales externi
|Anatomical terms of muscle
The muscles extend from the tubercles of the ribs behind, to the cartilages of the ribs in front, where they end in thin membranes, the external intercostal membranes, which are continued forward to the sternum. These muscles work in unison when inhalation occurs. The internal intercostal muscles relax while the external muscles contract causing the expansion of the chest cavity and an influx of air into the lungs.
Each arises from the lower border of a rib, and is inserted into the upper border of the rib below. In the two lower spaces they extend to the ends of the cartilages, and in the upper two or three spaces they do not quite reach the ends of the ribs.
- Position of the external intercostal muscles (shown in red). Animation.
- Deep muscles of the chest and front of the arm, with the boundaries of the axilla. (Intercostalis externus labeled at bottom center.)
- A central rib of the left side. Inferior aspect.
- Diagram of the course and branches of a typical intercostal nerve.
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