<unk> s are several groups of muscles that run between the ribs, and help form and move the chest wall.
There are three principal layers;
the external<unk> s, which aid in quiet and forced inhalation. They originate on ribs 1-11 and have their insertion on ribs 2-12. The external<unk> s are responsible for the elevation of the ribs, and expanding the transverse dimensions of the thoracic cavity.
Located around the the ribs
the internal<unk> s, 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<unk> s are responsible for the depression of the ribs decreasing the transverse dimensions of the thoracic cavity.
the innermost<unk> 
Both the external and internal muscles are innervated by the<unk>  nerves (the ventral rami of thoracic spinal nerves), and are provided by the<unk>  arteries and<unk>  veins. Their fibers run in opposite directions.
Intercostal muscle
Internal intercostal muscles
Subcostales muscle
External intercostal muscles
Diaphragm 
Mediastinal pleura
Levatores costarum muscles
Serous pericardium