Intercostal muscles

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

There are three principal layers; Located around the the ribs
 * the external intercostal muscles, which aid in quiet and forced inhalation. They 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, which aid in forced expiration (quiet expiration is a passive process). They 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.
 * the innermost intercostal muscle

Both the external and internal muscles are innervated by the intercostal nerves (=ventral rami of thoracic spinal nerves), and are provided by the intercostal arteries and intercostal veins. Their fibers run in opposite directions.

Other intercostal muscles
Besides the scaleni, who also move the chest wall and have a function in inhalation, other intercostal muscles are:
 * Subcostal muscles
 * Transversus thoracis muscle