Intercostal muscle

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Intercostal muscles are several groups of muscles that run between the ribs, and help form and move the chest wall.

There are three principle layers;

the external intercostal muscles, which aid in quiet and forced inhalation.
the internal intercostal muscles, which aid in forced expiration (quiet expiration is a passive process).
the innermost intercostal muscle

Their fibers run in opposite directions.

The external intercostal muscles originate on ribs 1-11 and have their insertion on ribs 2-12. The external intercostals are responsible for the elevation of the ribs, and expanding the transverse dimensions of the thoracic cavity.

The internal intercostal muscles originate on ribs 1-11 and have their insertions on ribs 2-12. The internal intercostals are responsible for the depression of the ribs decreasing the transverse dimensions of the thoracic cavity.

Both these muscles are innervated by the intercostal nerves (=ventral rami of thoracic spinal nerves), and are provided by the intercostal arteries and veins.

Besides the scaleni, who also move the chest wall and have a function in inhalation, other intercostal muscles are:

Subcostal muscles
Transversus thoracis muscle

[edit] Exercise

Side Bends: Stand erect, with your feet set a little wider than the width of your shoulders on each side. Place a broom stick, or an unloaded barbell bar across your neck and grasp the ends of it with your hands. Stiffen your legs and keep stiffened throughout the set you are doing. From this basic starting position, bend at the waist as far to the left as possible. Immediately bend as far to the right as possible. Rhythmically move back and forth between these two positions for a full set of the required number of repetitions, counting each complete cycle of bending both to the left and right as one repetition.