Intercostal muscle

The intercostal muscles ("intercostales interni") are a group of skeletal muscles between the ribs. These muscles help expand and shrink the size of the chest cavity to help breathing.
When breathing in, the muscles of the diaphragm contract. This pulls the diaphragm downwards, and increases the volume of the thorax. At the same time, the external intercostal muscles contract. This pulls the ribcage upwards and outwards. This also increases the volume of the thorax. 
There are three main layers;